Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Feb. 15, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Entity Registrant Name	AMICUS THERAPEUTICS, INC.
Entity Central Index Key	0001178879
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,325,721,562
Entity Common Stock, Shares Outstanding		223,708,827
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	false

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 79,749	$ 49,060
Investments in marketable securities	424,403	309,502
Accounts receivable	21,962	9,464
Inventories	8,390	4,623
Prepaid expenses and other current assets	16,592	19,316
Total current assets	551,096	391,965
Property and equipment, less accumulated depreciation of $15,671 and $12,515 at December 31, 2018 and 2017, respectively	11,375	9,062
In-process research & development	23,000	23,000
Goodwill	197,797	197,797
Other non-current assets	6,683	5,200
Total Assets	789,951	627,024
Current liabilities:
Accounts payable, accrued expenses, and other current liabilities	80,625	53,890
Deferred reimbursements	5,500	7,750
Contingent consideration payable	0	8,400
Total current liabilities	86,125	70,040
Deferred reimbursements	10,156	14,156
Convertible notes	175,006	164,167
Senior secured term loan	146,734	0
Contingent consideration payable	19,700	17,000
Deferred income taxes	6,465	6,465
Other non-current liabilities	2,853	2,346
Total Liabilities	447,039	274,174
Commitments and contingencies
Stockholders' equity:
Common stock, $.01 par value, 500,000,000 shares authorized, 189,383,924 shares issued and outstanding at December 31, 2018 Common stock, $.01 par value, 250,000,000 shares authorized, 166,989,790 shares issued and outstanding at December 31, 2017	1,942	1,721
Additional paid-in capital	1,740,061	1,400,758
Accumulated other comprehensive loss:
Foreign currency translation adjustment	495	(1,659)
Unrealized loss on available-for securities	(427)	(436)
Warrants	13,063	16,076
Accumulated deficit	(1,412,222)	(1,063,610)
Total stockholders' equity	342,912	352,850
Total Liabilities and Stockholders' Equity	$ 789,951	$ 627,024

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accumulated depreciation of property and equipment (in dollars)	$ 15,671	$ 12,515
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	500,000,000	250,000,000
Common stock, shares issued	189,383,924	166,989,790
Common stock, shares outstanding	189,383,924	166,989,790

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Net product sales	$ 91,245	$ 36,930	$ 4,958
Cost of goods sold	14,404	6,236	833
Gross profit	76,841	30,694	4,125
Operating expenses:
Research and development	270,902	149,310	104,793
Selling, general and administrative	127,200	88,671	71,151
Unrealized change in fair value change during the period, included in Statement of Operations	3,300	(234,322)	6,760
Loss on impairment of assets	0	465,427	0
Restructuring charges	0	0	69
Depreciation	4,216	3,593	3,242
Total operating expenses	405,618	472,679	186,015
Loss from operations	(328,777)	(441,985)	(181,890)
Other income (expenses):
Interest income	10,461	4,096	1,602
Interest expense	(22,402)	(17,240)	(5,398)
Change in fair value of derivatives	(2,739)	0	0
Loss on extinguishment of debt	0	0	(13,302)
Other income (expense)	(5,632)	6,008	(4,793)
Loss before income tax	(349,089)	(449,121)	(203,781)
Income tax benefit	94	165,119	3,739
Net loss attributable to common stockholders	$ (348,995)	$ (284,002)	$ (200,042)
Net loss attributable to common stockholders per common share - basic and diluted (in dollars per share)	$ (1.88)	$ (1.85)	$ (1.49)
Weighted-average common shares outstanding - basic and diluted (in shares)	185,790,021	153,355,144	134,401,588

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (348,995)	$ (284,002)	$ (200,042)
Other comprehensive gain (loss):
Foreign currency translation adjustment gain (loss), net of tax impact of $0, $0, $0, respectively	2,537	(3,604)	1,945
Unrealized gain (loss) on available-for-sale securities	9	(538)	217
Other comprehensive income (loss)	2,546	(4,142)	2,162
Comprehensive loss	$ (346,449)	$ (288,144)	$ (197,880)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Warrants	Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance at Dec. 31, 2015	$ 347,834	$ 1,306	$ 917,454	$ 8,755	$ (115)	$ (579,566)
Balance (in shares) at Dec. 31, 2015		125,027,034
Increase (Decrease) in Stockholders' Equity
Stock issued from exercise of stock options, net	3,036	$ 7	3,029
Stock issued from exercise of stock options, net (in shares)		723,102
Stock issued from ATM transactions	97,068	$ 150	96,918
Stock issued from ATM transactions (in shares)		14,989,027
Stock issued for MiaMed acquisition	4,607	$ 8	4,599
Stock issued for MiaMed acquisition (in shares)		825,603
Restricted stock tax vesting	(1,282)		(1,282)
Restricted stock tax vesting (in shares)		268,425
Stock issued for contingent consideration	6,115	$ 9	6,106
Stock issued for contingent consideration (in shares)		858,795
Receivable from investor	932		932
Warrants issued in debt financing	7,321			7,321
Equity component of the Convertible Notes issuance, net of issuance costs of $2,709	88,346		88,346
Premium paid for Capped Call Confirmations	(13,450)		(13,450)
Stock-based compensation	17,504		17,504
Unrealized holding gain on available-for-sale securities	217				217
Foreign currency translation adjustment	1,945				1,945
Net loss	(200,042)					(200,042)
Balance at Dec. 31, 2016	360,151	$ 1,480	1,120,156	16,076	2,047	(779,608)
Balance (in shares) at Dec. 31, 2016		142,691,986
Increase (Decrease) in Stockholders' Equity
Stock issued from exercise of stock options, net	16,301	$ 29	16,272
Stock issued from exercise of stock options, net (in shares)		2,878,681
Stock issued from equity financing	243,037	$ 212	242,825
Stock issued from equity financing (in shares)		21,122,449
Restricted stock tax vesting	(1,596)		(1,596)
Restricted stock tax vesting (in shares)		296,674
Stock-based compensation	23,101		23,101
Unrealized holding gain on available-for-sale securities	(538)				(538)
Foreign currency translation adjustment	(3,604)				(3,604)
Net loss	(284,002)					(284,002)
Balance at Dec. 31, 2017	$ 352,850	$ 1,721	1,400,758	16,076	(2,095)	(1,063,610)
Balance (in shares) at Dec. 31, 2017	166,989,790	166,989,790
Increase (Decrease) in Stockholders' Equity
Reclassification upon ASU 2018-02 adoption	$ 0				(383)	383
Stock issued from exercise of stock options, net	9,144	$ 14	9,130
Stock issued from exercise of stock options, net (in shares)		1,397,908
Stock issued from equity financing	294,583	$ 202	294,381
Stock issued from equity financing (in shares)		20,239,839
Restricted stock tax vesting	(2,832)		(2,832)
Restricted stock tax vesting (in shares)		303,173
Stock-based compensation	29,260		29,260
Unrealized holding gain on available-for-sale securities	9				9
Foreign currency translation adjustment	2,537				2,537
Net loss	(348,995)					(348,995)
Warrants exercised	3,617	$ 5	6,625	(3,013)
Warrants exercised (in shares)		453,214
Change in fair value of derivatives	2,739		2,739
Balance at Dec. 31, 2018	$ 342,912	$ 1,942	$ 1,740,061	$ 13,063	$ 68	$ (1,412,222)
Balance (in shares) at Dec. 31, 2018	189,383,924	189,383,924

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (348,995)	$ (284,002)	$ (200,042)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of debt discount and deferred financing	10,976	9,703	2,689
Depreciation	4,216	3,593	3,242
Stock-based compensation	29,260	23,101	17,504
Restructuring charges	0	0	69
Change in fair value of derivatives	2,739	(265)	265
Non-cash changes in the fair value of contingent consideration payable	3,300	(234,322)	6,760
Charges to research expense for stock issued in asset acquisition	0	0	4,607
Loss on extinguishment of debt	0	0	13,302
Foreign currency remeasurement (gain) loss	3,217	(5,620)	3,660
Non-cash deferred taxes	0	(167,305)	(3,742)
(Gain) loss on disposal of assets	59	(8)	17
Loss on impairment	0	465,427	0
Changes in operating assets and liabilities:
Accounts receivable	(13,294)	(7,725)	(1,419)
Inventories	(4,205)	(897)	(3,651)
Prepaid expenses and other current assets	2,488	(15,329)	(394)
Other non-current assets	(1,039)	(729)	(970)
Account payable and accrued expenses	17,115	12,563	7,131
Non-current liabilities	458	720	825
Deferred reimbursements	(6,250)	(12,600)	0
Net cash used in operating activities	(299,955)	(213,695)	(150,147)
Investing activities
Sale and redemption of marketable securities	463,502	323,753	221,374
Purchases of marketable securities	(578,394)	(490,468)	(219,932)
Capital expenditures	(6,308)	(4,526)	(5,951)
Net cash used in investing activities	(121,200)	(171,241)	(4,509)
Financing activities
Proceeds from issuance of common stock and warrants, net of issuance costs	294,584	243,037	97,068
Payments of secured loan agreement	0	0	(80,000)
Payment of capital leases	(334)	(308)	(193)
Purchase of vested restricted stock units	(2,832)	(1,596)	(1,282)
Proceeds from exercise of stock options	9,144	16,301	3,036
Proceeds from exercise of warrants	3,617	0	0
Payment of contingent consideration in cash	0	(10,000)	(5,000)
Proceeds from issuance of convertible notes, net of issuance costs	0	0	242,536
Premiums paid for Capped Call Confirmations	0	0	(13,450)
Proceeds from loan agreements, net of issuance costs	146,596	0	30,000
Net cash provided by financing activities	450,775	247,434	272,715
Effect of exchange rate changes on cash, cash equivalents and restricted cash	1,518	1,326	(131)
Net increase (decrease) in cash and cash equivalents and restricted cash	31,138	(136,176)	117,928
Cash and cash equivalents and restricted cash at beginning of year/period	51,237	187,413	69,485
Cash and cash equivalents and restricted cash at end of year/period	82,375	51,237	187,413
Supplemental disclosures of cash flow information
Cash paid during the period for interest	7,500	7,424	2,990
Contingent consideration paid in shares	0	0	6,115
Capital expenditures unpaid at the end of period	106	0	0
Capital expenditures funded by capital lease borrowings	$ 208	$ 0	$ 944

Description of Business	12 Months Ended
Description of Business	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business Amicus Therapeutics, Inc. (the “Company”) is a global patient-dedicated biotechnology company engaged in the discovery, development and commercialization of a diverse set of novel treatments for patients living with rare metabolic diseases. With one medicine for Fabry disease that has achieved widespread global approval, a differentiated biologic for Pompe disease in the clinic and the recent addition of fourteen new gene therapy programs into the pipeline, including two clinical stage gene therapies for Batten disease, the Company has a leading portfolio of medicines for lysosomal storage disorders (“LSDs”). The cornerstone of the Company’s portfolio is Galafold^® (also referred to as “migalastat”), the first and only approved oral precision medicine for people living with Fabry disease who have amenable genetic variants. Migalastat is currently approved under the trade name Galafold^® in the United States (“U.S.”), European Union (“EU”) and Japan, with additional approvals granted and applications pending in several other geographies. During the third quarter of 2018, the Company initiated the commercial launch of Galafold^® in the U.S. for the treatment of adult patients with a confirmed diagnosis of Fabry disease and an amenable genetic variant. The lead biologics program of the Company’s pipeline is Amicus Therapeutics GAA (“AT-GAA”, also known as ATB200/AT2221), a novel, clinical-stage, potential best-in-class treatment paradigm for Pompe disease. The Company’s Chaperone-Advanced Replacement Therapy (“CHART^®”) platform technology is leveraged to develop novel products for Pompe disease and potentially future other LSDs. During the second half of 2018, the Company has expanded its portfolio to include fourteen new gene therapy programs. In September 2018, the Company acquired worldwide development and commercial rights for ten gene therapy programs for neurologic LSDs developed at The Center for Gene Therapy at The Research Institute at Nationwide Children’s Hospital and The Ohio State University through the acquisition of Celenex, Inc., (“Celenex”) a private, clinical stage gene therapy company, for cash consideration of $100.0 million and additional consideration payable upon the achievement of certain development and approval milestones. The acquisition establishes the Company as a leading company in neurologic LSDs. The lead programs in CLN6, CLN3, and CLN8 Batten disease are potential first-to-market curative therapies for these rare, devastating diseases. For additional information see "—Note 3. Acquisitions." In October 2018, the Company further expanded its gene therapy portfolio through a collaboration agreement with the Gene Therapy Program in the Perelman School of Medicine at the University of Pennsylvania (“Penn”) to pursue the research and development of novel gene therapies for four additional indications, including Pompe disease, Fabry disease, CDKL5 deficiency disorder (“CDD”) and one additional undisclosed rare metabolic disorder. This relationship will combine the Company's protein engineering and glycobiology expertise with Penn’s adeno associated virus (“AAV”) gene transfer technologies to develop AAV gene therapies designed for optimal cellular uptake, targeting, dosing, safety and manufacturability. The Company believes that its platform technologies and its product pipeline uniquely positions it and drives its commitment to advancing and expanding a robust pipeline of cutting-edge, first- or best-in-class medicines for rare metabolic diseases. During the third quarter of 2018, the Company entered into a loan agreement with BioPharma Credit PLC, as the lender, for a $150.0 million non-dilutive senior secured term loan (the “Senior Secured Term Loan”) with an interest rate equal to 3-month LIBOR plus 7.50% per annum, subject to a floor and ceiling on the rate, which matures in five years. The Company received net proceeds of $146.6 million in September 2018, after deducting fees and estimated expenses payable by the Company. There are no warrants or any equity conversion features associated with the Senior Secured Term Loan. The proceeds from this financing were used to support the cost of the Celenex acquisition, its related development costs and other general corporate purposes. For additional information, see " —Note 12. Debt." During the first quarter of 2018, the Company issued 20,239,839 shares of its common stock through an underwritten offering resulting in net proceeds of $294.6 million after deducting underwriting discounts and commissions and offering expenses payable by the Company. The Company expects to use the net proceeds of the offering for investment in the U.S. and international commercial infrastructure for Galafold^®, investment in manufacturing capabilities for the ERT ATB200, the continued clinical development of its product candidates, research and development expenditures, clinical and pre-clinical trial expenditures, commercialization expenditures and for other general corporate purposes. For additional information, see “—Note 9. Stockholders’ Equity.” The Company had an accumulated deficit of approximately $1.4 billion as of December 31, 2018 and anticipates incurring losses through the fiscal year ending December 31, 2019 and beyond. The Company has been able to fund its operating losses to date through stock offerings, debt issuances, payments from partners during the terms of the collaboration agreements, and other financing arrangements. The current cash position, including expected Galafold^® revenues, is sufficient to fund ongoing Fabry, Pompe and gene therapy program operations into at least mid-2021. Potential future business development collaborations, pipeline expansion, and investment in manufacturing capabilities could impact the Company’s future capital requirements.

