Intercept Pharmaceuticals (ICPT) 10-Q 5 Nov 19 2019 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information	9 Months Ended
	Sep. 30, 2019shares
Document and Entity Information [Abstract]
Entity Registrant Name	INTERCEPT PHARMACEUTICALS, INC.
Entity Central Index Key	0001270073
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Trading Symbol	ICPT
Entity Common Stock, Shares Outstanding	32,733,447
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Period Focus	Q3
Document Fiscal Year Focus	2019
Entity Address, Address Line One	10 Hudson Yards, 37th Floor
Entity Address, Postal Zip Code	10001
Entity Address, State or Province	NY
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Small Business	false
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Entity File Number	001-35668
Entity Incorporation, State or Country Code	DE
Entity Address, City or Town	New York
Entity Tax Identification Number	22-3868459
City Area Code	646
Local Phone Number	747-1000
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common Stock

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 63,377	$ 43,248
Investment debt securities, available-for-sale	648,994	392,912
Accounts receivable, net	32,822	25,694
Prepaid expenses and other current assets	23,891	20,571
Total current assets	769,084	482,425
Fixed assets, net	5,973	10,411
Inventory, net	9,348	7,108
Security deposits	8,205	9,223
Other assets	9,484
Total assets	802,094	509,167
Current liabilities:
Accounts payable, accrued expenses and other liabilities	133,295	105,109
Short-term interest payable	5,463	7,475
Short-term portion of deferred revenue	1,216	1,621
Total current liabilities	139,974	114,205
Long-term liabilities:
Long-term debt	525,339	371,250
Long-term other liabilities	6,046	3,771
Long-term portion of deferred revenue		811
Total liabilities	671,359	490,037
Commitments and contingencies (Note 15)
Stockholders' equity:
Common stock par value $0.001 per share; 45,000,000 shares authorized; 32,733,447 and 29,693,876 shares issued and outstanding as of September 30, 2019 and December 31, 2018, respectively	33	30
Additional paid-in capital	2,157,080	1,800,144
Accumulated other comprehensive loss, net	(1,070)	(2,259)
Accumulated deficit	(2,025,308)	(1,778,785)
Total stockholders' equity	130,735	19,130
Total liabilities and stockholders' equity	$ 802,094	$ 509,167

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Condensed Consolidated Balance Sheets
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized	45,000,000	45,000,000
Common stock, shares, issued	32,733,447	29,693,876
Common stock, shares, outstanding	32,733,447	29,693,876

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue	$ 61,950	$ 46,986	$ 180,502	$ 126,524
Operating expenses:
Cost of sales	487	519	1,738	1,512
Selling, general and administrative	76,828	56,812	223,738	184,503
Research and development	60,168	47,941	178,163	144,028
Total operating expenses	137,483	105,272	403,639	330,043
Operating loss	(75,533)	(58,286)	(223,137)	(203,519)
Other income (expense):
Interest expense	(11,795)	(7,671)	(29,518)	(22,769)
Other income, net	2,495	1,503	6,132	5,051
Total other income (expense)	(9,300)	(6,168)	(23,386)	(17,718)
Net loss	$ (84,833)	$ (64,454)	$ (246,523)	$ (221,237)
Net loss per common and potential common share:
Basic and diluted	$ (2.59)	$ (2.18)	$ (7.88)	$ (7.89)
Weighted average common and potential common shares outstanding:
Basic and diluted	32,717	29,615	31,275	28,057
Product [Member]
Revenue	$ 61,545	$ 46,581	$ 179,286	$ 124,908
License [Member]
Revenue	$ 405	$ 405	$ 1,216	$ 1,616