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles ("U.S. GAAP") and include all adjustments necessary for the fair presentation of the Company's financial position for the periods presented. Consolidation The consolidated financial statements include the accounts of the Company and its subsidiaries. Intercompany accounts and transactions are eliminated in consolidation. Foreign Currency Transactions The functional currency for most of the Company's foreign subsidiaries is their local currency. For non-U.S. subsidiaries that transact in a functional currency other than the U.S. dollar, assets and liabilities are translated at current rates of exchange at the balance sheet date. Income and expense items are translated at the average foreign exchange rates for the period. Adjustments resulting from the translation of the financial statements of the Company's foreign operations into U.S. dollars are excluded from the determination of net income and are recorded in accumulated other comprehensive income, a separate component of stockholders' equity. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates. Reclassification Certain prior year amounts have been reclassified for comparative purposes. The reclassifications did not affect results of operations, net assets or cash flows. Cash, Cash Equivalents, Restricted Cash and Marketable Securities The Company considers all highly liquid investments purchased with a maturity of three months or less at the date of acquisition, to be cash equivalents. Marketable securities consist of fixed income investments with a maturity of greater than three months and other highly liquid investments that can be readily purchased or sold using established markets. These investments are classified as available-for-sale and are reported at fair value on the Company’s consolidated balance sheet. Unrealized holding gains and losses are reported within comprehensive income (loss) in the statements of comprehensive loss. Fair value is based on available market information including quoted market prices, broker or dealer quotations or other observable inputs. Restricted cash consists primarily of funds held to satisfy the requirements of certain agreements that are restricted in their use and is included in non-current assets on the Company’s consolidated balance sheet. Concentration of Credit Risk The Company's financial instruments that are exposed to concentration of credit risk consist primarily of cash and cash equivalents and marketable securities. The Company maintains its cash and cash equivalents in bank accounts, which, at times, exceed federally insured limits. The Company invests its marketable securities in high-quality commercial financial instruments. The Company has not recognized any losses from credit risks on such accounts during any of the periods presented. The Company believes it is not exposed to significant credit risk on cash and cash equivalents or its marketable securities. The Company is subject to credit risk from its accounts receivable related to its product sales of Galafold^®. The Company's accounts receivable at December 31, 2018 have arisen from product sales primarily in the EU and the United States. The Company will periodically assess the financial strength of its customers to establish allowances for anticipated losses, if any. For accounts receivable that have arisen from named patient sales, the payment terms are predetermined and the Company evaluates the creditworthiness of each customer on a regular basis. To date, the Company has not incurred any credit losses. Property and Equipment Property and equipment are stated at cost, less accumulated depreciation. Depreciation is calculated over the estimated useful lives of the respective assets, which range from three to five years, or the lesser of the related initial term of the lease or useful life for leasehold improvements. The initial cost of property and equipment consists of its purchase price and any directly attributable costs of bringing the asset to its working condition and location for its intended use. Expenditures incurred after the fixed assets have been put into operation, such as repairs and maintenance, are charged to income in the period in which the costs are incurred. Major replacements, improvements and additions are capitalized in accordance with Company policy. Revenue Recognition The Company’s net product sales consist of sales of Galafold^® for the treatment of Fabry disease. The Company has recorded revenue on sales where Galafold^® is available either on a commercial basis or through a reimbursed early access program (“EAP”). Orders for Galafold^® are generally received from distributors and pharmacies, with the ultimate payor often a government authority. The Company recognizes revenue when its performance obligations to its customers have been satisfied, which occurs at a point in time when the pharmacies or distributors obtain control of Galafold^®. The transaction price is determined based on fixed consideration in the Company’s customer contracts and is recorded net of estimates for variable consideration, which are third party discounts and rebates. The identified variable consideration is recorded as a reduction of revenue at the time revenues from sales of Galafold^® are recognized. The Company recognizes revenue to the extent that it is probable that a significant revenue reversal will not occur in a future period. These estimates may differ from actual consideration received. The Company evaluates these estimates each reporting period to reflect known changes. The Company elected the portfolio approach practical expedient in applying ASC Topic 606, Revenue from Contracts with Customers, to its identified revenue streams. Contracts within each revenue stream have similar characteristics and the Company believes this approach would not differ materially from applying ASC Topic 606 to each individual contract. Inventories and Cost of Goods Sold Prior to regulatory approval of Galafold^®, the Company expensed all manufacturing costs related to Galafold^® as research and development expense. Upon regulatory approval, the Company began capitalizing costs related to the purchase and manufacture of Galafold^®. Inventories are stated at the lower of cost and net realizable value, determined by the first-in, first-out method. Inventories are reviewed periodically to identify slow-moving or obsolete inventory based on projected sales activity as well as product shelf-life. In evaluating the recoverability of inventories produced, the probability that revenue will be obtained from the future sale of the related inventory is considered and inventory value is written down for inventory quantities in excess of expected requirements. Expired inventory is disposed of and the related costs are recognized as cost of product sales in the consolidated statements of operations. Cost of goods sold includes the cost of inventory sold, manufacturing and supply chain costs, product shipping and handling costs, provisions for excess and obsolete inventory, as well as royalties payable. A portion of the inventory available for sale was expensed as research and development costs prior to regulatory approval and as such the cost of goods sold and related gross margins are not necessarily indicative of future cost of goods sold and gross margin. Fair Value Measurements The Company records certain asset and liability balances under the fair value measurements as defined by the FASB guidance. Current FASB fair value guidance emphasizes that fair value is a market-based measurement, not an entity-specific measurement. Therefore, a fair value measurement should be determined based on the assumptions that market participants would use in pricing the asset or liability. As a basis for considering market participant assumptions in fair value measurements, current FASB guidance establishes a fair value hierarchy that distinguishes between market participant assumptions based on market data obtained from sources independent of the reporting entity (observable inputs that are classified within Levels 1 and 2 of the hierarchy) and the reporting entity's own assumptions that market participants assumptions would use in pricing assets or liabilities (unobservable inputs classified within Level 3 of the hierarchy). Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at measurement date. Level 2 inputs are inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. Level 2 inputs may include quoted prices for similar assets and liabilities in active markets, as well as inputs that are observable for the asset or liability (other than quoted prices), such as interest rates, foreign exchange rates, and yield curves that are observable at commonly quoted intervals. Level 3 inputs are unobservable inputs for the asset or liability, which is typically based on an entity's own assumptions, as there is little, if any, related market activity. In instances where the determination of the fair value measurement is based on inputs from different levels of the fair value hierarchy, the level in the fair value hierarchy within which the entire fair value measurement falls is based on the lowest level input that is significant to the fair value measurement in its entirety. The Company's assessment of the significance of a particular input to the fair value measurement in its entirety requires judgment, and considers factors specific to the asset or liability. Contingent Liabilities On an ongoing basis, the Company may be involved in various claims, and legal proceedings. On a quarterly basis, the Company reviews the status of each significant matter and assesses its potential financial exposure. If the potential loss from any claim, asserted or unasserted, or legal proceeding is considered probable and the amount can be reasonably estimated, the Company will accrue a liability for the estimated loss. Because of uncertainties related to claims and litigation, accruals will be based on the Company's best estimates based on available information. On a periodic basis, as additional information becomes available, or based on specific events such as the outcome of litigation or settlement of claims, the Company may reassess the potential liability related to these matters and may revise these estimates, which could result in material adverse adjustments to the Company's operating results. Research and Development Costs Research and development costs are expensed as incurred. Research and development expense consists primarily of costs related to personnel, including salaries and other personnel related expenses, consulting fees and the cost of facilities and support services used in drug development. Assets acquired that are used for research and development and have no future alternative use are expensed as in-process research and development. Interest Income and Interest Expense Interest income consists of interest earned on the Company's cash and cash equivalents and marketable securities. Interest expense consists of interest incurred on debt and capital leases. Income Taxes The Company accounts for income taxes under the liability method. Under this method deferred income tax liabilities and assets are determined based on the difference between the financial statement carrying amounts and tax basis of assets and liabilities and for operating losses and tax credit carry forwards, using enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance is recorded if it is "more likely than not" that a portion or all of a deferred tax asset will not be realized. Other Comprehensive Income (Loss) Components of other comprehensive income (loss) include unrealized gains and losses on available-for-sale securities and gain (loss) on foreign currency transactions, and are included in the statements of comprehensive loss. Leases In the ordinary course of business, the Company enters into lease agreements for office space as well as leases for certain property and equipment. The leases have varying terms and expirations and have provisions to extend or renew the lease agreement, among other terms and conditions, as negotiated. Once the agreement is executed, the lease is assessed to determine whether the lease qualifies as a capital or operating lease. When a non-cancelable operating lease includes any fixed escalation clauses and lease incentives for rent holidays or build-out contributions, rent expense is recognized on a straight-line basis over the initial term of the lease. The excess between the average rental amount charged to expense and amounts payable under the lease is recorded in accrued expenses. Nonqualified Cash Deferral Plan The Company's Cash Deferral Plan (the "Deferral Plan"), provides certain key employees and members of the Board of Directors as selected by the Compensation Committee of the Board of Directors of the Company (the "Compensation Committee"), with an opportunity to defer the receipt of such participant's base salary, bonus and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code (the "Code"). All of the investments held in the Deferral Plan are classified as investments held-to-maturity and recorded at fair value with changes in the investments' fair value recognized as earnings in the period they occur. The corresponding liability for the Deferral Plan is included in other non-current liability in the consolidated balance sheets. Equity-based Compensation At December 31, 2018, the Company had three equity-based employee compensation plans, which are described more fully in "— Note 9. Stockholders' Equity." The Company applies the fair value method of measuring equity-based compensation, which requires a public entity to measure the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award. Loss per Common Share The Company calculates net loss per share as a measurement of the Company's performance while giving effect to all dilutive potential common shares that were outstanding during the reporting period. The Company had a net loss for all periods presented; accordingly, the inclusion of common stock options, unvested RSUs and warrants would be anti-dilutive. Therefore, the weighted average shares used to calculate both basic and diluted earnings per share are the same. See "— Note 17. Basic and Diluted Net Loss per Common Share" for further discussion on net loss per share. Segment Information The Company currently operates in one business segment focused on the discovery, development and commercialization of advanced therapies to treat a range of devastating rare and orphan diseases. The Company is not organized by market and is managed and operated as one business. A single management team reports to the chief operating decision maker who comprehensively manages the entire business. The Company does not operate any separate lines of business or separate business entities with respect to its products. Accordingly, the Company does not accumulate discrete financial information with respect to separate service lines, and thus there is one reporting unit. Business Combinations The Company assigns fair value to the tangible and intangible assets acquired and liabilities assumed based upon their estimated fair values on the acquisition date from acquired businesses. The purchase price allocation process requires management to make significant estimates and assumptions, especially at the acquisition date with respect to intangible assets and in-process research and development ("IPR&D"). In connection with the purchase price allocations for acquisitions, the Company estimates the fair value of contingent payments utilizing a probability-based income approach inclusive of an estimated discount rate. Contingent Consideration Payable The Company determines the fair value of contingent acquisition consideration payable on the acquisition date using a probability-based income approach utilizing an appropriate discount rate. Contingent acquisition consideration payable is shown as a non-current liability on the Company's consolidated balance sheets. The fair value of the contingent consideration payable will be determined each period end and the resulting change will be recorded on the consolidated statements of operations. Intangible Assets and Goodwill The Company records goodwill in a business combination when the total consideration exceeds the fair value of the net tangible and identifiable intangible assets acquired. Purchased IPR&D is accounted for as an indefinite lived intangible asset until the underlying project is completed, at which point the intangible asset will be accounted for as a definite lived intangible asset, or abandoned, at which point the intangible asset will be written off or partially impaired. Goodwill and indefinite lived intangible assets are assessed annually for impairment and whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. If it is determined that the full carrying amount of an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the asset exceeds its fair value. Recent Accounting Developments - Guidance Adopted in 2018 ASU 2018-07 - In June 2018, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting (“ASU 2018-07”).These amendments expand the scope of Topic 718, Compensation-Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to non-employees for goods or services. Consequently, the accounting for share-based payments to non-employees and employees will be substantially aligned. ASU 2018-07 supersedes Subtopic 505-50, Equity-Equity-Based Payments to Non-Employees, and is effective for all public entities for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted, but no earlier than a company’s adoption date of Topic 606, Revenue from Contracts with Customers. The Company early adopted ASU 2018-07 in the second quarter of 2018 and there was no material impact on its consolidated financial statements from the adoption. ASU 2018-02 - In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (“ASU 2018-02”). Prior to ASU 2018-02, U.S. GAAP required the remeasurement of deferred tax assets and liabilities as a result of a change in tax laws or rates to be presented in net income from continuing operations, even in situations in which the related income tax effects of items in accumulated other comprehensive income were originally recognized in other comprehensive income. As a result, such items, referred to as stranded tax effects, did not reflect the appropriate tax rate. Under ASU 2018-02, entities are permitted, but not required, to reclassify from accumulated other comprehensive income to retained earnings those stranded tax effects resulting from the Tax Cuts and Jobs Act of 2017. ASU 2018-02 is effective for all entities for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted. The Company adopted the new standard effective January 1, 2018. As a result of the adoption, the Company reclassified a gain of $383,000 from the foreign currency translation adjustment in accumulated other comprehensive loss to accumulated deficit in the consolidated balance sheet as of December 31, 2018. ASU 2017-09 - In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”). The amendments in ASU 2017-09 provide guidance on determining which changes to the terms and conditions of share-based payment awards require an entity to apply modification accounting under ASU 2017-09. An entity should account for the effects of a modification unless all the following are met: (1) The fair value (or calculated value or intrinsic value, if such an alternative measurement method is used) of the modified award is the same as the fair value (or calculated value or intrinsic value, if such an alternative measurement method is used) of the original award immediately before the original award is modified. If the modification does not affect any of the inputs to the valuation technique that the entity uses to value the award, the entity is not required to estimate the value immediately before and after the modification; (2) The vesting conditions of the modified award are the same as the vesting conditions of the original award immediately before the original award is modified; and (3) The classification of the modified award as an equity instrument or a liability instrument is the same as the classification of the original award immediately before the original award is modified. ASU 2017-09 is effective for all entities for annual periods, including interim periods within those annual periods, beginning after December 15, 2017. The Company adopted this standard on January 1, 2018 and the adoption did not have a material impact on its consolidated financial statements. ASU 2017-01 - In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (“ASU 2017-01”). This ASU clarifies the definition of a business. The amendments affect all companies and other reporting organizations that must determine whether they have acquired or sold a business. The amendments in ASU 2017-01 are effective for public companies for annual periods beginning after December 15, 2017, including interim periods within those periods. The amendments should be applied prospectively as of the beginning of the period of adoption. The Company adopted ASU 2017-01 on January 1, 2018. The adoption of the standard did not have a material impact on its consolidated financial statements. For additional information see "—Note 3. Acquisitions." ASU 2016-18 - In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”). The amendments of ASU 2016-18 require an entity to include amounts generally described as restricted cash and restricted cash equivalents with cash and cash equivalents when reconciling the beginning of period and end of period total amounts on the statement of cash flows. The amendments of ASU 2016-18 are effective for annual reporting periods beginning after December 15, 2017, and interim periods within those annual periods. The Company adopted the guidance in ASU 2016-18 effective January 1, 2018. In connection with the adoption of the standard, the Company applied the guidance retrospectively which resulted in an increase in cash flows from operations of $1.8 million on the statement of cash flows for the year ended December 31, 2017. ASU 2016-16 - In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory (“ASU 2016-16”). ASU 2016-16 requires an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The amendments eliminate the exception for an intra-entity transfer of an asset other than inventory. The amendments in ASU 2016-16 are effective for public business entities for annual periods beginning after December 15, 2017, including interim reporting periods within those annual reporting periods. The Company adopted ASU 2016-16 on January 1, 2018 and there was no material impact from the adoption. ASU 2016-01 - In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”). ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option, and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. In addition, the FASB clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to measure equity investments without readily determinable fair values at either fair value or at cost adjusted for changes in observable prices minus impairment. Companies that elect the fair value option for financial liabilities must recognize changes in fair value related to instrument-specific credit risk in other comprehensive income. Companies must assess valuation allowances for deferred tax assets related to available-for-sale debt securities in combination with their other deferred tax assets. ASU 2016-01 is effective beginning in the first quarter of 2018 and the Company adopted it in the first quarter of 2018. There was no impact on the Company’s consolidated financial statements and related disclosures upon adoption, as the Company does not have equity investments or liabilities with credit risk. In addition, the guidance relating to deferred tax assets did not result in a change in accounting treatment for the Company. ASU 2014-09 - In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) which along with amendments issued in 2015 and 2016, replaced substantially all current U.S. GAAP guidance on this topic and eliminated industry-specific guidance. The new revenue recognition standard requires entities to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Effective January 1, 2018, the Company adopted the new revenue recognition standard using the modified retrospective approach and applied this approach only to contracts that were not completed as of January 1, 2018. The timing of revenue recognition and treatment of contract costs remains unchanged under the new revenue recognition standard. As such, the adoption of the new revenue recognition standard did not have a material impact on the Company's consolidated financial statements. The information presented for the periods prior to January 1, 2018 has not been restated and is reported under the accounting standard in effect for those periods. Recent Accounting Developments - Guidance Not Yet Adopted In August 2018, the Securities Exchange Commission (“SEC”) issued Final Rule 33-10532, Disclosure Update and Simplification, which amends certain disclosure requirements that were redundant, duplicative, overlapping or superseded by other SEC disclosure requirements. The amendments generally eliminated or otherwise reduced certain disclosure requirements of various SEC rules and regulations. However, in some cases, the amendments require additional information to be disclosed, including changes in stockholders’ equity in interim periods. The rule is effective 30 days after its publication in the Federal Register. The rule was posted on October 4, 2018. On September 25, 2018, the SEC released guidance advising it will not object to a registrant adopting the requirement to include changes in stockholders’ equity in the Form 10-Q for the first quarter beginning after the effective date of the rule. The Company is currently assessing the impact that this standard will have on its consolidated financial statements upon adoption and expects to adopt the guidance in it's Form 10-Q for the period ended March 31, 2019. ASU 2018-13 - In August 2018, the FASB issued ASU 2018-03, Fair Value Measurement (Topic 820): Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). The amendments modify the disclosure requirements in Topic 820. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. Early adoption is permitted. An entity is permitted to early adopt any removed or modified disclosures upon issuance of ASU 2018-13 and delay adoption of the additional disclosures until their effective date. The Company is currently assessing the impact that this standard will have on its consolidated financial statements upon adoption. ASU 2017-08 - In March 2017, the FASB issued ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20), Premium Amortization on Purchased Callable Debt Securities (“ASU 2017-08”). The amendments in ASU 2017-08 shorten the amortization period for certain callable debt securities held at a premium. Specifically, the amendments require the premium to be amortized to the earliest call date. The amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. ASU 2017-08 is effective for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted, including adoption in an interim period. If an entity early adopts in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes that interim period. The amendments should be applied on a modified retrospective basis, with a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company is currently assessing the impact that this standard will have on its consolidated financial statements. ASU 2017-04 - In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment (“ASU 2017-04”). To simplify the subsequent measurement of goodwill, ASU 2017-04 eliminates Step 2 from the goodwill impairment test. The annual, or interim, goodwill impairment test is performed by comparing the fair value of a reporting unit with its carrying amount. An impairment charge should be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. In addition, income tax effects from any tax deductible goodwill on the carrying amount of the reporting unit should be considered when measuring the goodwill impairment loss, if applicable. ASU 2017-04 also eliminates the requirements for any reporting unit with a zero or negative carrying amount to perform a qualitative assessment and, if it fails that qualitative test, to perform Step 2 of the goodwill impairment test. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. ASU 2017-04 should be applied on a prospective basis. The nature of and reason for the change in accounting principle should be disclosed upon transition. A public business entity that is a U.S. SEC filer should adopt ASU 2017-04 for its annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company is currently assessing the impact that this standard will have on its consolidated financial statements. ASU 2016-02 - In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires the recognition of lease assets and lease liabilities on the balance sheet for all lease obligations and disclosing key information about leasing arrangements. ASU 2016-02 requires the recognition of lease assets and lease liabilities by lessees for those leases classified as operating leases under previous generally accepted accounting principles. ASU 2016-02 will be effective for the Company for all annual and interim periods beginning after December 15, 2018, including interim periods within those fiscal years. In August 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements, (“ASU 2018-11”). ASU 2018-11 provide entities with an additional transition method for adoption, whereby, an entity initially applies the new leases standard at the adoption date and recognizes a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company will adopt these standards effective January 1, 2019 and elect the transition method in ASU 2018-11. The Company will also elect certain of the practical expedients permitted, including the expedient that permits the Company to retain its existing lease assessment and classification. The Company is currently working through an adoption plan which includes the evaluation of lease contracts compared to the new standard and estimates that the impact that this standard will have on its consolidated financial statements will be an increase to lease liabilities and right-of-use assets of approximately $15 million to $18 million.

Acquisitions	12 Months Ended
Acquisitions	Dec. 31, 2018
Business Combinations [Abstract]
Acquisitions	Acquisitions Acquisition of Celenex In September 2018, the Company expanded its pipeline by acquiring the rights and related intellectual property of ten gene therapy programs through its acquisition of Celenex. Celenex is a private, clinical stage gene therapy company whose lead programs are ten gene therapy programs including CLN6 and CLN3, which are in clinical stage, and several programs in pre-clinical stage. Pursuant to the terms of the agreement, the Company acquired Celenex for cash consideration of $100 million. The Company has also agreed to pay up to an additional $15 million in connection with the achievement of certain development milestones, $262 million in connection with the achievement of certain regulatory approval milestones across multiple programs and up to $75 million in tiered sales milestone payments. Celenex has an exclusive license agreement with Nationwide Children’s Hospital (“NCH”). Under this license agreement, NCH is eligible to receive development and sales based milestones of up to $7.8 million for each product. The Company evaluated the Celenex transaction and concluded that the transaction did not meet the definition of a business and was an asset acquisition. Given the fact that the license has no alternative future use, the $100.0 million upfront payment was expensed to research and development expense in the Consolidated Statements of Operations for the year ended December 31, 2018. Acquisition of MiaMed, Inc. In July 2016, the Company entered into an Agreement and Plan of Merger (the "MiaMed Agreement") with MiaMed, Inc., ("MiaMed"). MiaMed is a pre-clinical biotechnology company focused on developing protein replacement therapy for CDD and related diseases. Under the terms of the MiaMed Agreement, the former holders of MiaMed's capital stock received an aggregate of $6.5 million, comprised of (i) approximately $1.8 million in cash (plus MiaMed's cash and cash equivalents at closing and less any of MiaMed's unpaid third-party fees and expenses related to the transaction), and (ii) 825,603 shares of the Company's common stock. In addition, the Company also agreed to pay up to an additional $83.0 million in connection with the achievement of certain clinical, regulatory and commercial milestones, for a potential aggregate deal value of $89.5 million. The Company evaluated the transaction and concluded that it only acquired inputs and did not acquire any processes. The Company will need to develop its own processes in order to produce an output. Therefore, the Company accounted for the transaction as an asset acquisition and accordingly $6.5 million was expensed to research and development expense in the Consolidated Statements of Operation for the year ended December 31, 2016. Acquisition of Scioderm, Inc. In September 2015, the Company acquired Scioderm Inc., ("Scioderm"), a privately-held biopharmaceutical company focused on developing innovative therapies for treating the rare disease, Epidermolysis Bullosa ("EB"). The acquisition potentially leveraged the Scioderm development team's EB expertise with the Company's global clinical infrastructure to advance SD-101toward regulatory approvals and the Company's commercial, patient advocacy, and medical affairs infrastructure to support a successful global launch. The acquisition of Scioderm was accounted for as a purchase of a business in accordance with ASC 805 Business Combinations. At the end of the first quarter of 2017, the Company achieved 100% enrollment in the Phase 3 clinical study of SD-101 and the milestone payment of $10 million due for this event, was paid in April 2017. On September 13, 2017, the Company reported that top-line data from the randomized, double-blind, placebo-controlled Phase 3 clinical study ("ESSENCE, SD-5") to assess the efficacy and safety of the novel topical wound-healing agent SD-101 did not meet the primary endpoints or secondary endpoints in participants with EB. Based on these top-line data, the Company has no current plans to invest in any additional clinical studies or commercial preparation activities for SD-101. The associated impairment of Scioderm IPR&D is discussed in "— Note 4. Goodwill and Intangible Assets." The Company was previously developing SD-101 in late-stage development as a potential first-to-market therapy for the chronic, rare connective tissue disorder Epidermolysis Bullosa ("EB"). On September 13, 2017, the Company reported that top-line data from the randomized, double-blind, placebo-controlled Phase 3 clinical study ("ESSENCE" or "SD-005") to assess the efficacy and safety of the novel topical wound-healing agent SD-101 did not meet the primary endpoints or secondary endpoints in participants with EB. The Company plans to further analyze and share the Phase 3 ESSENCE results with key stakeholders in the EB community including physicians, patient organizations and regulators. In the interim, in consultation with their physicians, participants in the ongoing extension studies (SD-004 and -006) will have the opportunity to continue being treated with SD-101. Based on the top-line data, the Company has no current plans to invest in any additional clinical studies or commercial preparation activities for SD-101. This event led the Company to assess the carrying amount of the program's tangible and intangible assets against their respective fair values. Based on the assessment, the Company recognized a loss on impairment of intangible assets in the amount of $463.7 million and $1.7 million in fixed assets recorded within Loss on Impairment of Assets within the Consolidated Statements of Operations. Since the study did not meet the primary and secondary endpoints, the Company has concluded that they will not make the potential milestone payments indicated in the Asset Purchase Agreement to the former Scioderm holders. Accordingly, the Company recognized a gain of $254.7 million in Changes in Fair Value of Contingent Consideration Payable in the third quarter of 2017, in order to decrease the liability to zero. The Company also recognized $0.4 million in selling, general and administrative costs and $8.1 million in research and development expenses related to the wind-down of operations for the Phase 3 ESSENCE study and ongoing extension studies SD-004 and SD-006, as well as income tax benefit of $164.7 million due to the reduction of the deferred tax liability related to Scioderm IPR&D, in the Consolidated Statements of Operations in the third quarter of 2017. Acquisition of Callidus Biopharma, Inc. In November 2013, the Company acquired Callidus a privately-held biologics company focused on developing best-in-class ERTs for LSDs with its lead ERT ATB200 for Pompe disease in late preclinical development. The acquisition of the Callidus assets and technology complements Amicus' CHART^® platform for the development of next generation ERTs. The fair value of the contingent acquisition consideration payments was estimated by applying a probability-based income approach utilizing an appropriate discount rate. Key assumptions include discount rate and various probability factors. This estimation was based on significant inputs that are not observable in the market, referred to as Level 3 inputs. Some of the more significant assumptions used in the valuation include (i) the probability and timing related to the achievement of certain developmental milestones and (ii) and the discount rate. See "— Note 11. Assets and Liabilities Measured at Fair Value", for additional discussion regarding fair value measurements of the contingent acquisition consideration payable. The Company determined the fair value of the contingent consideration to be $28.7 million at December 31, 2018, of which $9.0 million is payable over the next twelve months and $19.7 million is payable beyond the next twelve months, resulting in an increase in the contingent consideration payable and related expense of $3.3 million in the year ended December 31, 2018. The expense is recorded in the Consolidated Statement of Operations within the changes in fair value of contingent consideration line item. During the fourth quarter of 2018, the Company reached a clinical milestone for Callidus, which was the dosing of the first patient in a Phase 3 study. The milestone payment for this event was $9.0 million which was paid in the Company's stock during the first quarter of 2019. For further information, see "— Note 4. Goodwill and Intangible Assets."

Goodwill and Intangible Assets

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

In connection with the acquisitions, the Company initially recognized IPR&D of $486.7 million and goodwill of $197.8 million. Intangible assets related to IPR&D assets are considered to be indefinite-lived until the completion or abandonment of the associated research and development efforts. During the period the assets are considered indefinite-lived, they will not be amortized but will be tested for impairment on an annual basis and between annual tests if the Company becomes aware of any events occurring or changes in circumstances that would indicate a reduction in the fair value of the IPR&D assets below their respective carrying amounts.

Goodwill and intangible assets are assessed annually for impairment on October 1 and whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. If it is determined that the full carrying amount of an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the asset exceeds its fair value.

As discussed in "— Note 3. Acquisitions", in September 2017, the Company reported that top-line data from the randomized, double-blind, placebo-controlled Phase 3 clinical study ("ESSENCE, SD-005") to assess the efficacy and safety of the novel topical wound-healing agent SD-101 did not meet the primary endpoints or secondary endpoints in participants with EB. This event led to an assessment to determine if an impairment had occurred for goodwill and IPR&D. Based on tests for impairment, the Company determined that IPR&D had been impaired, however goodwill was not impaired based on qualitative and market capitalization tests performed. The loss on impairment of IPR&D of 463.7 million was recorded within Loss on Impairment of Assets in the Consolidated Statements of Operations for the year ended December 31, 2017.

The following table represents the changes in IPR&D for the years ended December 31, 2018 and 2017, respectively:



	(in millions)
Beginning balance	$	486.7
Impairment in IPR&D related to Scioderm	(463.7		)
Balance at December 31, 2017	$	23.0
Change in IPR&D	—
Balance at December 31, 2018	$	23.0

The following table represents the changes in Goodwill for the years ended December 31, 2018 and 2017, respectively:



	(in millions)
Beginning balance	$	197.8
Change in goodwill	—
Balance at December 31, 2017	$	197.8
Change in goodwill	—
Balance at December 31, 2018	$	197.8

Cash, Cash Equivalents, Marketable Securities and Restricted Cash

12 Months Ended

Dec. 31, 2018

Cash, Cash Equivalents, and Short-term Investments [Abstract]

Cash, Cash Equivalents, Marketable Securities and Restricted Cash

As of December 31, 2018, the Company held $79.7 million in cash and cash equivalents and $424.4 million of available- for-sale debt securities which are reported at fair value on the Company’s consolidated balance sheets. Unrealized holding gains and losses are reported within accumulated other comprehensive loss in the statements of comprehensive loss. If a decline in the fair value of a marketable security below the Company’s cost basis is determined to be other than temporary, such marketable security is written down to its estimated fair value as a new cost basis and the amount of the write-down is included in earnings as an impairment charge.

The Company regularly invests excess operating cash in deposits with major financial institutions, money market funds, notes issued by the U.S. government, as well as fixed income investments and U.S. bond funds, both of which can be readily purchased and sold using established markets. The Company believes that the market risk arising from its holdings of these financial instruments is mitigated as many of these securities are either government backed or of the highest credit rating. Investments that have original maturities greater than three months but less than one year are classified as current, while investments that have maturities greater than one year are classified as long-term.

Cash, cash equivalents and marketable securities are classified as current unless mentioned otherwise below and consisted of the following:



	As of December 31, 2018
(in thousands)	Cost		Unrealized Gain		Unrealized Loss			Fair Value
Cash and cash equivalents	$	79,749	$	—	$	—		$	79,749
Corporate debt securities, current portion	240,969		7		(250		)	240,726
Commercial paper	115,245		—		(104		)	115,141
Asset-backed securities	68,215		4		(84		)	68,135
Money market	350		—		—			350
Certificate of deposit	51		—		—			51
	$	504,579	$	11	$	(438	)	$	504,152
Included in cash and cash equivalents	$	79,749	$	—	$	—		$	79,749
Included in marketable securities, current	424,830		11		(438		)	424,403
Total cash, cash equivalents and marketable securities	$	504,579	$	11	$	(438	)	$	504,152



	As of December 31, 2017
(in thousands)	Cost		Unrealized Gain		Unrealized Loss			Fair Value
Cash and cash equivalents	$	49,060	$	—	$	—		$	49,060
Corporate debt securities, current portion	199,314		1		(303		)	199,012
Commercial paper	79,878		—		(75		)	79,803
Asset-backed securities	30,346		—		(59		)	30,287
Money market	350		—		—			350
Certificate of deposit	50		—		—			50
	$	358,998	$	1	$	(437	)	$	358,562
Included in cash and cash equivalents	$	49,060	$	—	$	—		$	49,060
Included in marketable securities, current	309,938		1		(437		)	309,502
Total cash, cash equivalents and marketable securities	$	358,998	$	1	$	(437	)	$	358,562

For the years ended December 31, 2018 there were nominal realized gains. For the fiscal year ended December 31, 2017, there were no realized gains or losses. The cost of securities sold is based on the specific identification method.

Unrealized loss positions in the available for sale debt securities as of December 31, 2018 and December 31, 2017 reflect temporary impairments that have been in a loss position for less than twelve months and as such are recognized in other comprehensive gain (loss). The fair value of these available for sale debt securities in unrealized loss positions was $403.1 million and $295.1 million as of December 31, 2018 and 2017, respectively.

The following table provides a reconciliation of cash and cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the Statement of Cash Flows:



(in thousands)	December 31, 2018		December 31, 2017		December 31, 2016
Cash and cash equivalents	$	79,749	$	49,060	$	187,026
Restricted cash	2,626		2,177		387
Cash and cash equivalents and restricted cash shown in the statement of cash flows	$	82,375	$	51,237	$	187,413

Stockholders' Equity	12 Months Ended
Stockholders' Equity	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	Stockholders' Equity Common Stock and Warrants As of December 31, 2018, the Company was authorized to issue 500 million shares of common stock. Dividends on common stock will be paid when, and if, declared by the board of directors. Each holder of common stock is entitled to vote on all matters that are appropriate for stockholder voting and is entitled to one vote for each share held. On February 15, 2018, the Company announced the pricing of an underwritten offering of 19,354,839 shares of its common stock at $15.50 per share, resulting in gross proceeds of $300.0 million. Under the terms of the underwriting agreement, the Company granted the underwriters an option, exercisable for 30 days after February 16, 2018, to purchase up to an additional 2,903,225 shares of the Company’s common stock, which was exercised with respect to 885,000 shares of the Company’s common stock at a purchase price of $15.50 per share. The Company received net proceeds of $294.6 million from these offerings, after deducting underwriting discounts and commissions and offering expenses payable by the Company. In April 2018, 453,214 warrants were exercised at $7.98 per share of common stock resulting in gross cash proceeds of $3.6 million. On June 7, 2018, the Company’s stockholders approved an amendment to the Company’s Restated Certificate of Incorporation to increase the number of shares of common stock, par value $0.01 per share, that the Company is authorized to issue from 250,000,000 shares to 500,000,000 shares. In July 2017, the Company entered into an underwriting agreement whereby the Company issued and sold 21,122,449 shares at a price to the public of $12.25 per share, resulting in gross proceeds of $258.8 million, before deducting underwriting discounts and commissions and offering expenses payable by the Company. The offering closed on July 18, 2017 and the Company received net proceeds, after deducting underwriting discounts and commissions and offering expenses payable by the Company of $243.0 million. As discussed in ‘‘— Note 12. Debt’’, in December 2016, the Company issued $250 million aggregate principal amount of 3.0% unsecured Convertible Senior Notes due 2023 (the ‘‘Convertible Notes), in a private offering. The Notes will mature on December 15, 2023, unless earlier repurchased, redeemed, or converted in accordance with their terms. The Notes are convertible at the option of the holders, under certain circumstances and during certain periods, into cash, shares of the Company’s common stock, par value $0.01 per share (‘‘common stock’’), or a combination thereof. Prior to the close of business on the business day immediately preceding September 15, 2023, the Notes are convertible at the option of the holders of the Notes only under certain conditions. On or after September 15, 2023, until the close of business on the second business day immediately preceding the maturity date, holders of the Notes may convert their Notes at their option at the conversion rate then in effect, irrespective of these conditions. The Company will settle conversions of the Notes by paying or delivering, as the case may be, cash, shares of common stock, or a combination of cash and shares of common stock, at the Company’s election. The conversion rate will initially be 163.3987 shares of common stock per $1,000 principal amount of Notes (equivalent to an initial conversion price of approximately $6.12 per share of common stock). The conversion rate is subject to customary adjustments upon the occurrence of certain events. During the first quarter of 2019, the Company entered into separate, privately negotiated exchange agreements with a limited number of holders (the “Holders”) of the Convertible Notes. Under the terms of the exchange agreements (the “Exchange Agreements”), the Holders agreed to exchange an aggregate principal amount of approximately $184.6 million of Convertible Notes held by them in exchange for an aggregate of approximately 33.0 million shares of the our common stock, par value $0.01 per share. In addition, pursuant to the Exchange Agreements, the Company made aggregate cash payments of approximately $0.7 million to the Holders to satisfy accrued and unpaid interest to the closing date of the transaction, along with cash in lieu of fractional shares. Nonqualified Cash Plan The Company's Deferral Plan, (the "Deferral Plan") provides certain key employees and members of the Board of Directors as selected by the Compensation Committee, with an opportunity to defer the receipt of such participant's base salary, bonus and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code of 1986 as amended. The Company had a deferred compensation investment balance of $2.7 million and $2.2 million as of December 31, 2018 and 2017, respectively, with corresponding approximate amounts of liability. Deferral Plan investment assets are classified as trading securities and are recorded at fair value with changes in the investments' fair value recognized in AOCI in the period they occur. Deferred compensation liability amounts under the Deferral Plan are included in other long-term liabilities. Equity Incentive Plans The Company’s Equity Incentive Plans consist of the Amended and Restated 2007 Equity Incentive Plan (the ‘‘Plan’’) and the 2007 Director Option Plan (the ‘‘2007 Director Plan’’). The Plan provides for the granting of restricted stock and options to purchase common stock in the Company to employees, advisors and consultants at a price to be determined by the Company’s board of directors. The Plan is intended to encourage ownership of stock by employees and consultants of the Company and to provide additional incentives for them to promote the success of the Company’s business. The 2007 Director Plan is intended to promote the recruiting and retention of highly qualified eligible directors and strengthen the commonality of interest between directors and stockholders by encouraging ownership of common stock of the Company. Under the provisions of each plan, no option will have a term in excess of 10 years. The Board of Directors, or its committee, is responsible for determining the individuals to be granted options, the number of options each individual will receive, the option price per share, and the exercise period of each option. Options granted pursuant to the Plan generally vest 25% on the first year anniversary date of grant plus an additional 1/48th for each month thereafter and may be exercised in whole or in part for 100% of the shares vested at any time after the date of grant. Options under the 2007 Director Plan may be granted to new directors upon joining the Board and vest in the same manner as options under the Plan. In addition, options are granted to independent directors at each annual meeting of stockholders and vest on the date of the annual meeting of stockholders of the Company in the year following the year during which the options were granted. As of December 31, 2018, the Company has reserved up to 9,457,741 shares for issuance under the Plan and the 2007 Director Plan.

Share based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share based Compensation

The Company’s Equity Incentive Plans consist of the Amended and Restated 2007 Equity Incentive Plan (the “Plan”) and the 2007 Director Option Plan (the “2007 Director Plan”). The Plan provides for the granting of restricted stock units and options to purchase common stock in the Company to employees, advisors and consultants at a price to be determined by the Company’s Board of Directors. The Plan is intended to encourage ownership of stock by employees and consultants of the Company and to provide additional incentives for them to promote the success of the Company’s business. The 2007 Director Plan is intended to promote the recruiting and retention of highly qualified eligible directors and strengthen the commonality of interest between directors and stockholders by encouraging ownership of common stock of the Company. The Board of Directors, or its committee, is responsible for determining the individuals to be granted options, the number of options each individual will receive, the option price per share, and the exercise period of each option.

On December 21, 2018, the Board of Directors of the Company approved an amendment (the “Amendment”) to the Plan. The Amendment provides for certain benefits to qualifying Plan Participants who separate from service with the Company due to death, disability or “Retirement” (as such term is defined under the Plan) (“Qualified Participants”). Options granted under the Plan (“Options”) to a Qualified Participant shall continue to vest until the 2nd anniversary of the Qualified Participant’s separation and all vested Options held by such Qualified Participant shall remain exercisable until the earlier of the 4th anniversary of the Qualified Participant’s separation or the original expiration date of the Option. Options that are not exercised during this exercise period shall be forfeited. Time-based restricted stock units and restricted stock granted to a Qualified Participant under the Plan that was scheduled to vest within the two year period following the Qualified Participant's separation shall accelerate and be delivered upon such separation. Any time-based restricted stock units or restricted stock that would have vested after such two year period will be forfeited upon the Qualified Participant's separation. Also per the Amendment, any performance-based restricted stock units under the Plan (“PRSUs”) received by the Qualified Participant, shall remain eligible to vest after the Qualified Participant’s separation based on the actual performance of the Company through the end of the performance period applicable to any such PRSU.

Stock Option Grants

The Company adopted the fair value method of measuring stock-based compensation, using the fair value of each equity award granted. The Company chose the ‘‘straight-line’’ attribution method for allocating compensation costs and recognized the fair value of each stock option on a straight-line basis over the vesting period of the related awards.

The Company uses the Black-Scholes option pricing model when estimating the grant date fair value for stock-based awards. Use of a valuation model requires management to make certain assumptions with respect to selected model inputs. Expected volatility was based on our historical volatility since our initial public offering in May 2007. Beginning in the third quarter of 2017, the average expected life was determined using our actual historical data versus a ‘‘simplified’’ method used in prior quarters. The ‘‘simplified’’ method of estimating the Company did not have sufficient reliable exercise data to justify a change from the use of the ‘‘simplified’’ method of estimating the expected exercise term of employee stock option grants. The impact from this change was not material. The risk-free interest rate is based on U.S. Treasury, zero-coupon issues with a remaining term equal to the expected life assumed at the date of grant. Forfeitures are estimated based on voluntary termination behavior, as well as a historical analysis of actual option forfeitures.

The fair value of the stock options granted is estimated on the date of grant using a Black-Scholes option pricing model with the following weighted-average assumptions:



	Years Ended December 31,
	2018			2017			2016
Expected stock price volatility	78.6		%	82.8		%	81.3		%
Risk free interest rate	2.4		%	2.0		%	1.5		%
Expected life of options (years)	5.62			6.18			6.25
Expected annual dividend per share	$	0.00		$	0.00		$	0.00

The weighted average grant-date fair value per share of options granted during 2018, 2017 and 2016 were $10.19, $5.09 and $5.28, respectively.

A summary of the Company's stock options for the year ended December 31, 2018 were as follows:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life	Aggregate Intrinsic Value
	(in thousands)					(in millions)
Options outstanding, December 31, 2015	11,729.2		$	7.11
Granted	5,114.1		$	7.67
Exercised	(723.1	)	$	4.20
Forfeited	(622.7	)	$	8.62
Options outstanding, December 31, 2016	15,497.5		$	7.37
Granted	3,695.3		$	7.17
Exercised	(2,878.7	)	$	5.67
Forfeited	(1,133.0	)	$	9.55
Options outstanding, December 31, 2017	15,181.1		$	7.48
Granted	2,348.0		$	14.96
Exercised	(1,398.0	)	$	6.54
Forfeited	(313.1	)	$	9.55
Expired	(8.0	)	10.76
Options outstanding, December 31, 2018	15,810.0		$	8.63	6.7 years	$	37.6
Vested and unvested expected to vest, December 31, 2018	15,152.6		$	8.51	6.6 years	$	36.8
Exercisable at December 31, 2018	9,977.0		$	7.43	5.8 years	$	28.8

The aggregate intrinsic value of options exercised during the years ended December 31, 2018, 2017 and 2016 was $11.9 million, $20.8 million and $2.6 million respectively. Cash proceeds from stock options exercised during the years ended December 31, 2018, 2017 and 2016 were $9.1 million, $16.3 million, and $3.0 million, respectively. As of December 31, 2018, the total unrecognized compensation cost related to non-vested stock options granted was $33.3 million and is expected to be recognized over a weighted average period of 2.4 years.

Restricted Stock Units (“RSUs”) and Performance-Based Restricted Stock Units

RSUs awarded under the Plan are generally subject to graded vesting and are contingent on an employee’s continued service. RSUs are generally subject to forfeiture if employment terminates prior to the release of vesting restrictions. The Company expenses the cost of the RSUs, which is determined to be the fair market value of the shares of common stock underlying the RSUs at the date of grant, ratably over the period during which the vesting restrictions lapse. A summary of non-vested RSU activity under the Plan for the year ended December 31, 2018 is as follows:



	Number of Share		Weighted Average Grant Date Fair Value		Weighted Average Remaining Years	Aggregate Intrinsic Value
	(in thousands)					(in millions)
Non-vested units as of December 31, 2016	744.4		$	7.86
Granted	2,348.7		$	5.69
Vested	(318.2	)	$	9.23
Forfeited	(199.8	)	$	6.24
Non-vested units as of December 31, 2017	2,575.1		$	5.85
Granted	1,811.9		$	16.11
Vested	(530.0	)	$	6.01
Forfeited	(145.0	)	$	9.65
Non-vested units as of December 31, 2018	3,712.0		$	10.59	2.57	$	36.3

On December 30, 2016, the Compensation Committee approved a form of Performance-Based Restricted Stock Unit Award Agreement (the “Performance-Based RSU Agreement”), to be used for performance-based RSUs granted to participants under the Amended and Restated 2007 Equity Incentive Plan, including named executive officers. Certain awards under the Performance- Based RSU Agreement were granted in January 2017 and 2018. The grants include performance-based restricted stock units and market performance-based restricted stock units (“MPRSUs”). The performance-based awards vest over three years based on the Company achieving certain clinical milestones. Each reporting period the Company estimates the amount of the award that will vest based on the probability of achieving the clinical milestones and adjusts the expense, prospectively, when a change occurs. For the MPRSUs, vesting of these awards is contingent upon the Company meeting certain total shareholder return (“TSR”) levels as compared to a select peer group over three years. The MPRSUs cliff vest at the end of the three-year period and have a maximum potential to vest at 200% based on TSR performance. The related share-based compensation expense is determined based on the estimated fair value of the underlying shares on the date of grant and is recognized on a straight-line basis over the vesting term, irrespective of the actual TSR performance. The estimated fair value per share of the MPRSUs was calculated using a Monte Carlo simulation model.

For the year ended December 31, 2018, 530,035 RSUs have vested and all non-vested units are expected to vest over their normal term. As of December 31, 2018, there was $25.8 million of total unrecognized compensation cost related to unvested RSUs with service-based vesting conditions. These costs are expected to be recognized over a weighted average period of 2.5 years.

Compensation Expense Related to Equity Awards

The following table summarizes information related to compensation expense recognized in the statements of operations related to the equity awards:



	Years Ended December 31,
	2018		2017		2016
Equity compensation expense recognized in:
Research and development expense	$	11,740	$	10,328	$	8,071
Selling, general and administrative expense	17,520		12,773		9,433
Total equity compensation expense	$	29,260	$	23,101	$	17,504

Assets and Liabilities Measured at Fair Value

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Assets and Liabilities Measured at Fair Value

The Company's financial assets and liabilities are measured at fair value and classified within the fair value hierarchy which is defined as follows:

Level 1 — Quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date.

Level 2 — Inputs other than quoted prices in active markets that are observable for the asset or liability, either directly or indirectly.

Level 3 — Inputs that are unobservable for the asset or liability.

A summary of the fair value of the Company's recurring assets and liabilities aggregated by the level in the fair value hierarchy within which those measurements fall as of December 31, 2018 are identified in the following table:



(in thousands)	Level 2		Total
Assets:
Commercial paper	$	115,141	$	115,141
Asset-back securities	68,135		68,135
Corporate debt securities	240,726		240,726
Money market funds	3,082		3,082
	$	427,084	$	427,084



	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	19,700	$	19,700
Deferred compensation plan liability	2,732		—		2,732
	$	2,732	$	19,700	$	22,432

A summary of the fair value of the Company's assets and liabilities aggregated by the level in the fair value hierarchy within which those measurements fall as of December 31, 2017 are identified in the following table:



(in thousands)	Level 2		Total
Assets:
Commercial paper	$	79,803	$	79,803
Asset-back securities	30,287		30,287
Corporate debt securities	199,012		199,012
Money market funds	2,598		2,598
	$	311,700	$	311,700



	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	25,400	$	25,400
Deferred compensation plan liability	2,258		—		2,258
	$	2,258	$	25,400	$	27,658

The Company’s Convertible Notes are classified in Level 2 category within the fair value level hierarchy. The fair value was determined using broker quotes in a non-active market for valuation. The fair value of the debt at December 31, 2018 was approximately $437.1 million.

The Company’s Senior Secured Term Loan are classified in Level 2 category within the fair value level hierarchy and the fair value was determined using quoted prices for similar liabilities in active markets, as well as inputs that are observable for the liability (other than quoted prices), such as interest rates that are observable at commonly quoted intervals. The carrying value of the Senior Secured Term Loan approximates the fair value.

The Company did not have any Level 3 assets as of December 31, 2018 or December 31, 2017.

Cash, Money Market Funds and Marketable Securities

The Company classifies its cash and money market funds within the fair value hierarchy as Level 1 as these assets are valued using quoted prices in active market for identical assets at the measurement date. The Company considers its investments in marketable securities as available for sale and classifies these assets within the fair value hierarchy as Level 2 primarily utilizing broker quotes in a non-active market for valuation of these securities. No changes in valuation techniques or inputs occurred during the year ended December 31, 2018. No transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy occurred during the year ended December 31, 2018.

Contingent Consideration Payable

The contingent consideration payable resulted from the acquisition of Callidus, as discussed in "— Note 3. Acquisitions." The most recent valuation was determined using a probability weighted discounted cash flow valuation approach. Using this approach, expected future cash flows are calculated over the expected life of the agreement, are discounted, and then exercise scenario probabilities are applied. The valuation is performed quarterly. Gains and losses are included in the statement of operations.

The contingent consideration payable for Callidus has been classified as a Level 3 recurring liability as its valuation requires substantial judgment and estimation of factors that are not currently observable in the market. If different assumptions were used for the various inputs to the valuation approach the estimated fair value could be significantly higher or lower than the fair value the Company determined. The Company may be required to record losses in future periods, including expenses related to CDD.

The following significant unobservable inputs were used in the valuation of the contingent consideration payable of Callidus for the ATB-200 Pompe program:



Contingent Consideration Liability	Fair value as of December 31, 2018	Valuation Technique	Unobservable Input	Range
			Discount rate	10%
Clinical and regulatory milestones	$19.2 million	Probability weighted discounted cash flow	Probability of achievement of milestones	71.0% - 100.0%
			Projected year of payments	2021 - 2022

Contingent consideration liabilities are remeasured to fair value each reporting period using discount rates, probabilities of payment and projected payment dates. Projected contingent payment amounts related to clinical and regulatory based milestones are discounted back to the current period using a discounted cash flow model. Increases in discount rates and the time to payment may result in lower fair value measurements. Increases or decreases in any of those inputs together, or in isolation, may result in a significantly lower or higher fair value measurement. There is no assurance that any of the conditions for the milestone payments will be met.

The following table shows the change in the balance of contingent consideration payable for the year ended December 31, 2018 and 2017, respectively:



	Year ended December 31,
(in thousands)	2018			2017
Balance, beginning of the period	$	25,400		$	269,722
Payment of contingent consideration in cash	—			(10,000		)
Milestone payable, included in accrued expenses	(9,000		)	—
Unrealized change in fair value change during the period, included in Statement of Operations	3,300			(234,322		)
Balance, end of the period	$	19,700		$	25,400

Deferred Compensation Plan - Investment and Liability

The Deferred Compensation Plan (the “Deferral Plan”) provides certain key employees and members of the Board of Directors with an opportunity to defer the receipt of such participant’s base salary, bonus and director’s fees, as applicable. Deferral Plan assets are classified as trading securities and recorded at fair value with changes in the investment’s fair value recognized in the period they occur. The asset investments consist of market exchanged mutual funds. The Company considers its investments in marketable securities as available-for-sale and classifies these assets and related liability within the fair value hierarchy as Level 2, primarily utilizing broker quotes in a non-active market for valuation of these securities.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Debt

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Debt

Senior Secured Term Loan due 2023

In September 2018, the Company entered into a loan agreement with BioPharma Credit PLC as the lender. The loan agreement provides for a $150 million senior secured term loan (“Senior Secured Term Loan”) with an interest rate equal to the 3-month LIBOR plus 7.50% per annum and matures in five years. The Senior Secured Term Loan will be repaid in four quarterly payments equal to 12.50% thereof starting on the forty-eight month anniversary of the date of the first credit extension with the balance due on the Maturity Date. Interest is payable quarterly in arrears. The Senior Secured Term Loan contains certain customary representations and warranties, affirmative and negative covenants and events of default applicable to the Company and certain of its subsidiaries, but does not include any financial covenants relating to the achievement or maintenance of revenue or cash flow. If an event of default occurs and is continuing, the lender may declare all amounts outstanding under the Senior Secured Term Loan to be immediately due and payable. The Company received net proceeds of $146.6 million in September 2018, after deducting fees and estimated expenses payable by the Company.

Convertible Notes due 2023

In December 2016, the Company issued at par value $250 million aggregate principal amount of unsecured Convertible Senior Notes due 2023 (the “Convertible Notes”), which included the exercise in full of the $25 million over-allotment option granted to the initial purchasers of the Convertible Notes in a private offering to qualified institutional buyers pursuant to Rule 144A under the Securities Act (the “Note Offering”). Interest is payable semiannually on June 15 and December 15 of each year, beginning on June 15, 2017. The Convertible Notes will mature on December 15, 2023, unless earlier repurchased, redeemed, or converted in accordance with their terms. The Convertible Notes are convertible at the option of the holders, under certain circumstances and during certain periods, into cash, shares of the Company’s common stock or a combination thereof. The net proceeds from the Note Offering were $243.0 million, after deducting fees and estimated expenses payable by the Company. In addition, the Company used approximately $13.5 million of the net proceeds from the issuance of the Convertible Notes to pay the cost of the capped call transactions (“Capped Call Confirmations”) that the Company entered into in connection with the issuance of the Convertible Notes. In accounting for the issuance of the Convertible Notes, the Company separated the Convertible Notes into liability and equity components based on their relative values.

The Convertible Notes are initially convertible into approximately 40,849,675 shares of the Company’s common stock under certain circumstances prior to maturity at a conversion rate of 163.3987 shares per $1,000 principal amount of Convertible Notes, which represents a conversion price of approximately $6.12 per share of Common Stock, subject to adjustment under certain conditions. Holders may convert their Convertible Notes at their option at specified times prior to the maturity date of December 15, 2023, only if:

• during any fiscal quarter commencing after March 31, 2017, if the last reported sale price of the Company’s common stock for at least 20 trading days in the period of 30 consecutive trading days ending on the last trading day of the immediately preceding fiscal quarter is equal to or more than 130% of the conversion price of the Convertible Notes on the last day of such preceding fiscal quarter;

• a Holder submits its Convertible Notes for conversion during the five business day period following any five consecutive trading day period in which the trading price for the Convertible Notes, per $1,000 principal amount of the Convertible Notes, for each such trading day was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate of the Convertible Notes on such date;

• the Company issues to all or substantially all of the holders of common stock rights options or warrants entitling then them for a period of not more than 60 calendar days after the date of such issuance to subscribe for or purchase shares of the common stock, at a price per share less than the average of the Last Reported Sale Prices of the common stock for the 10 consecutive Trading Day period ending on, and including, the Trading Day immediately preceding the date of announcement of such issuance or distributes to all or substantially all holders of the common stock the Company’s assets, debt securities or rights to purchase the Company’s securities which distribution has a per share value of exceeding 10% of the Last Reported Sale Price of the common stock on the Trading Day immediately preceding the date of announcement of such distribution;

• the Company enters into specified corporate transactions; or

• the Company has had a call for redemption, the holder can convert up until the second trading day immediately preceding the redemption date.

The Convertible Notes will be convertible, at the option of the note holders, regardless of whether any of the foregoing conditions have been satisfied, on or after September 15, 2023 at any time prior to the close of business on the second scheduled trading day immediately preceding the stated maturity date of December 15, 2023.

The last reported sale price of the Company’s common stock was equal to or more than 130% of the conversion price of the Convertible Notes for at least 20 trading days of the 30 consecutive trading days ending on the last day of the second quarter. As a result, the Convertible Notes are currently convertible into the Company’s common stock.

As further described in “Note 9. Stockholders’ Equity,” on February 15, 2018, the Company entered into an underwriting agreement relating to an underwritten public offering of 19,354,839 shares of the Company’s common stock. Under the terms of the underwriting agreement, the Company granted the underwriters an option, exercisable for 30 days after February 16, 2018, to purchase up to an additional 2,903,225 shares of the Company’s common stock, which was exercised with respect to 885,000 shares of the Company’s common stock.

Subsequent to the underwritten public offering on February 15, 2018, the Company did not have sufficient unissued authorized shares to cover a conversion of the Convertible Notes. As a result, the Company accounted for the portion of the bifurcated conversion feature and of the Capped Call Confirmations that would not be able to be net share settled as a current derivative liability and as a derivative asset, respectively. The fair value of the derivative liability for the conversion feature and derivative asset for the Capped Call Confirmations at February 15, 2018 was determined to be $507.4 million and $13.6 million, respectively, of which the portion that was determined to not be able to be net share settled was recorded with a corresponding impact to additional-paid-in-capital. Subsequent changes to fair value of the derivatives were recorded in the second quarter of 2018 through earnings on the Company’s consolidated statements of operations resulting in a change in fair value of derivatives for the year ended December 31, 2018 of $2.7 million.

Following the approval by the stockholders of the Company on June 7, 2018, to increase the authorized shares of common stock to 500,000,000, the Company has sufficient unissued authorized shares to cover a conversion of the Convertible Notes. As a result, the derivative liability and derivative asset were reclassified into additional-paid-in-capital. The fair value of the derivative liability for the conversion feature and derivative asset for the Capped Call Confirmations at June 7, 2018 was determined to be $88.3 million and $2.4 million, respectively.

The Convertible Notes and Senior Secured Term Loan consist of the following:



Liability component (in thousands)	2018			2017
Principal	$	400,000		$	250,000
Less: debt discount ⁽¹⁾	(74,145		)	(81,566		)
Less: deferred financing ⁽¹⁾	(4,115		)	(4,267		)
Net carrying value of the debt	$	321,740		$	164,167

_____________________________________________________________________


(1)	Included in the consolidated balance sheets within convertible notes and senior secured term loan and amortized to interest expense over the remaining life of the Convertible Notes and Senior Secured Term Loan using the effective interest rate method.

The following table sets forth total interest expense recognized related to the Convertible Notes and Senior Secured Term Loan for the year ended December 31, 2018, respectively:



Components (in thousands)	2018			2017
Contractual interest expense	$	11,426		$	7,528
Amortization of deferred financing	555			470
Amortization of debt discount	10,421			9,241
Total	$	22,402		$	17,239
Effective interest rate of the liability component, convertible debt	10.85		%	10.85		%
Effective interest rate of the liability component, senior secured term loan	10.48		%	—		%

The Capped Call Confirmations of $13.5 million are expected generally to reduce the potential dilution to the common stock upon any conversion of the Convertible Notes and/or offset the cash payments the Company is required to make in excess of the principal amount upon conversion of the Notes in the event that the market price of the common stock is greater than the strike price of the Capped Call Confirmations (which initially corresponds to the initial conversion price of the Convertible Notes and is subject to certain adjustments under the terms of the Capped Call Confirmations), with such reduction and/or offset subject to a cap based on the cap price of the Capped Call Confirmations. The Capped Call Confirmations have an initial cap price of $7.20 per share, which represents a premium of approximately 50% over the closing price of the Company’s common stock on The NASDAQ Global Market on December 15, 2016, and is subject to certain adjustments under the terms of the Capped Call Confirmations. The Capped Call Confirmations will cover, subject to anti-dilution adjustments substantially similar to those applicable to the Convertible Notes, the number of shares of common stock that will underlie the Convertible Notes. The Capped Call Confirmations do not meet the criteria for separate accounting as a derivative as they are indexed to the Company’s common stock. The premiums paid for the Capped Call Confirmations have been included as a net reduction to additional paid-in capital.

During the first quarter of 2019, the Company entered into separate, privately negotiated exchange agreements with a limited number of holders (the “Holders”) of the Convertible Notes. Under the terms of the exchange agreements (the “Exchange Agreements”), the Holders agreed to exchange an aggregate principal amount of approximately $184.6 million of Convertible Notes held by them in exchange for an aggregate of approximately 33.0 million shares of the our common stock, par value $0.01 per share. In addition, pursuant to the Exchange Agreements, the Company made aggregate cash payments of approximately $0.7 million to the Holders to satisfy accrued and unpaid interest to the closing date of the transaction, along with cash in lieu of fractional shares.

During the first quarter of 2019, the Company also terminated the proportion of the Capped Call Confirmations related to the exchange of the Convertible Notes in 2019 for proceeds of approximately $14.6 million.

Leases

12 Months Ended

Dec. 31, 2018

Leases [Abstract]

Leases

Operating Leases

The Company currently leases office and research laboratory space, equipment and vehicles in various facilities under operating agreements expiring at various dates through 2028.

The following table contains information about our current significant leased properties as of December 31, 2018:



Location	Approximate Square Feet	Use	Lease expiry date
Cranbury, New Jersey	90,000	Office and laboratory	September 2025
United Kingdom	46,617	Office	August 2028
Princeton, New Jersey	21,922	Office	January 2022

In addition to the above, we also maintain offices in Germany, Netherlands, Italy, Spain, France, Japan, Canada, Denmark, and Australia. We believe that our current office and laboratory facilities are adequate and suitable for our current and anticipated needs. We believe that, to the extent required, we will be able to lease or buy additional facilities at commercially reasonable rates.

Rent expenses for the Company's facilities are recognized over the term of the lease. The Company recognizes rent starting when possession of the facility is taken from the landlord. When a lease contains a predetermined fixed escalation of the minimum rent, the Company recognizes the related rent expense on a straight-line basis and records the difference between the recognized rental expense and the amounts payable under the lease as deferred rent liability. Tenant leasehold improvement allowances are reflected in accrued expenses on the consolidated balance sheets and are amortized as a reduction to rent expense in the statement of operations over the term of the lease.

At December 31, 2018, aggregate annual future minimum lease payments under these leases are as follows:



(in thousands)	2019		2020		2021		2022		2023 and beyond		Total
Minimum lease payments	$	6,244	$	4,063	$	3,560	$	3,371	$	13,649	$	30,887

Rent expense, including fees for utilities and common area maintenances for the years ended December 31, 2018, 2017 and 2016 were $5.7 million, $3.9 million and $3.5 million, respectively.

Subsequent to December 31, 2018, the Company entered into several new lease arrangements for additional office and research laboratory space in the U.S. and internationally. These leases will be accounted for under ASU 2016-02, Leases (Topic 842) during the first quarter of 2019. The future minimum lease payments of these leases range from approximately $1.5 million to $6.1 million annually over the next five years.

Capital Leases

In 2018, the Company purchased vehicles of approximately $0.2 million through financing arrangements. These financing arrangements include interest of approximately 5.0%-7.0%, and lease terms of 36-48 months.

In 2016, the Company purchased equipment of approximately $0.9 million through financing arrangements. These financing arrangements include interest of approximately 2.0%-5.7%, and lease terms of 36-48 months.

At December 31, 2018, aggregate annual future minimum lease payments under these leases, including interest, are as follows (in thousands):



Years ending December 31:
2019	$	194
2020	60
2021	47
2022	30
2023 and beyond	—
Total principal obligation	$	331

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

For financial reporting purposes, income (loss) before income taxes includes the following components:



	Years Ended December 31,
(in thousands)	2018			2017			2016
United States	$	(309,183	)	$	(440,696	)	$	(174,913	)
Foreign	(39,906		)	(8,425		)	(28,868		)
Total	$	(349,089	)	$	(449,121	)	$	(203,781	)

Following were the components of income tax expense (benefit) for the years ended December 31, 2018, 2017 and 2016:



(in thousands)	2018			2017			2016
Current
Federal	$	—		$	—		$	—
State	6			9			7
Foreign	(100		)	2,276			—
Deferred
Federal	—			(150,015		)	(1,101		)
State	—			(17,389		)	(2,645		)
Foreign	—			—			—
Total	$	(94	)	$	(165,119	)	$	(3,739	)

A reconciliation of the statutory tax rates and the effective tax rates for the years ended December 31, 2018, 2017 and 2016 are as follows:



	Years Ended December 31,
	2018		2017		2016
Statutory rate	(21	)%	(34	)%	(34	)%
State taxes, net of federal benefit	(4	)	(5	)	(5	)
Nondeductible IPR&D	6		(1	)	3
Contingent consideration	1		(18	)	—
Tax credits	(10	)	(2	)	(3	)
Foreign income tax rate differential	2		5		2
Impact of 2017 Act	—		27		—
Other	—		5		(1	)
Valuation allowance	26		(14	)	36
Net	—	%	(37	)%	(2	)%

On December 22, 2017, the U.S. government enacted the Tax Act. The Tax Act significantly revises U.S. tax law by, among other provisions, lowering the U.S. federal statutory income tax rate to 21%, imposing a mandatory one-time transition tax on previously deferred foreign earnings, and eliminating or reducing certain income tax deductions.

ASC 740, Income Taxes, requires the effects of changes in tax laws to be recognized in the period in which the legislation is enacted. However, due to the complexity and significance of the Tax Act's provisions, the SEC staff issued SAB 118, which allows companies to record the tax effects of the Tax Act on a provisional basis based on a reasonable estimate, and then, if necessary, subsequently adjust such amounts during a limited measurement period as more information becomes available. The measurement period ends when a company has obtained, prepared, and analyzed the information necessary to finalize its accounting, but cannot extend beyond one year from enactment. The Tax Act did not have a material impact on the Company's financial statements because its deferred temporary differences are fully offset by a valuation allowance and the Company does not have any significant offshore earnings from which to record the mandatory transition tax. The Company recorded an income tax benefit of $0.1 million in the Consolidated Statement of Operations, in connection with the reduction in the statutory corporate income tax rate. The Company operates in a consolidated loss position in its foreign operations, and does not have a one-time tax on accumulated earnings of foreign subsidiaries.

As of December 31, 2017, we provisionally recorded certain impacts of the Tax Act including the adjustment to our net deferred tax liability arising from the reduction in the federal tax rate as well as the impact of mandatory deemed repatriation. Adjustments to these provisional amounts that we recorded in 2018 did not have a significant impact on our consolidated financial statements. Our accounting for the effects of the enactment of the Tax Act is now complete.

The Company recorded an income tax benefit of $0.1 million in 2018 for taxes in foreign and state jurisdictions.

The Company did not recognize interest or penalties related to income tax during the period ended December 31, 2018 and did not accrue for interest or penalties as of December 31, 2018. The Company does not have an accrual for uncertain tax positions as of December 31, 2018. Tax returns for all years 2010 and thereafter are subject to future examination by tax authorities.

Deferred income taxes reflect the net effect of temporary difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The significant components of the deferred tax assets and liabilities are as follows:



	For Years Ended December 31,
(in thousands)	2018			2017
Deferred tax assets
Intellectual property	$	48,339		$	44,573
Amortization/depreciation	3,732			3,082
Research tax credit	96,509			43,382
Net operating loss carry forwards	248,398			221,912
Deferred revenue	4,401			6,158
Non-cash stock issue	16,850			10,751
Interest carry forward limitation	1,032			—
Others	10,852			7,328
Gross deferred tax assets	430,113			337,186
Deferred tax liabilities
Business acquisition	(6,465		)	(6,465		)
Royalty payable	(48,339		)	(44,573		)
Convertible notes	(16,666		)	(18,991		)
Advanced R&D payments	(2,103		)	(3,069		)
Total net deferred tax assets	356,540			264,088
Less: valuation allowance	(363,005		)	(270,553		)
Net deferred tax liability	$	(6,465	)	$	(6,465	)

The Company records a valuation allowance for temporary differences for which it is more likely than not that the Company will not receive future tax benefits. At December 31, 2018 and 2017, the Company recorded valuation allowances of $363.0 million and $270.6 million, respectively, representing an increase in the valuation allowance of $92.4 million in 2018 due to the uncertainty regarding the realization of such deferred tax assets, to offset the benefits of net operating losses generated during those years. The deferred tax liability related to business acquisitions pertains to the basis difference in IPR&D acquired by the Company. The Company's policy is to record a deferred tax liability related to acquired IPR&D that may eventually be realized either upon amortization of the asset when the research is completed and a product is successfully launched or the write-off of the asset if it is abandoned or unsuccessful.

As of December 31, 2018, the Company had federal, state, and foreign net operating loss carry forwards ("NOLs") of approximately $858.4 million, $943.5 million, and $35.0 million, respectively. The federal carry forward for losses generated prior to 2018 will expire in 2030 through 2037. Federal net operating losses incurred in 2018 and onward have an indefinite expiration under the 2017 Tax Cut & Jobs Act. Most of the state carry forwards generated prior to 2009 have expired through 2016. The remaining state carry forwards including those generated in 2009 through 2018 will expire in 2030 through 2038. The foreign NOLs have indefinite expiration. Utilization of NOLs may be subject to a substantial limitation pursuant to Section 382 of the Code as well as similar state statutes in the event of an ownership change. Such ownership changes have occurred in the past, and could occur again in the future. Under Section 382 of the Internal Revenue Code of 1986, as amended, or Section 382, if a corporation undergoes an "ownership change," generally defined as a greater than 50% change (by value) in its equity ownership over a three-year period, the corporation's ability to use its pre-change NOLs and other pre-change tax attributes (such as research and development tax credits) to offset its post-change income may be limited. We may experience ownership changes in the future as a result of shifts in our stock ownership some of which are outside our control. We completed a detailed study of our NOLs and determined that there was not an ownership change in excess of 50%. Ownership changes in future periods may place additional limits on our ability to utilize net operating loss and tax credit carry forwards. In addition, at the state level, there may be periods during which the use of NOLs is suspended or otherwise limited, which could accelerate or permanently increase state taxes owed.

The Company also has research and experimentation and orphan drug credit carryforwards of approximately $23.7 million and $72.8 million, respectively, which will expire in the years 2023 through 2037. Deferred tax assets for these carryforwards are subject to a full valuation allowance.

Licenses	12 Months Ended
Licenses	Dec. 31, 2018
Research and Development [Abstract]
Licenses	Licenses The Company acquired rights to develop and commercialize its product candidates through licenses granted by various parties. The following summarizes the Company's material rights and obligations under those licenses: Nationwide Children's Hospital — As discussed in "— Note 3. Acquisitions," Celenex has an exclusive license agreement with Nationwide Children’s Hospital (“NCH”). Under this license agreement, NCH is eligible to receive development and sales based milestones of up to $7.8 million for each product. University of Pennsylvania — For discussion of the royalties and milestone payments potentially due to University of Pennsylvania (“Penn”), see "— Note 16. Collaborative Agreements." GSK — For discussion of the royalties and milestone payments potentially due to GSK, see "— Note 16. Collaborative Agreements." Mt. Sinai School of Medicine of New York University ("MSSM") — The Company acquired exclusive worldwide patent rights to develop and commercialize Galafold^® and other pharmacological chaperones for the prevention or treatment of human diseases or clinical conditions by increasing the activity of wild-type and mutant enzymes pursuant to a license agreement with Mt. Sinai School of Medicine ("MSSM") of New York University. Under this agreement, to date, the Company has paid no upfront or annual license fees and there are no milestone or future payments other than royalties on net sales. This agreement expires upon expiration of the last of the licensed patent rights, which will be in 2019, subject to any patent term extension that may be granted, or 2024 if the Company develops a product for combination therapy (pharmacological chaperone plus/ERT) and a patent issues from the pending application covering the combination therapy, subject to any patent term extension that may be granted. Under its license agreements, if the Company owes royalties on net sales for one of its products to more than one of the above licensors, then the Company has the right to reduce the royalties owed to one licensor for royalties paid to another. The amount of royalties to be offset is generally limited in each license and can vary under each agreement. For the year ended December 31, 2018, under the MSSM and GSK license and collaboration agreements, we paid $7.6 million in royalties. The Company's rights with respect to these agreements to develop and commercialize Galafold^® may terminate, in whole or in part, if the Company fails to meet certain development or commercialization requirements or if the Company does not meet its obligations to make royalty payments.

Collaborative Agreements

12 Months Ended

Dec. 31, 2018

Collaborative Agreements

Collaborative Agreements

University of Pennsylvania

In October 2018, the Company further expanded its gene therapy portfolio through a collaboration agreement with Penn to pursue research and development of novel gene therapies for four additional indications, including Pompe disease, Fabry disease, CDD and one additional undisclosed rare metabolic disorder. This relationship will combine the Company's protein engineering and glycobiology expertise with Penn’s AAV gene transfer technologies to develop AAV gene therapies designed for optimal cellular uptake, targeting, dosing, safety and manufacturability. In connection with the collaboration agreement, the Company made an upfront payment of $7 million in cash to Penn in October 2018, which was expensed to research and development expense in the Consolidated Statements of Operations. The Company agreed to certain milestone payments following the achievement of certain developmental and commercial milestone events by a licensed product in each indication up to an aggregate of $86.5 million per indication.

GSK

In November 2013, Amicus entered into the Revised Agreement with GSK, pursuant to which Amicus has obtained global rights to develop and commercialize Galafold^® as a monotherapy and in combination with ERT for Fabry disease. The Revised Agreement amends and replaces in its entirety the earlier agreement entered into between Amicus and GSK in July 2012. Under the terms of the Revised Agreement, there was no upfront payment from Amicus to GSK. For Galafold^® monotherapy, GSK is eligible to receive post-approval and sales-based milestones up to $40 million, as well as tiered royalties in the mid-teens in eight major markets outside the U.S. For the year ended December 31, 2018, the Company incurred approximately $6.9 million of royalty expenses under the revised agreement with GSK.

Under the terms of the Revised Agreement, GSK will no longer jointly fund development costs for all formulations of Galafold^®.

In evaluating the impact of both the Expanded Collaboration Agreement and the Revised Agreement, the Company applied the accounting guidance regarding the impact of potential future payments it may make in its role as a vendor (i.e., Amicus) to its customer (i.e., GSK) and evaluated if these potential future payments could be a reduction of revenue from GSK. If the potential future payments to GSK are as follows:


•	a payment for an identifiable benefit, and


•	the identifiable benefit is separable from the existing relationship between the Company and GSK, and


•	the identifiable benefit can be obtained from a party other than GSK, and


•	the Company can reasonably estimate the fair value of the identifiable benefit,

then the potential future payments would be treated separately from the collaboration and research revenue. However, if all these criteria are not satisfied, then the potential future payments are treated as a reduction of revenue.

Accordingly, the Company did not believe that, for accounting purposes, the new U.S. licensing rights to Galafold^® obtained from GSK under the Expanded Collaboration Agreement, nor the ex U.S. licensing rights to Galafold^® obtained from GSK under the Revised Agreement, represented a separate, identifiable benefit from the licenses in the Original Collaboration Agreement entered into between Amicus and GSK in 2010. The contingent amounts payable to GSK were not sufficiently separable from GSK's original license and the research and development reimbursements such that Amicus could not have entered into a similar exchange transaction with another party. Additionally, the Company cannot reasonably estimate the fair value of the worldwide licensing rights to Galafold^®.

The Company determined that the potential future payments to GSK would be treated as a reduction of revenue and that the total amount of revenue to be received under the arrangement is no longer fixed or determinable as the contingent milestone payments are subject to significant uncertainty.

As a result, the Company no longer recognized any of the upfront license fees and premiums on the equity purchase from GSK until such time as the arrangement consideration becomes fixed or determinable, because an indeterminable amount may ultimately be payable back to GSK. These amounts (the balance of the unrecognized upfront license fee and the premium on the equity purchases) are classified as deferred reimbursements on the balance sheet.

For the year ended December 31, 2018, under the GSK collaboration agreements, we paid $1.3 million in sales-based milestones. As of December 31, 2018, the Company recognized a liability of $15.7 million as deferred reimbursements, in addition to $3.4 million related to royalties payable to GSK in accounts payable, accrued expenses, and other current liabilities in the Consolidated Balance Sheets.

The recognition of Research Revenue was also affected by the determination that the overall total arrangement consideration was no longer fixed and determinable, despite the fact that the research activities continued and that the research expense reimbursements by GSK to Amicus were received as the research activities related to the reimbursement had been completed. Therefore the research reimbursements from GSK were recorded as deferred reimbursements on the balance sheet and would not recognized until the total arrangement consideration becomes fixed and determinable.

As a result, all revenue recognition was suspended until the total arrangement consideration would become fixed and determinable. In addition, future milestone payments made by the Company will be applied against the balance of this deferred reimbursements account. Revenue recognition for research expense reimbursements, the original upfront license fee, and the equity premiums will resume once the total arrangement consideration becomes fixed and determinable which will occur when the balance of the deferred reimbursements account is sufficient to cover all the remaining contingent milestone payments.

Basic and Diluted Net Loss per Common Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Basic and Diluted Net Loss per Common Share

The following table provides a reconciliation of the numerator and denominator used in computing basic and diluted net loss attributable to common stockholders per common share:



(in thousands, except per share amounts)	Years Ended December 31,
Historical	2018			2017			2016
Numerator:
Net loss attributable to common stockholders	$	(348,995	)	$	(284,002	)	$	(200,042	)
Denominator:
Weighted average common shares outstanding — basic and diluted	185,790,021			153,355,144			134,401,588

Dilutive common stock equivalents would include the dilutive effect of common stock options, convertible debt units, RSUs and warrants for common stock equivalents. Potentially dilutive common stock equivalents were excluded from the diluted earnings per share denominator for all periods because of their anti-dilutive effect. For the year ended 2018 there was 40.9 million potential common shares outstanding as a result of the convertible debt that was excluded from the diluted net loss per share calculation because their effect would have been anti-dilutive.

The table below presents potential shares of common stock that were excluded from the computation as they were anti-dilutive using the treasury stock method:



	Year ended December 31,
(in thousands)	2018	2017	2016
Options to purchase common stock	15,810	15,181	15,528
Convertible debt	40,850	40,850	40,850
Outstanding warrants, convertible to common stock	2,657	3,110	3,110
Unvested restricted stock units	3,712	2,575	744
Vested restricted stock units, unissued	91	50	—
Total number of potentially issuable shares	63,120	61,766	60,232

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles ("U.S. GAAP") and include all adjustments necessary for the fair presentation of the Company's financial position for the periods presented.
Consolidation	Consolidation The consolidated financial statements include the accounts of the Company and its subsidiaries. Intercompany accounts and transactions are eliminated in consolidation.
Foreign Currency Transactions	Foreign Currency Transactions The functional currency for most of the Company's foreign subsidiaries is their local currency. For non-U.S. subsidiaries that transact in a functional currency other than the U.S. dollar, assets and liabilities are translated at current rates of exchange at the balance sheet date. Income and expense items are translated at the average foreign exchange rates for the period. Adjustments resulting from the translation of the financial statements of the Company's foreign operations into U.S. dollars are excluded from the determination of net income and are recorded in accumulated other comprehensive income, a separate component of stockholders' equity.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates
Cash, Cash Equivalent, Restricted Cash and Marketable Securities	Cash, Cash Equivalents, Restricted Cash and Marketable Securities The Company considers all highly liquid investments purchased with a maturity of three months or less at the date of acquisition, to be cash equivalents. Marketable securities consist of fixed income investments with a maturity of greater than three months and other highly liquid investments that can be readily purchased or sold using established markets. These investments are classified as available-for-sale and are reported at fair value on the Company’s consolidated balance sheet. Unrealized holding gains and losses are reported within comprehensive income (loss) in the statements of comprehensive loss. Fair value is based on available market information including quoted market prices, broker or dealer quotations or other observable inputs. Restricted cash consists primarily of funds held to satisfy the requirements of certain agreements that are restricted in their use and is included in non-current assets on the Company’s consolidated balance sheet.
Concentration of Credit Risk	Concentration of Credit Risk The Company's financial instruments that are exposed to concentration of credit risk consist primarily of cash and cash equivalents and marketable securities. The Company maintains its cash and cash equivalents in bank accounts, which, at times, exceed federally insured limits. The Company invests its marketable securities in high-quality commercial financial instruments. The Company has not recognized any losses from credit risks on such accounts during any of the periods presented. The Company believes it is not exposed to significant credit risk on cash and cash equivalents or its marketable securities. The Company is subject to credit risk from its accounts receivable related to its product sales of Galafold^®. The Company's accounts receivable at December 31, 2018 have arisen from product sales primarily in the EU and the United States. The Company will periodically assess the financial strength of its customers to establish allowances for anticipated losses, if any. For accounts receivable that have arisen from named patient sales, the payment terms are predetermined and the Company evaluates the creditworthiness of each customer on a regular basis. To date, the Company has not incurred any credit losses.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, less accumulated depreciation. Depreciation is calculated over the estimated useful lives of the respective assets, which range from three to five years, or the lesser of the related initial term of the lease or useful life for leasehold improvements. The initial cost of property and equipment consists of its purchase price and any directly attributable costs of bringing the asset to its working condition and location for its intended use. Expenditures incurred after the fixed assets have been put into operation, such as repairs and maintenance, are charged to income in the period in which the costs are incurred. Major replacements, improvements and additions are capitalized in accordance with Company policy.
Revenue Recognition	Revenue Recognition The Company’s net product sales consist of sales of Galafold^® for the treatment of Fabry disease. The Company has recorded revenue on sales where Galafold^® is available either on a commercial basis or through a reimbursed early access program (“EAP”). Orders for Galafold^® are generally received from distributors and pharmacies, with the ultimate payor often a government authority. The Company recognizes revenue when its performance obligations to its customers have been satisfied, which occurs at a point in time when the pharmacies or distributors obtain control of Galafold^®. The transaction price is determined based on fixed consideration in the Company’s customer contracts and is recorded net of estimates for variable consideration, which are third party discounts and rebates. The identified variable consideration is recorded as a reduction of revenue at the time revenues from sales of Galafold^® are recognized. The Company recognizes revenue to the extent that it is probable that a significant revenue reversal will not occur in a future period. These estimates may differ from actual consideration received. The Company evaluates these estimates each reporting period to reflect known changes. The Company elected the portfolio approach practical expedient in applying ASC Topic 606, Revenue from Contracts with Customers, to its identified revenue streams. Contracts within each revenue stream have similar characteristics and the Company believes this approach would not differ materially from applying ASC Topic 606 to each individual contract.
Inventories and Cost of Goods Sold	Inventories and Cost of Goods Sold Prior to regulatory approval of Galafold^®, the Company expensed all manufacturing costs related to Galafold^® as research and development expense. Upon regulatory approval, the Company began capitalizing costs related to the purchase and manufacture of Galafold^®. Inventories are stated at the lower of cost and net realizable value, determined by the first-in, first-out method. Inventories are reviewed periodically to identify slow-moving or obsolete inventory based on projected sales activity as well as product shelf-life. In evaluating the recoverability of inventories produced, the probability that revenue will be obtained from the future sale of the related inventory is considered and inventory value is written down for inventory quantities in excess of expected requirements. Expired inventory is disposed of and the related costs are recognized as cost of product sales in the consolidated statements of operations. Cost of goods sold includes the cost of inventory sold, manufacturing and supply chain costs, product shipping and handling costs, provisions for excess and obsolete inventory, as well as royalties payable. A portion of the inventory available for sale was expensed as research and development costs prior to regulatory approval and as such the cost of goods sold and related gross margins are not necessarily indicative of future cost of goods sold and gross margin.
Fair Value Measurements	Fair Value Measurements The Company records certain asset and liability balances under the fair value measurements as defined by the FASB guidance. Current FASB fair value guidance emphasizes that fair value is a market-based measurement, not an entity-specific measurement. Therefore, a fair value measurement should be determined based on the assumptions that market participants would use in pricing the asset or liability. As a basis for considering market participant assumptions in fair value measurements, current FASB guidance establishes a fair value hierarchy that distinguishes between market participant assumptions based on market data obtained from sources independent of the reporting entity (observable inputs that are classified within Levels 1 and 2 of the hierarchy) and the reporting entity's own assumptions that market participants assumptions would use in pricing assets or liabilities (unobservable inputs classified within Level 3 of the hierarchy). Level 1 inputs utilize quoted prices (unadjusted) in active markets for identical assets or liabilities that the Company has the ability to access at measurement date. Level 2 inputs are inputs other than quoted prices included in Level 1 that are observable for the asset or liability, either directly or indirectly. Level 2 inputs may include quoted prices for similar assets and liabilities in active markets, as well as inputs that are observable for the asset or liability (other than quoted prices), such as interest rates, foreign exchange rates, and yield curves that are observable at commonly quoted intervals. Level 3 inputs are unobservable inputs for the asset or liability, which is typically based on an entity's own assumptions, as there is little, if any, related market activity. In instances where the determination of the fair value measurement is based on inputs from different levels of the fair value hierarchy, the level in the fair value hierarchy within which the entire fair value measurement falls is based on the lowest level input that is significant to the fair value measurement in its entirety. The Company's assessment of the significance of a particular input to the fair value measurement in its entirety requires judgment, and considers factors specific to the asset or liability.
Contingent Liabilities	Contingent Liabilities On an ongoing basis, the Company may be involved in various claims, and legal proceedings. On a quarterly basis, the Company reviews the status of each significant matter and assesses its potential financial exposure. If the potential loss from any claim, asserted or unasserted, or legal proceeding is considered probable and the amount can be reasonably estimated, the Company will accrue a liability for the estimated loss. Because of uncertainties related to claims and litigation, accruals will be based on the Company's best estimates based on available information. On a periodic basis, as additional information becomes available, or based on specific events such as the outcome of litigation or settlement of claims, the Company may reassess the potential liability related to these matters and may revise these estimates, which could result in material adverse adjustments to the Company's operating results.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development expense consists primarily of costs related to personnel, including salaries and other personnel related expenses, consulting fees and the cost of facilities and support services used in drug development. Assets acquired that are used for research and development and have no future alternative use are expensed as in-process research and development.
Interest Income and Interest Expense	Interest Income and Interest Expense Interest income consists of interest earned on the Company's cash and cash equivalents and marketable securities. Interest expense consists of interest incurred on debt and capital leases.
Income Taxes	Income Taxes The Company accounts for income taxes under the liability method. Under this method deferred income tax liabilities and assets are determined based on the difference between the financial statement carrying amounts and tax basis of assets and liabilities and for operating losses and tax credit carry forwards, using enacted tax rates in effect in the years in which the differences are expected to reverse. A valuation allowance is recorded if it is "more likely than not" that a portion or all of a deferred tax asset will not be realized.
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) Components of other comprehensive income (loss) include unrealized gains and losses on available-for-sale securities and gain (loss) on foreign currency transactions, and are included in the statements of comprehensive loss.
Leases	Leases In the ordinary course of business, the Company enters into lease agreements for office space as well as leases for certain property and equipment. The leases have varying terms and expirations and have provisions to extend or renew the lease agreement, among other terms and conditions, as negotiated. Once the agreement is executed, the lease is assessed to determine whether the lease qualifies as a capital or operating lease. When a non-cancelable operating lease includes any fixed escalation clauses and lease incentives for rent holidays or build-out contributions, rent expense is recognized on a straight-line basis over the initial term of the lease. The excess between the average rental amount charged to expense and amounts payable under the lease is recorded in accrued expenses.
Nonqualified Cash Deferral Plan	Nonqualified Cash Deferral Plan The Company's Cash Deferral Plan (the "Deferral Plan"), provides certain key employees and members of the Board of Directors as selected by the Compensation Committee of the Board of Directors of the Company (the "Compensation Committee"), with an opportunity to defer the receipt of such participant's base salary, bonus and director's fees, as applicable. The Deferral Plan is intended to be a nonqualified deferred compensation plan that complies with the provisions of Section 409A of the Internal Revenue Code (the "Code"). All of the investments held in the Deferral Plan are classified as investments held-to-maturity and recorded at fair value with changes in the investments' fair value recognized as earnings in the period they occur. The corresponding liability for the Deferral Plan is included in other non-current liability in the consolidated balance sheets.
Equity-based Compensation	Equity-based Compensation At December 31, 2018, the Company had three equity-based employee compensation plans, which are described more fully in "— Note 9. Stockholders' Equity." The Company applies the fair value method of measuring equity-based compensation, which requires a public entity to measure the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award.
Loss per Common Share	Loss per Common Share The Company calculates net loss per share as a measurement of the Company's performance while giving effect to all dilutive potential common shares that were outstanding during the reporting period. The Company had a net loss for all periods presented; accordingly, the inclusion of common stock options, unvested RSUs and warrants would be anti-dilutive. Therefore, the weighted average shares used to calculate both basic and diluted earnings per share are the same. See "— Note 17. Basic and Diluted Net Loss per Common Share" for further discussion on net loss per share.
Segment Information	Segment Information The Company currently operates in one business segment focused on the discovery, development and commercialization of advanced therapies to treat a range of devastating rare and orphan diseases. The Company is not organized by market and is managed and operated as one business. A single management team reports to the chief operating decision maker who comprehensively manages the entire business. The Company does not operate any separate lines of business or separate business entities with respect to its products. Accordingly, the Company does not accumulate discrete financial information with respect to separate service lines, and thus there is one reporting unit.
Business Combinations	Business Combinations The Company assigns fair value to the tangible and intangible assets acquired and liabilities assumed based upon their estimated fair values on the acquisition date from acquired businesses. The purchase price allocation process requires management to make significant estimates and assumptions, especially at the acquisition date with respect to intangible assets and in-process research and development ("IPR&D"). In connection with the purchase price allocations for acquisitions, the Company estimates the fair value of contingent payments utilizing a probability-based income approach inclusive of an estimated discount rate.
Contingent Consideration Payable	Contingent Consideration Payable The Company determines the fair value of contingent acquisition consideration payable on the acquisition date using a probability-based income approach utilizing an appropriate discount rate. Contingent acquisition consideration payable is shown as a non-current liability on the Company's consolidated balance sheets. The fair value of the contingent consideration payable will be determined each period end and the resulting change will be recorded on the consolidated statements of operations.
Intangible Assets and Goodwill	Intangible Assets and Goodwill The Company records goodwill in a business combination when the total consideration exceeds the fair value of the net tangible and identifiable intangible assets acquired. Purchased IPR&D is accounted for as an indefinite lived intangible asset until the underlying project is completed, at which point the intangible asset will be accounted for as a definite lived intangible asset, or abandoned, at which point the intangible asset will be written off or partially impaired. Goodwill and indefinite lived intangible assets are assessed annually for impairment and whenever events or circumstances indicate that the carrying amount of an asset may not be recoverable. If it is determined that the full carrying amount of an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the asset exceeds its fair value.
Recent Accounting Pronouncements	Recent Accounting Developments - Guidance Adopted in 2018 ASU 2018-07 - In June 2018, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (“ASU”) 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting (“ASU 2018-07”).These amendments expand the scope of Topic 718, Compensation-Stock Compensation (which currently only includes share-based payments to employees) to include share-based payments issued to non-employees for goods or services. Consequently, the accounting for share-based payments to non-employees and employees will be substantially aligned. ASU 2018-07 supersedes Subtopic 505-50, Equity-Equity-Based Payments to Non-Employees, and is effective for all public entities for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted, but no earlier than a company’s adoption date of Topic 606, Revenue from Contracts with Customers. The Company early adopted ASU 2018-07 in the second quarter of 2018 and there was no material impact on its consolidated financial statements from the adoption. ASU 2018-02 - In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (“ASU 2018-02”). Prior to ASU 2018-02, U.S. GAAP required the remeasurement of deferred tax assets and liabilities as a result of a change in tax laws or rates to be presented in net income from continuing operations, even in situations in which the related income tax effects of items in accumulated other comprehensive income were originally recognized in other comprehensive income. As a result, such items, referred to as stranded tax effects, did not reflect the appropriate tax rate. Under ASU 2018-02, entities are permitted, but not required, to reclassify from accumulated other comprehensive income to retained earnings those stranded tax effects resulting from the Tax Cuts and Jobs Act of 2017. ASU 2018-02 is effective for all entities for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted. The Company adopted the new standard effective January 1, 2018. As a result of the adoption, the Company reclassified a gain of $383,000 from the foreign currency translation adjustment in accumulated other comprehensive loss to accumulated deficit in the consolidated balance sheet as of December 31, 2018. ASU 2017-09 - In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting (“ASU 2017-09”). The amendments in ASU 2017-09 provide guidance on determining which changes to the terms and conditions of share-based payment awards require an entity to apply modification accounting under ASU 2017-09. An entity should account for the effects of a modification unless all the following are met: (1) The fair value (or calculated value or intrinsic value, if such an alternative measurement method is used) of the modified award is the same as the fair value (or calculated value or intrinsic value, if such an alternative measurement method is used) of the original award immediately before the original award is modified. If the modification does not affect any of the inputs to the valuation technique that the entity uses to value the award, the entity is not required to estimate the value immediately before and after the modification; (2) The vesting conditions of the modified award are the same as the vesting conditions of the original award immediately before the original award is modified; and (3) The classification of the modified award as an equity instrument or a liability instrument is the same as the classification of the original award immediately before the original award is modified. ASU 2017-09 is effective for all entities for annual periods, including interim periods within those annual periods, beginning after December 15, 2017. The Company adopted this standard on January 1, 2018 and the adoption did not have a material impact on its consolidated financial statements. ASU 2017-01 - In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (“ASU 2017-01”). This ASU clarifies the definition of a business. The amendments affect all companies and other reporting organizations that must determine whether they have acquired or sold a business. The amendments in ASU 2017-01 are effective for public companies for annual periods beginning after December 15, 2017, including interim periods within those periods. The amendments should be applied prospectively as of the beginning of the period of adoption. The Company adopted ASU 2017-01 on January 1, 2018. The adoption of the standard did not have a material impact on its consolidated financial statements. For additional information see "—Note 3. Acquisitions." ASU 2016-18 - In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”). The amendments of ASU 2016-18 require an entity to include amounts generally described as restricted cash and restricted cash equivalents with cash and cash equivalents when reconciling the beginning of period and end of period total amounts on the statement of cash flows. The amendments of ASU 2016-18 are effective for annual reporting periods beginning after December 15, 2017, and interim periods within those annual periods. The Company adopted the guidance in ASU 2016-18 effective January 1, 2018. In connection with the adoption of the standard, the Company applied the guidance retrospectively which resulted in an increase in cash flows from operations of $1.8 million on the statement of cash flows for the year ended December 31, 2017. ASU 2016-16 - In October 2016, the FASB issued ASU 2016-16, Income Taxes (Topic 740): Intra-Entity Transfers of Assets Other Than Inventory (“ASU 2016-16”). ASU 2016-16 requires an entity to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. The amendments eliminate the exception for an intra-entity transfer of an asset other than inventory. The amendments in ASU 2016-16 are effective for public business entities for annual periods beginning after December 15, 2017, including interim reporting periods within those annual reporting periods. The Company adopted ASU 2016-16 on January 1, 2018 and there was no material impact from the adoption. ASU 2016-01 - In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU 2016-01”). ASU 2016-01 changes accounting for equity investments, financial liabilities under the fair value option, and presentation and disclosure requirements for financial instruments. ASU 2016-01 does not apply to equity investments in consolidated subsidiaries or those accounted for under the equity method of accounting. In addition, the FASB clarified guidance related to the valuation allowance assessment when recognizing deferred tax assets resulting from unrealized losses on available-for-sale debt securities. Equity investments with readily determinable fair values will be measured at fair value with changes in fair value recognized in net income. Companies have the option to measure equity investments without readily determinable fair values at either fair value or at cost adjusted for changes in observable prices minus impairment. Companies that elect the fair value option for financial liabilities must recognize changes in fair value related to instrument-specific credit risk in other comprehensive income. Companies must assess valuation allowances for deferred tax assets related to available-for-sale debt securities in combination with their other deferred tax assets. ASU 2016-01 is effective beginning in the first quarter of 2018 and the Company adopted it in the first quarter of 2018. There was no impact on the Company’s consolidated financial statements and related disclosures upon adoption, as the Company does not have equity investments or liabilities with credit risk. In addition, the guidance relating to deferred tax assets did not result in a change in accounting treatment for the Company. ASU 2014-09 - In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) which along with amendments issued in 2015 and 2016, replaced substantially all current U.S. GAAP guidance on this topic and eliminated industry-specific guidance. The new revenue recognition standard requires entities to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Effective January 1, 2018, the Company adopted the new revenue recognition standard using the modified retrospective approach and applied this approach only to contracts that were not completed as of January 1, 2018. The timing of revenue recognition and treatment of contract costs remains unchanged under the new revenue recognition standard. As such, the adoption of the new revenue recognition standard did not have a material impact on the Company's consolidated financial statements. The information presented for the periods prior to January 1, 2018 has not been restated and is reported under the accounting standard in effect for those periods. Recent Accounting Developments - Guidance Not Yet Adopted In August 2018, the Securities Exchange Commission (“SEC”) issued Final Rule 33-10532, Disclosure Update and Simplification, which amends certain disclosure requirements that were redundant, duplicative, overlapping or superseded by other SEC disclosure requirements. The amendments generally eliminated or otherwise reduced certain disclosure requirements of various SEC rules and regulations. However, in some cases, the amendments require additional information to be disclosed, including changes in stockholders’ equity in interim periods. The rule is effective 30 days after its publication in the Federal Register. The rule was posted on October 4, 2018. On September 25, 2018, the SEC released guidance advising it will not object to a registrant adopting the requirement to include changes in stockholders’ equity in the Form 10-Q for the first quarter beginning after the effective date of the rule. The Company is currently assessing the impact that this standard will have on its consolidated financial statements upon adoption and expects to adopt the guidance in it's Form 10-Q for the period ended March 31, 2019. ASU 2018-13 - In August 2018, the FASB issued ASU 2018-03, Fair Value Measurement (Topic 820): Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). The amendments modify the disclosure requirements in Topic 820. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. Early adoption is permitted. An entity is permitted to early adopt any removed or modified disclosures upon issuance of ASU 2018-13 and delay adoption of the additional disclosures until their effective date. The Company is currently assessing the impact that this standard will have on its consolidated financial statements upon adoption. ASU 2017-08 - In March 2017, the FASB issued ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20), Premium Amortization on Purchased Callable Debt Securities (“ASU 2017-08”). The amendments in ASU 2017-08 shorten the amortization period for certain callable debt securities held at a premium. Specifically, the amendments require the premium to be amortized to the earliest call date. The amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity. ASU 2017-08 is effective for public business entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted, including adoption in an interim period. If an entity early adopts in an interim period, any adjustments should be reflected as of the beginning of the fiscal year that includes that interim period. The amendments should be applied on a modified retrospective basis, with a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company is currently assessing the impact that this standard will have on its consolidated financial statements. ASU 2017-04 - In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment (“ASU 2017-04”). To simplify the subsequent measurement of goodwill, ASU 2017-04 eliminates Step 2 from the goodwill impairment test. The annual, or interim, goodwill impairment test is performed by comparing the fair value of a reporting unit with its carrying amount. An impairment charge should be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. In addition, income tax effects from any tax deductible goodwill on the carrying amount of the reporting unit should be considered when measuring the goodwill impairment loss, if applicable. ASU 2017-04 also eliminates the requirements for any reporting unit with a zero or negative carrying amount to perform a qualitative assessment and, if it fails that qualitative test, to perform Step 2 of the goodwill impairment test. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. ASU 2017-04 should be applied on a prospective basis. The nature of and reason for the change in accounting principle should be disclosed upon transition. A public business entity that is a U.S. SEC filer should adopt ASU 2017-04 for its annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company is currently assessing the impact that this standard will have on its consolidated financial statements. ASU 2016-02 - In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (“ASU 2016-02”). ASU 2016-02 requires the recognition of lease assets and lease liabilities on the balance sheet for all lease obligations and disclosing key information about leasing arrangements. ASU 2016-02 requires the recognition of lease assets and lease liabilities by lessees for those leases classified as operating leases under previous generally accepted accounting principles. ASU 2016-02 will be effective for the Company for all annual and interim periods beginning after December 15, 2018, including interim periods within those fiscal years. In August 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements, (“ASU 2018-11”). ASU 2018-11 provide entities with an additional transition method for adoption, whereby, an entity initially applies the new leases standard at the adoption date and recognizes a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company will adopt these standards effective January 1, 2019 and elect the transition method in ASU 2018-11. The Company will also elect certain of the practical expedients permitted, including the expedient that permits the Company to retain its existing lease assessment and classification. The Company is currently working through an adoption plan which includes the evaluation of lease contracts compared to the new standard and estimates that the impact that this standard will have on its consolidated financial statements will be an increase to lease liabilities and right-of-use assets of approximately $15 million to $18 million.

Goodwill and Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2018

Goodwill and Intangible Assets Disclosure [Abstract]

Schedule of changes in IPR&D

The following table represents the changes in IPR&D for the years ended December 31, 2018 and 2017, respectively:



	(in millions)
Beginning balance	$	486.7
Impairment in IPR&D related to Scioderm	(463.7		)
Balance at December 31, 2017	$	23.0
Change in IPR&D	—
Balance at December 31, 2018	$	23.0

Schedule of changes in goodwill

The following table represents the changes in Goodwill for the years ended December 31, 2018 and 2017, respectively:



	(in millions)
Beginning balance	$	197.8
Change in goodwill	—
Balance at December 31, 2017	$	197.8
Change in goodwill	—
Balance at December 31, 2018	$	197.8

Cash, Cash Equivalents, Marketable Securities and Restricted Cash (Tables)

12 Months Ended

Dec. 31, 2018

Cash, Cash Equivalents, and Short-term Investments [Abstract]

Schedule of cash and available for sale securities

Cash, cash equivalents and marketable securities are classified as current unless mentioned otherwise below and consisted of the following:



	As of December 31, 2018
(in thousands)	Cost		Unrealized Gain		Unrealized Loss			Fair Value
Cash and cash equivalents	$	79,749	$	—	$	—		$	79,749
Corporate debt securities, current portion	240,969		7		(250		)	240,726
Commercial paper	115,245		—		(104		)	115,141
Asset-backed securities	68,215		4		(84		)	68,135
Money market	350		—		—			350
Certificate of deposit	51		—		—			51
	$	504,579	$	11	$	(438	)	$	504,152
Included in cash and cash equivalents	$	79,749	$	—	$	—		$	79,749
Included in marketable securities, current	424,830		11		(438		)	424,403
Total cash, cash equivalents and marketable securities	$	504,579	$	11	$	(438	)	$	504,152



	As of December 31, 2017
(in thousands)	Cost		Unrealized Gain		Unrealized Loss			Fair Value
Cash and cash equivalents	$	49,060	$	—	$	—		$	49,060
Corporate debt securities, current portion	199,314		1		(303		)	199,012
Commercial paper	79,878		—		(75		)	79,803
Asset-backed securities	30,346		—		(59		)	30,287
Money market	350		—		—			350
Certificate of deposit	50		—		—			50
	$	358,998	$	1	$	(437	)	$	358,562
Included in cash and cash equivalents	$	49,060	$	—	$	—		$	49,060
Included in marketable securities, current	309,938		1		(437		)	309,502
Total cash, cash equivalents and marketable securities	$	358,998	$	1	$	(437	)	$	358,562

Schedule of reconciliation of cash and cash equivalents and restricted cash



(in thousands)	December 31, 2018		December 31, 2017		December 31, 2016
Cash and cash equivalents	$	79,749	$	49,060	$	187,026
Restricted cash	2,626		2,177		387
Cash and cash equivalents and restricted cash shown in the statement of cash flows	$	82,375	$	51,237	$	187,413

Share based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Fair Value of Options

The fair value of the stock options granted is estimated on the date of grant using a Black-Scholes option pricing model with the following weighted-average assumptions:



	Years Ended December 31,
	2018			2017			2016
Expected stock price volatility	78.6		%	82.8		%	81.3		%
Risk free interest rate	2.4		%	2.0		%	1.5		%
Expected life of options (years)	5.62			6.18			6.25
Expected annual dividend per share	$	0.00		$	0.00		$	0.00

Schedule of Stock Options Activity

A summary of the Company's stock options for the year ended December 31, 2018 were as follows:



	Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life	Aggregate Intrinsic Value
	(in thousands)					(in millions)
Options outstanding, December 31, 2015	11,729.2		$	7.11
Granted	5,114.1		$	7.67
Exercised	(723.1	)	$	4.20
Forfeited	(622.7	)	$	8.62
Options outstanding, December 31, 2016	15,497.5		$	7.37
Granted	3,695.3		$	7.17
Exercised	(2,878.7	)	$	5.67
Forfeited	(1,133.0	)	$	9.55
Options outstanding, December 31, 2017	15,181.1		$	7.48
Granted	2,348.0		$	14.96
Exercised	(1,398.0	)	$	6.54
Forfeited	(313.1	)	$	9.55
Expired	(8.0	)	10.76
Options outstanding, December 31, 2018	15,810.0		$	8.63	6.7 years	$	37.6
Vested and unvested expected to vest, December 31, 2018	15,152.6		$	8.51	6.6 years	$	36.8
Exercisable at December 31, 2018	9,977.0		$	7.43	5.8 years	$	28.8

Schedule of Non-Vested RSU Activity under the Plan

A summary of non-vested RSU activity under the Plan for the year ended December 31, 2018 is as follows:



	Number of Share		Weighted Average Grant Date Fair Value		Weighted Average Remaining Years	Aggregate Intrinsic Value
	(in thousands)					(in millions)
Non-vested units as of December 31, 2016	744.4		$	7.86
Granted	2,348.7		$	5.69
Vested	(318.2	)	$	9.23
Forfeited	(199.8	)	$	6.24
Non-vested units as of December 31, 2017	2,575.1		$	5.85
Granted	1,811.9		$	16.11
Vested	(530.0	)	$	6.01
Forfeited	(145.0	)	$	9.65
Non-vested units as of December 31, 2018	3,712.0		$	10.59	2.57	$	36.3

Schedule of Equity Compensation Expenses

The following table summarizes information related to compensation expense recognized in the statements of operations related to the equity awards:



	Years Ended December 31,
	2018		2017		2016
Equity compensation expense recognized in:
Research and development expense	$	11,740	$	10,328	$	8,071
Selling, general and administrative expense	17,520		12,773		9,433
Total equity compensation expense	$	29,260	$	23,101	$	17,504

Assets and Liabilities Measured at Fair Value (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of assets and liabilities subject to fair value measurements



(in thousands)	Level 2		Total
Assets:
Commercial paper	$	115,141	$	115,141
Asset-back securities	68,135		68,135
Corporate debt securities	240,726		240,726
Money market funds	3,082		3,082
	$	427,084	$	427,084



	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	19,700	$	19,700
Deferred compensation plan liability	2,732		—		2,732
	$	2,732	$	19,700	$	22,432



(in thousands)	Level 2		Total
Assets:
Commercial paper	$	79,803	$	79,803
Asset-back securities	30,287		30,287
Corporate debt securities	199,012		199,012
Money market funds	2,598		2,598
	$	311,700	$	311,700



	Level 2		Level 3		Total
Liabilities:
Contingent consideration payable	$	—	$	25,400	$	25,400
Deferred compensation plan liability	2,258		—		2,258
	$	2,258	$	25,400	$	27,658

Schedule of significant unobservable inputs used in the valuation of the contingent consideration payable

The following significant unobservable inputs were used in the valuation of the contingent consideration payable of Callidus for the ATB-200 Pompe program:



Contingent Consideration Liability	Fair value as of December 31, 2018	Valuation Technique	Unobservable Input	Range
			Discount rate	10%
Clinical and regulatory milestones	$19.2 million	Probability weighted discounted cash flow	Probability of achievement of milestones	71.0% - 100.0%
			Projected year of payments	2021 - 2022

Schedule of changes in contingent consideration payable

The following table shows the change in the balance of contingent consideration payable for the year ended December 31, 2018 and 2017, respectively:



	Year ended December 31,
(in thousands)	2018			2017
Balance, beginning of the period	$	25,400		$	269,722
Payment of contingent consideration in cash	—			(10,000		)
Milestone payable, included in accrued expenses	(9,000		)	—
Unrealized change in fair value change during the period, included in Statement of Operations	3,300			(234,322		)
Balance, end of the period	$	19,700		$	25,400

- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

+ References + Details

Debt (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of liability components of the Convertible Notes

The Convertible Notes and Senior Secured Term Loan consist of the following:



Liability component (in thousands)	2018			2017
Principal	$	400,000		$	250,000
Less: debt discount ⁽¹⁾	(74,145		)	(81,566		)
Less: deferred financing ⁽¹⁾	(4,115		)	(4,267		)
Net carrying value of the debt	$	321,740		$	164,167

_____________________________________________________________________


(1)	Included in the consolidated balance sheets within convertible notes and senior secured term loan and amortized to interest expense over the remaining life of the Convertible Notes and Senior Secured Term Loan using the effective interest rate method.

Components of total interest expense recognized related to the Convertible Notes

The following table sets forth total interest expense recognized related to the Convertible Notes and Senior Secured Term Loan for the year ended December 31, 2018, respectively:



Components (in thousands)	2018			2017
Contractual interest expense	$	11,426		$	7,528
Amortization of deferred financing	555			470
Amortization of debt discount	10,421			9,241
Total	$	22,402		$	17,239
Effective interest rate of the liability component, convertible debt	10.85		%	10.85		%
Effective interest rate of the liability component, senior secured term loan	10.48		%	—		%

Leases (Tables)

12 Months Ended

Dec. 31, 2018

Leases [Abstract]

Schedule of current significant leased properties

The following table contains information about our current significant leased properties as of December 31, 2018:



Location	Approximate Square Feet	Use	Lease expiry date
Cranbury, New Jersey	90,000	Office and laboratory	September 2025
United Kingdom	46,617	Office	August 2028
Princeton, New Jersey	21,922	Office	January 2022

Schedule of aggregate annual future minimum lease payments under the operating leases

At December 31, 2018, aggregate annual future minimum lease payments under these leases are as follows:



(in thousands)	2019		2020		2021		2022		2023 and beyond		Total
Minimum lease payments	$	6,244	$	4,063	$	3,560	$	3,371	$	13,649	$	30,887

Schedule of aggregate annual future minimum lease payments under the capital leases, including interest

At December 31, 2018, aggregate annual future minimum lease payments under these leases, including interest, are as follows (in thousands):



Years ending December 31:
2019	$	194
2020	60
2021	47
2022	30
2023 and beyond	—
Total principal obligation	$	331

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of income (loss) before income taxes

For financial reporting purposes, income (loss) before income taxes includes the following components:



	Years Ended December 31,
(in thousands)	2018			2017			2016
United States	$	(309,183	)	$	(440,696	)	$	(174,913	)
Foreign	(39,906		)	(8,425		)	(28,868		)
Total	$	(349,089	)	$	(449,121	)	$	(203,781	)

Schedule of income tax expense (benefit)

Following were the components of income tax expense (benefit) for the years ended December 31, 2018, 2017 and 2016:



(in thousands)	2018			2017			2016
Current
Federal	$	—		$	—		$	—
State	6			9			7
Foreign	(100		)	2,276			—
Deferred
Federal	—			(150,015		)	(1,101		)
State	—			(17,389		)	(2,645		)
Foreign	—			—			—
Total	$	(94	)	$	(165,119	)	$	(3,739	)

Schedule of reconciliation of the statutory tax rates and the effective tax rates

A reconciliation of the statutory tax rates and the effective tax rates for the years ended December 31, 2018, 2017 and 2016 are as follows:



	Years Ended December 31,
	2018		2017		2016
Statutory rate	(21	)%	(34	)%	(34	)%
State taxes, net of federal benefit	(4	)	(5	)	(5	)
Nondeductible IPR&D	6		(1	)	3
Contingent consideration	1		(18	)	—
Tax credits	(10	)	(2	)	(3	)
Foreign income tax rate differential	2		5		2
Impact of 2017 Act	—		27		—
Other	—		5		(1	)
Valuation allowance	26		(14	)	36
Net	—	%	(37	)%	(2	)%

Schedule of significant components of the deferred tax assets and liabilities

The significant components of the deferred tax assets and liabilities are as follows:



	For Years Ended December 31,
(in thousands)	2018			2017
Deferred tax assets
Intellectual property	$	48,339		$	44,573
Amortization/depreciation	3,732			3,082
Research tax credit	96,509			43,382
Net operating loss carry forwards	248,398			221,912
Deferred revenue	4,401			6,158
Non-cash stock issue	16,850			10,751
Interest carry forward limitation	1,032			—
Others	10,852			7,328
Gross deferred tax assets	430,113			337,186
Deferred tax liabilities
Business acquisition	(6,465		)	(6,465		)
Royalty payable	(48,339		)	(44,573		)
Convertible notes	(16,666		)	(18,991		)
Advanced R&D payments	(2,103		)	(3,069		)
Total net deferred tax assets	356,540			264,088
Less: valuation allowance	(363,005		)	(270,553		)
Net deferred tax liability	$	(6,465	)	$	(6,465	)

Basic and Diluted Net Loss per Common Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Schedule of reconciliation of the numerator and denominator used in computing basic and diluted net loss per common share

The following table provides a reconciliation of the numerator and denominator used in computing basic and diluted net loss attributable to common stockholders per common share:



(in thousands, except per share amounts)	Years Ended December 31,
Historical	2018			2017			2016
Numerator:
Net loss attributable to common stockholders	$	(348,995	)	$	(284,002	)	$	(200,042	)
Denominator:
Weighted average common shares outstanding — basic and diluted	185,790,021			153,355,144			134,401,588

Schedule of potential shares of common stock that were excluded from the computation as they were anti-dilutive using the treasury stock method

The table below presents potential shares of common stock that were excluded from the computation as they were anti-dilutive using the treasury stock method:



	Year ended December 31,
(in thousands)	2018	2017	2016
Options to purchase common stock	15,810	15,181	15,528
Convertible debt	40,850	40,850	40,850
Outstanding warrants, convertible to common stock	2,657	3,110	3,110
Unvested restricted stock units	3,712	2,575	744
Vested restricted stock units, unissued	91	50	—
Total number of potentially issuable shares	63,120	61,766	60,232

Description of Business (Details)		1 Months Ended	3 Months Ended		12 Months Ended
Description of Business (Details)	Jul. 18, 2017 USD ($) shares	Sep. 30, 2018 USD ($) program	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($) shares	Dec. 31, 2018 USD ($) medicine program	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 08, 2018 program
Corporate Information, Status of Operations and Management Plans
Number of approved medicines for Farby disease \| medicine					1
Number of gene therapy programs \| program					14			4
Number of gene therapy programs for Batten disease \| program					2
Number of gene therapy programs for neurologic LSDs developed \| program		10
Number of common stock shares issued from underwriting agreement \| shares	21,122,449			20,239,839
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions	$ 243,000,000			$ 294,600,000	$ 294,584,000	$ 243,037,000	$ 97,068,000
Accumulated deficit					$ (1,412,222,000)	$ (1,063,610,000)
Line of Credit
Corporate Information, Status of Operations and Management Plans
Maximum borrowing capacity		$ 150,000,000.0	$ 150,000,000.0
Debt instrument term		5 years	5 years
Proceeds from issuance of debt		$ 146,600,000
Line of Credit \| LIBOR
Corporate Information, Status of Operations and Management Plans
Basis spread on variable rate		7.50%	7.50%
Celenex
Corporate Information, Status of Operations and Management Plans
Research and development expense		$ 100,000,000

Summary of Significant Accounting Policies - Recent Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle
Total stockholders' equity	$ 342,912	$ 352,850	$ 360,151		$ 347,834
Selling, general and administrative	127,200	88,671	71,151
Net cash used in operating activities	$ (299,955)	(213,695)	$ (150,147)
ASU 2016-18
New Accounting Pronouncements or Change in Accounting Principle
Net cash used in operating activities		1,800
ASU 2016-02 \| Forecast \| Minimum
New Accounting Pronouncements or Change in Accounting Principle
Right-of-use asset				$ 15,000
Operating lease liability				15,000
ASU 2016-02 \| Forecast \| Maximum
New Accounting Pronouncements or Change in Accounting Principle
Right-of-use asset				18,000
Operating lease liability				$ 18,000
Accumulated Foreign Currency Adjustment Attributable to Parent \| ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle
Total stockholders' equity		$ (383)

Acquisitions (Details) $ in Thousands	1 Months Ended			3 Months Ended		12 Months Ended
Acquisitions (Details) $ in Thousands	Sep. 30, 2018 USD ($) program	Apr. 30, 2017 USD ($)	Jul. 31, 2016 USD ($) shares	Sep. 30, 2017 USD ($)	Mar. 31, 2017	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Acquisitions
Number of gene therapy programs for neurologic LSDs developed \| program	10
Charges to research expense for stock issued in asset acquisition						$ 0	$ 0	$ 4,607
Loss on impairment of assets						0	465,427	0
Changes in fair value of contingent consideration payable						3,300	(234,322)	6,760
Contingent consideration payable						19,700	25,400
Contingent consideration payable						0	8,400
Contingent consideration payable, non-current portion						19,700	17,000
Milestone Payment\ \| Callidus
Acquisitions
Milestone payment						9,000
Fixed assets
Acquisitions
Loss on impairment of assets							1,700
MiaMed Inc
Acquisitions
Asset acquisition, total consideration, stock and cash			$ 6,500
Asset acquisition, cash consideration paid			1,800
Asset acquisition, potential aggregate deal value			$ 89,500
Charges to research expense for stock issued in asset acquisition								$ 6,500
MiaMed Inc \| Amicus \| Common Stock
Acquisitions
Consideration paid in common stock \| shares			825,603
Scioderm
Acquisitions
Enrollment rate					100.00%
Milestone payment		$ 10,000
Loss on impairment of assets							$ 463,700
Changes in fair value of contingent consideration payable				$ 254,700
Selling , general and administrative costs due to wind-down				400
Research and development expenses due to wind-down				8,100
Income tax expense (benefit) due to wind-down				$ (164,700)
Callidus
Acquisitions
Contingent consideration payable						28,700
Contingent consideration payable						9,000
Contingent consideration payable, non-current portion						19,700
Callidus \| Changes in fair value of contingent consideration payable
Acquisitions
Changes in fair value of contingent consideration payable						$ 3,300
Clinical, Regulatory and Commercial milestones \| MiaMed Inc \| Maximum
Acquisitions
Contingent consideration payable upon achievement of milestones			$ 83,000
Celenex
Acquisitions
Research and development expense	$ 100,000
Contingent consideration based on the achievement of certain milestones	15,000
Contingent consideration based on the achievement of regulatory milestones	262,000
Maximum payment under contingent consideration over the next 4 years	75,000
Celenex \| Nationwide Children's Hospital
Acquisitions
Contingent consideration based on the achievement of certain milestones	$ 7,800

Goodwill and Intangible Assets - IPR&D (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
December 31, 2017	$ 486,700
Loss on impairment	0	$ 465,427	$ 0
Changes in IPR&D
December 31, 2017	23,000
Change in IPR&D	0
December 31, 2018	$ 23,000	23,000
Scioderm
Goodwill [Line Items]
Loss on impairment		$ 463,700

Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill in connection with acquisitions		$ 197,800
Change in goodwill		0
Changes in goodwill
Balance at end of the period	$ 197,797
Change in goodwill	0
Balance at end of the period	$ 197,797	$ 197,797

Cash, Cash Equivalents, Marketable Securities and Restricted Cash (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Cost, Cash balances	$ 79,749,000	$ 49,060,000	$ 187,026,000
Cost, Debt securities	424,830,000	309,938,000
Cost, Cash balances and Debt securities	504,579,000	358,998,000
Unrealized Gain, Debt securities	11,000	1,000
Unrealized Gain, Cash balances and Debt securities	11,000	1,000
Unrealized Loss, Debt securities	(438,000)	(437,000)
Unrealized Loss, Cash balances and Debt securities	(438,000)	(437,000)
Fair Value, Cash balances	79,749,000	49,060,000
Fair Value, Debt Securities	424,403,000	309,502,000
Fair Value, Cash balances and Debt securities	504,152,000	358,562,000
Debt securities, realized gain (loss)	0	0
Fair value of available-for-sale debt securities in unrealized loss positions	403,100,000	295,100,000
Corporate debt securities, current portion
Debt Securities, Available-for-sale [Line Items]
Cost, Debt securities	240,969,000	199,314,000
Unrealized Gain, Debt securities	7,000	1,000
Unrealized Loss, Debt securities	(250,000)	(303,000)
Fair Value, Debt Securities	240,726,000	199,012,000
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Cost, Debt securities	115,245,000	79,878,000
Unrealized Gain, Debt securities	0	0
Unrealized Loss, Debt securities	(104,000)	(75,000)
Fair Value, Debt Securities	115,141,000	79,803,000
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost, Debt securities	68,215,000	30,346,000
Unrealized Gain, Debt securities	4,000	0
Unrealized Loss, Debt securities	(84,000)	(59,000)
Fair Value, Debt Securities	68,135,000	30,287,000
Money market
Debt Securities, Available-for-sale [Line Items]
Cost, Debt securities	350,000	350,000
Unrealized Gain, Debt securities	0	0
Unrealized Loss, Debt securities	0	0
Fair Value, Debt Securities	350,000	350,000
Certificate of deposit
Debt Securities, Available-for-sale [Line Items]
Cost, Debt securities	51,000	50,000
Unrealized Gain, Debt securities	0	0
Unrealized Loss, Debt securities	0	0
Fair Value, Debt Securities	$ 51,000	$ 50,000

Cash, Cash Equivalents, Marketable Securities and Restricted Cash - Additional Information (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents	$ 79,749	$ 49,060	$ 187,026
Restricted cash	2,626	2,177	387
Cash and cash equivalents and restricted cash shown in the statement of cash flows	$ 82,375	$ 51,237	$ 187,413	$ 69,485

Inventories (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 1,291,000	$ 2,393,000
Work-in-process	3,485,000	1,450,000
Finished goods	3,614,000	780,000
Total inventories	8,390,000	4,623,000
Reserve for inventory	$ 200,000	$ 0

Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property and Equipment
Property and equipment, gross	$ 27,046	$ 21,577
Less accumulated depreciation	(15,671)	(12,515)
Property and equipment, net	11,375	9,062
Depreciation	4,216	3,593	$ 3,242
Computer equipment
Property and Equipment
Property and equipment, gross	4,691	3,746
Computer software
Property and Equipment
Property and equipment, gross	1,298	1,236
Research equipment
Property and Equipment
Property and equipment, gross	8,445	6,379
Furniture and fixtures
Property and Equipment
Property and equipment, gross	4,876	2,992
Leasehold improvements
Property and Equipment
Property and equipment, gross	7,425	7,193
Vehicles
Property and Equipment
Property and equipment, gross	209	0
Construction in progress
Property and Equipment
Property and equipment, gross	$ 102	$ 31

Accounts Payable and Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts payable and accrued expenses
Accounts payable	$ 6,606	$ 7,867
Accrued professional fees	2,276	5,845
Accrued contract manufacturing & contract research costs	5,890	4,632
Accrued compensation and benefits	21,731	19,620
Accrued facility costs	2,102	1,665
Accrued program fees	16,674	5,707
Royalties payable	4,463	2,529
Accrued interest	4,189	313
Milestone payments	9,000	0
Accrued sales rebates and discounts	3,636	1,957
Other	4,058	3,755
Accounts payable and accrued expenses	$ 80,625	$ 53,890

Stockholders' Equity - Common Stock and Warrants (Details)				1 Months Ended		2 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Feb. 16, 2018 $ / shares shares	Feb. 15, 2018 USD ($) $ / shares shares	Jul. 18, 2017 USD ($) $ / shares shares	Apr. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Feb. 28, 2019 USD ($) shares	Mar. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) vote $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Jun. 07, 2018 shares
Convertible Debt
Authorized number of shares of common stock \| shares									500,000,000	250,000,000		500,000,000
Voting right for each share held, number \| vote										1
Number of common stock shares issued from underwriting agreement \| shares			21,122,449				20,239,839
Price per share of common stock issued (in dollars per share) \| $ / shares			$ 12.25
Gross proceeds from issue of common stock before deducting underwriting discounts and commissions			$ 258,800,000						$ 294,583,000	$ 243,037,000
Gross cash proceeds from warrant exercises									$ 3,617,000	$ 0	$ 0
Common stock, par value (in dollars per share) \| $ / shares									$ 0.01	$ 0.01
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions			$ 243,000,000				$ 294,600,000		$ 294,584,000	$ 243,037,000	97,068,000
Subsequent Event
Convertible Debt
Exchange agreement, aggregate principal amount						$ 184,600,000
Exchange agreement, shares authorized \| shares						33,000,000
Accrued and unpaid interest on exchange agreement						$ 700,000
Convertible Notes \| 2016 Convertible Notes
Convertible Debt
Aggregate principal amount					$ 250,000,000						$ 250,000,000
Fixed interest rate					3.00%						3.00%
Conversion rate					16.33987%
Conversion price (in dollars per share) \| $ / shares					$ 6.12						$ 6.12
Warrants
Convertible Debt
Number of common stock shares issued from underwriting agreement \| shares				453,214
Gross cash proceeds from warrant exercises				$ 3,600,000
Common Stock
Convertible Debt
Number of common stock shares issued from underwriting agreement \| shares	885,000	19,354,839
Price per share of common stock issued (in dollars per share) \| $ / shares		$ 15.50
Gross proceeds from issue of common stock before deducting underwriting discounts and commissions		$ 300,000,000
Net proceeds from the issuance of common stock after deducting underwriting discounts and commissions								$ 294,600,000
Common Stock \| Warrants
Convertible Debt
Exercise price of warrants (in dollars per share) \| $ / shares				$ 7.98
Common Stock \| Over-allotment Option
Convertible Debt
Number of common stock shares issued from underwriting agreement \| shares	885,000
Price per share of common stock issued (in dollars per share) \| $ / shares	$ 15.50
Period granted to underwriters for purchase of shares		30 days
Maximum number of shares granted to underwriters \| shares	2,903,225

Stockholders' Equity - Nonqualified Cash Plan and Equity Incentive Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Stockholders' Equity
Deferred compensation investment	$ 2.7	$ 2.2
2007 Plan and 2007 Director Plan
Stockholders' Equity
Award term	P10Y
Vesting percentage	100.00%
2007 Plan and 2007 Director Plan \| First Anniversary
Stockholders' Equity
Vesting percentage	25.00%
Maximum \| 2007 Plan and 2007 Director Plan \| Options to purchase common stock
Stockholders' Equity
Number of shares reserved for issuance		9,457,741

Share based Compensation - Weighted-average Assumptions (Details) - Options to purchase common stock - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected stock price volatility	78.60%	82.80%	81.30%
Risk free interest rate	2.40%	2.00%	1.50%
Expected life of options (years)	5 years 7 months 12 days	6 years 2 months 5 days	6 years 3 months
Expected annual dividend per share	$ 0	$ 0	$ 0

Share based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Dec. 21, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Proceeds from exercise of stock options		$ 9,144	$ 16,301	$ 3,036
RSUs vested and non-vested units expected to vest (in shares)		530,035
Options to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value, options (in dollars per share)		$ 10.19	$ 5.09	$ 5.28
Aggregate intrinsic value of options exercised (in dollars)		$ 11,900	$ 20,800	$ 2,600
Proceeds from exercise of stock options		9,100	$ 16,300	$ 3,000
Total unrecognized compensation costs		$ 33,300
Unrecognized compensation costs, period for recognition (in years)		2 years 5 months
Market Performance Based Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)		3 years
Market Performance Based Restricted Stock Units \| Stock Option Plan Amended Restated 2007 Equity Incentive Plan \| Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)		3 years
Market Performance Based Restricted Stock Units \| Stock Option Plan Amended Restated 2007 Equity Incentive Plan \| Executive Officer \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights		200.00%
Market Performance Based Restricted Stock Units \| Stock Option Plan Amended Restated 2007 Equity Incentive Plan \| Executive Officer \| Cliff Vesting
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)		3 years
Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period following separation for accelerated vesting	2 years
Unvested restricted stock units \| Stock Option Plan Amended Restated 2007 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs, period for recognition (in years)		2 years 5 months 19 days
RSUs vested and non-vested units expected to vest (in shares)		530,000	318,200
Unrecognized compensation cost		$ 25,800

Share based Compensation - Stock Option Activity (Details) - Options to purchase common stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance at the beginning of the period (in shares)	15,181,100	15,497,500	11,729,200
Options, Granted (in shares)	2,348,000	3,695,300	5,114,100
Options, Exercised (in shares)	(1,398,000)	(2,878,700)	(723,100)
Options, Forfeited (in shares)	(313,100)	(1,133,000)	(622,700)
Options, Expired (in shares)	(8,000)
Balance at the end of the period (in shares)	15,810,000	15,181,100	15,497,500
Vested and unvested expected to vest as of the end of the period (in shares)	15,152,600
Exercisable at the end of the period (in shares)	9,977,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Balance at the beginning of the period (in dollars per share)	$ 7.48	$ 7.37	$ 7.11
Options, Granted (in dollars per share)	14.96	7.17	7.67
Options, Exercised (in dollars per share)	6.54	5.67	4.20
Options, Forfeited (in dollars per share)	9.55	9.55	8.62
Options, Expired (in dollars per share)	10.76
Balance at the end of the period (in dollars per share)	8.63	$ 7.48	$ 7.37
Vested and unvested expected to vest as of the end of the period (in dollars per share)	8.51
Exercisable at the end of the period (in dollars per share)	$ 7.43
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Balance at the end of the period	6 years 8 months
Vested and unvested expected to vest at the end of the period	6 years 7 months
Exercisable at the end of the period	5 years 9 months
Aggregate intrinsic value of options outstanding	$ 37.6
Aggregate intrinsic value of options vested and unvested expected to vest	36.8
Aggregate intrinsic value of options exercisable	$ 28.8

Share based Compensation - RSUs and PBRSUs Summary (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Vested (in shares)	(530,035)
Stock Option Plan Amended Restated 2007 Equity Incentive Plan \| Unvested restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested units as of the beginning of the period (in shares)	2,575,100	744,400
Granted (in shares)	1,811,900	2,348,700
Vested (in shares)	(530,000)	(318,200)
Forfeited (in shares)	(145,000)	(199,800)
Non-vested units as of the end of the period (in shares)	3,712,000	2,575,100
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Non-vested units as of the beginning of the period (in dollars per share)	$ 5.85	$ 7.86
Granted (in dollars per share)	16.11	5.69
Vested (in dollars per share)	6.01	9.23
Forfeited (in dollars per share)	9.65	6.24
Non-vested units as of the end of the period (in dollars per share)	$ 10.59	$ 5.85
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Non-vested units, weighted average remaining years	2 years 6 months 25 days
Non-vested units, aggregate intrinsic value	$ 36.3

Share based Compensation - Expense Summary (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total equity compensation expense	$ 29,260	$ 23,101	$ 17,504
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total equity compensation expense	11,740	10,328	8,071
Selling, general and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total equity compensation expense	$ 17,520	$ 12,773	$ 9,433

Assets and Liabilities Measured at Fair Value (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Assets:
Fair value of assets	$ 427,084	$ 311,700
Liabilities:
Contingent consideration payable	19,700	25,400
Deferred compensation plan liability	2,732	2,258
Fair value of liabilities	22,432	27,658
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of the period	25,400	269,722
Payment of contingent consideration in cash	0	(10,000)	$ (5,000)
Milestone payable, included in accrued expenses	(9,000)	0
Unrealized change in fair value change during the period, included in Statement of Operations	3,300	(234,322)	6,760
Balance, end of the period	19,700	25,400	$ 269,722
Callidus
Liabilities:
Contingent consideration payable	28,700
ATB-200 Pompe Program \| Clinical and Regulatory Approval milestones \| Callidus
Liabilities:
Contingent consideration payable	$ 19,200
ATB-200 Pompe Program \| Minimum \| Clinical and Regulatory Approval milestones \| Callidus \| Probability of Milestone Achievement
Liabilities:
Contingent consideration, measurement input	0.710
ATB-200 Pompe Program \| Maximum \| Clinical and Regulatory Approval milestones \| Callidus \| Probability of Milestone Achievement
Liabilities:
Contingent consideration, measurement input	1
Convertible Notes \| 2016 Convertible Notes
Liabilities:
Fair value of the debt	$ 437,100
Commercial paper
Assets:
Fair value of assets	115,141	79,803
Asset-backed securities
Assets:
Fair value of assets	68,135	30,287
Corporate debt securities
Assets:
Fair value of assets	240,726	199,012
Money market
Assets:
Fair value of assets	3,082	2,598
Level 2
Assets:
Fair value of assets	427,084	311,700
Liabilities:
Contingent consideration payable	0	0
Deferred compensation plan liability	2,732	2,258
Fair value of liabilities	2,732	2,258
Level 2 \| Commercial paper
Assets:
Fair value of assets	115,141	79,803
Level 2 \| Asset-backed securities
Assets:
Fair value of assets	68,135	30,287
Level 2 \| Corporate debt securities
Assets:
Fair value of assets	240,726	199,012
Level 2 \| Money market
Assets:
Fair value of assets	3,082	2,598
Level 3
Liabilities:
Contingent consideration payable	19,700	25,400
Deferred compensation plan liability	0	0
Fair value of liabilities	$ 19,700	$ 25,400
Level 3 \| ATB-200 Pompe Program \| Clinical and Regulatory Approval milestones \| Probability weighted discounted cash flow \| Callidus \| Discount Rate
Liabilities:
Contingent consideration, measurement input	0.10

Debt - Convertible Debt (Details)				1 Months Ended		2 Months Ended	3 Months Ended		12 Months Ended
Debt - Convertible Debt (Details)	Feb. 16, 2018 shares	Feb. 15, 2018 USD ($) shares	Jul. 18, 2017 shares	Sep. 30, 2018 USD ($)	Dec. 31, 2016 USD ($) day $ / shares shares	Feb. 28, 2019 USD ($) shares	Sep. 30, 2018 USD ($)	Mar. 31, 2018 shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares	Jun. 07, 2018 USD ($) shares
Convertible Debt
Proceeds for capped call confirmations									$ 0	$ 0	$ 13,450,000
Common stock, par value (in dollars per share) \| $ / shares									$ 0.01	$ 0.01
Common stock issued from underwriting agreement (in shares) \| shares			21,122,449					20,239,839
Derivative liability, noncurrent		$ 507,400,000
Change in fair value of derivatives									$ 2,739,000	$ 0	0
Common stock, shares authorized \| shares									500,000,000	250,000,000		500,000,000
Derivative liabilities												$ 88,300,000
Interest expense
Common stock, par value (in dollars per share) \| $ / shares									$ 0.01	$ 0.01
Common Stock Capped Call Confirmation
Convertible Debt
Derivative asset, noncurrent		$ 13,600,000
Interest expense
Capped call confirmations									$ 13,500,000
Capped call confirmations, initial cap price (in dollars per share) \| $ / shares									$ 7.20
Capped call confirmations, premium percentage									50.00%
Common Stock
Convertible Debt
Common stock issued from underwriting agreement (in shares) \| shares	885,000	19,354,839
Over-allotment Option \| Common Stock
Convertible Debt
Common stock issued from underwriting agreement (in shares) \| shares	885,000
Number of days granted underwriters for purchase of shares		30 days
Maximum number of additional shares granted to underwriters \| shares	2,903,225
Subsequent Event
Interest expense
Exchange agreement, aggregate principal amount						$ 184,600,000
Exchange agreement, shares authorized \| shares						33,000,000
Accrued and unpaid interest on exchange agreement						$ 700,000
Proceeds from capped call confirmations, exchange of convertible debt						$ 14,600,000
Convertible Notes \| Common Stock Capped Call Confirmation
Convertible Debt
Other assets, current												$ 2,400,000
Convertible Notes \| 2016 Convertible Notes
Convertible Debt
Aggregate principal amount					$ 250,000,000						$ 250,000,000
Net proceeds after deducting fees and estimated expenses					243,000,000
Proceeds for capped call confirmations					$ 13,500,000
Shares issued per increment of convertible debt \| shares					40,849,675
Debt conversion ratio (in shares)					0.1633987
Conversion price (in dollars per share) \| $ / shares					$ 6.12						$ 6.12
Threshold consecutive trading days \| day					30
Threshold percentage					130.00%				130.00%
Period following 5 consecutive trading day period, trading price less than 98% of product of sales price					5 years
Period following issuance of stock rights options or warrants with price per share less than last reported sales price					60 days
Period for measurement of sales price					10 days
Excess of value per share for triggering event, percent					10.00%
Liability component (in thousands)
Principal									$ 400,000,000	$ 250,000,000
Less: debt discount									(74,145,000)	(81,566,000)
Less: deferred financing									(4,115,000)	(4,267,000)
Net carrying value of the debt									321,740,000	164,167,000
Interest expense
Contractual interest expense									11,426,000	7,528,000
Amortization of deferred financing									555,000	470,000
Amortization of debt discount									10,421,000	9,241,000
Total									$ 22,402,000	$ 17,239,000
Effective interest rate of the liability component, convertible debt									10.85%	10.85%
Convertible Notes \| 2016 Convertible Notes \| Minimum
Convertible Debt
Threshold trading days					20 days
Convertible Notes \| 2016 Convertible Notes \| Over-allotment Option \| Private Placement Purchase Agreement
Convertible Debt
Aggregate principal amount					$ 25,000,000						$ 25,000,000
Senior Notes
Interest expense
Effective interest rate of the liability component, convertible debt									10.48%	0.00%
Line of Credit
Debt Instruments
New credit facility borrowing capacity				$ 150,000,000.0			$ 150,000,000.0
Debt instrument term				5 years			5 years
Periodic payment, percentage				12.50%
Proceeds from issuance of debt				$ 146,600,000
Line of Credit \| LIBOR
Debt Instruments
Basis spread on variable rate				7.50%			7.50%

Leases (Details) $ in Thousands	12 Months Ended
Leases (Details) $ in Thousands	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2019 USD ($)
Minimum lease payments
2019	$ 6,244
2020	4,063
2021	3,560
2022	3,371
2023 and beyond	13,649
Total	30,887
Rent expense, including fees for utilities and common area maintenances	5,700	$ 3,900	$ 3,500
Minimum \| Forecast
Minimum lease payments
Future lease payments, 2019				$ 1,500
Future lease payments, 2020				1,500
Future lease payments, 2021				1,500
Future lease payments, 2022				1,500
Future lease payments, 2023				1,500
Maximum \| Forecast
Minimum lease payments
Future lease payments, 2019				6,100
Future lease payments, 2020				6,000
Future lease payments, 2021				6,000
Future lease payments, 2022				6,000
Future lease payments, 2023				$ 6,000
Equipment
Capital Leases
Capital lease obligation	200		$ 900
2019	194
2020	60
2021	47
2022	30
2023 and beyond	0
Total principal obligation	$ 331
Equipment \| Minimum
Capital Leases
Fixed interest rate		5.00%	2.00%
Term of capital lease		36 months	36 months
Equipment \| Maximum
Capital Leases
Fixed interest rate		7.00%	5.70%
Term of capital lease		48 months	48 months
Cranbury, New Jersey \| Office and laboratory
Operating Leased Assets [Line Items]
Area of space leased (in square feet) \| ft²	90,000
United Kingdom \| Office
Operating Leased Assets [Line Items]
Area of space leased (in square feet) \| ft²	46,617
Princeton, New Jersey \| Office
Operating Leased Assets [Line Items]
Area of space leased (in square feet) \| ft²	21,922

Income Taxes - Statutory Rate (Details) - USD ($)		12 Months Ended
Income Taxes - Statutory Rate (Details) - USD ($)	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income (loss) before income taxes:
United States		$ (309,183,000)	$ (440,696,000)	$ (174,913,000)
Foreign		(39,906,000)	(8,425,000)	(28,868,000)
Loss before income tax		(349,089,000)	(449,121,000)	(203,781,000)
Current
Federal		0	0	0
State		6,000	9,000	7,000
Foreign		(100,000)	2,276,000	0
Deferred
Federal		0	(150,015,000)	(1,101,000)
State		0	(17,389,000)	(2,645,000)
Foreign		0	0	0
Total		$ (94,000)	$ (165,119,000)	$ (3,739,000)
Reconciliation of the statutory tax rates and the effective tax rates
Statutory rate	(21.00%)	(21.00%)	(34.00%)	(34.00%)
State taxes, net of federal benefit		(4.00%)	(5.00%)	(5.00%)
Nondeductible IPR&D		6.00%	(1.00%)	3.00%
Contingent consideration		1.00%	(18.00%)	(0.00%)
Tax credits		(10.00%)	(2.00%)	(3.00%)
Foreign income tax rate differential		2.00%	5.00%	2.00%
Impact of 2017 Act		(0.00%)	27.00%	(0.00%)
Other		(0.00%)	5.00%	(1.00%)
Valuation allowance		26.00%	(14.00%)	36.00%
Net		(0.00%)	(37.00%)	(2.00%)
Income tax benefit		$ (94,000)	$ (165,119,000)	$ (3,739,000)
Foreign and state income tax expense (benefits)		(100,000)
Accrual for interest and penalties		0
Accrual for uncertain tax positions		$ 0

Income Taxes - Deferred and Valuation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Intellectual property	$ 48,339	$ 44,573
Amortization/depreciation	3,732	3,082
Research tax credit	96,509	43,382
Net operating loss carry forwards	248,398	221,912
Deferred revenue	4,401	6,158
Non-cash stock issue	16,850	10,751
Interest carry forward limitation	1,032	0
Others	10,852	7,328
Gross deferred tax assets	430,113	337,186
Deferred tax liabilities
Business acquisition	(6,465)	(6,465)
Royalty payable	(48,339)	(44,573)
Convertible notes	(16,666)	(18,991)
Advanced R&D payments	(2,103)	(3,069)
Total net deferred tax assets	356,540	264,088
Less: valuation allowance	(363,005)	(270,553)
Net deferred tax liability	(6,465)	$ (6,465)
Increase in the valuation allowance	$ (92,400)

Income Taxes - Loss Carryforwards (Details) $ in Millions	Dec. 31, 2018 USD ($)
Research and experimentation tax credit carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward amount	$ 23.7
Orphan drug tax credit carryforwards
Operating Loss Carryforwards [Line Items]
Tax credit carryforward amount	72.8
Federal
Operating Loss Carryforwards [Line Items]
Amount of net operating loss carry forwards	858.4
State
Operating Loss Carryforwards [Line Items]
Amount of net operating loss carry forwards	943.5
Foreign
Operating Loss Carryforwards [Line Items]
Amount of net operating loss carry forwards	$ 35.0

Licenses (Details) - USD ($)	12 Months Ended
Licenses (Details) - USD ($)	Dec. 31, 2018	Sep. 30, 2018
License agreement \| MSSM
Licenses
Upfront and annual license fees paid	$ 0
Potential milestone payments	0
Royalty expense	$ 7,600,000
Celenex
Licenses
Contingent consideration based on the achievement of certain milestones		$ 15,000,000
Nationwide Children's Hospital \| Celenex
Licenses
Contingent consideration based on the achievement of certain milestones		$ 7,800,000

Collaborative Agreements (Details)	1 Months Ended		12 Months Ended
Collaborative Agreements (Details)	Oct. 31, 2018 USD ($)	Nov. 30, 2013 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Agreements
Revenue recognized			$ 91,245,000	$ 36,930,000	$ 4,958,000
Deferred reimbursements			10,156,000	14,156,000
Accounts payable, accrued expenses, and other current liabilities			80,625,000	$ 53,890,000
Collaborative Arrangements
Collaborative Agreements
Payments for collaborative agreement	$ 7,000,000
Conditional aggregate payments per indication	$ 86,500,000
Collaborative Arrangements \| GSK
Collaborative Agreements
Payments for collaborative agreement			1,300,000
Revised Agreement \| GSK
Collaborative Agreements
Upfront payment received		$ 0
Potential milestone payments upon achievement of post-approval and sales-based milestones		$ 40,000,000
Number of major markets outside the U.S. from whom parties to contractual arrangement is eligible to receive post-approval and sales-based milestones		8
Revenue recognized			6,900,000
Deferred reimbursements			15,700,000
Accounts payable, accrued expenses, and other current liabilities			$ 3,400,000

Basic and Diluted Net Loss per Common Share (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss attributable to common stockholders	$ (78,083)	$ (159,163)	$ (61,833)	$ (49,916)	$ (69,208)	$ (111,666)	$ (48,136)	$ (54,992)	$ (348,995)	$ (284,002)	$ (200,042)
Denominator:
Weighted-average common shares outstanding - basic and diluted (in shares)									185,790,021	153,355,144	134,401,588
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									63,120,000	61,766,000	60,232,000
Options to purchase common stock
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									15,810,000	15,181,000	15,528,000
Convertible debt
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									40,850,000	40,850,000	40,850,000
Outstanding warrants, convertible to common stock
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									2,657,000	3,110,000	3,110,000
Unvested restricted stock units
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									3,712,000	2,575,000	744,000
Vested restricted stock units, unissued
Antidilutive securities
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									91,000	50,000	0

Selected Quarterly Financial Data (Unaudited - in thousands except per share data) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Data
Net loss	$ (78,083)	$ (159,163)	$ (61,833)	$ (49,916)	$ (69,208)	$ (111,666)	$ (48,136)	$ (54,992)	$ (348,995)	$ (284,002)	$ (200,042)
Basic and diluted net loss per common share (in dollars per share)	$ (0.43)	$ (0.84)	$ (0.33)	$ (0.28)	$ (0.42)	$ (0.69)	$ (0.34)	$ (0.39)	$ (1.88)	$ (1.85)	$ (1.49)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) $ in Thousands	Dec. 31, 2016 USD ($)
Statement of Stockholders' Equity [Abstract]
Convertible notes issuance costs	$ 2,709

Summary of Significant Accounting Policies - Equity Incentive Plan and Stock-Based Compensation (Details)	Dec. 31, 2018 program
Equity-based Compensation
Number of equity-based employee compensation plans	3

Summary of Significant Accounting Policies - Segment Information and Recent Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2018 unit segment
Accounting Policies [Abstract]
Number of business segments \| segment	1
Number of reporting units \| unit	1

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Minimum
Property and Equipment
Estimated useful lives	3 years
Maximum
Property and Equipment
Estimated useful lives	5 years