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Condensed Consolidated Statements of Comprehensive Loss
Net loss	$ (84,833)	$ (64,454)	$ (246,523)	$ (221,237)
Net changes related to available-for-sale investment debt securities:
Unrealized gains on investment debt securities	451	368	1,646	119
Reclassification adjustment for realized gains on investment debt securities included in other income, net	2		6
Net unrealized gains on investment debt securities	453	368	1,652	119
Foreign currency translation gains (losses)	(687)	63	(463)	(979)
Other comprehensive (loss) income	(234)	431	1,189	(860)
Comprehensive loss	$ (85,067)	$ (64,023)	$ (245,334)	$ (222,097)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Changes in Stockholders' (Deficit) Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Total
Balance at Dec. 31, 2017	$ 25	$ 1,486,690	$ (786)	$ (1,469,543)	$ 16,386
Balance (in shares) at Dec. 31, 2017	25,173
Stock-based compensation		38,415			38,415
Issuance of common stock from public and private placement offerings, net of underwriting fees and issuance costs	$ 5	261,357			261,362
Issuance of common stock from public and private placement offerings, net of underwriting fees and issuance costs (in shares)	4,258
Value of stock options exercised		3,201			3,201
Net proceeds from exercise of stock options		3,201			3,201
Net proceeds from exercise of stock options (in shares)	223
Employee withholding taxes related to stock-based awards		(1,458)			(1,458)
Other comprehensive income			(860)		(860)
Net loss				(221,237)	(221,237)
Balance at Sep. 30, 2018	$ 30	1,788,205	(1,646)	(1,690,780)	95,809
Balance (in shares) at Sep. 30, 2018	29,654
Balance at Dec. 31, 2017	$ 25	1,486,690	(786)	(1,469,543)	$ 16,386
Balance (in shares) at Dec. 31, 2017	25,173
Net proceeds from exercise of stock options (in shares)					69
Balance at Sep. 30, 2019	$ 33	2,157,080	(1,070)	(2,025,308)	$ 130,735
Balance (in shares) at Sep. 30, 2019	32,733
Balance at Jun. 30, 2018	$ 29	1,774,955	(2,082)	(1,626,326)	146,576
Balance (in shares) at Jun. 30, 2018	29,565
Stock-based compensation		11,994			11,994
Value of stock options exercised	$ 1	2,715			2,716
Net proceeds from exercise of stock options	$ 1	2,715			2,716
Net proceeds from exercise of stock options (in shares)	89
Employee withholding taxes related to stock-based awards		(1,459)			(1,459)
Other comprehensive income			436		436
Net loss				(64,454)	(64,454)
Balance at Sep. 30, 2018	$ 30	1,788,205	(1,646)	(1,690,780)	95,809
Balance (in shares) at Sep. 30, 2018	29,654
Balance at Dec. 31, 2018	$ 30	1,800,144	(2,259)	(1,778,785)	19,130
Balance (in shares) at Dec. 31, 2018	29,694
Stock-based compensation		42,809			42,809
Recognition of debt discount on 2026 Convertible Notes		85,915			85,915
Issuance of common stock from public and private placement offerings, net of underwriting fees and issuance costs	$ 3	227,177			227,180
Issuance of common stock from public and private placement offerings, net of underwriting fees and issuance costs (in shares)	2,880
Value of stock options exercised		2,511			2,511
Net proceeds from exercise of stock options		2,511			2,511
Net proceeds from exercise of stock options (in shares)	159
Employee withholding taxes related to stock-based awards		(1,476)			(1,476)
Other comprehensive income			1,189		1,189
Net loss				(246,523)	(246,523)
Balance at Sep. 30, 2019	$ 33	2,157,080	(1,070)	(2,025,308)	130,735
Balance (in shares) at Sep. 30, 2019	32,733
Balance at Jun. 30, 2019	$ 33	2,116,481	(836)	(1,940,475)	175,203
Balance (in shares) at Jun. 30, 2019	32,696
Stock-based compensation		13,130			13,130
Recognition of debt discount on 2026 Convertible Notes		26,577			26,577
Value of stock options exercised		773			773
Net proceeds from exercise of stock options		773			773
Net proceeds from exercise of stock options (in shares)	37
Employee withholding taxes related to stock based awards		(119)			(119)
Other comprehensive income			(234)		(234)
Net loss				(84,833)	(84,833)
Balance at Sep. 30, 2019	$ 33	$ 2,157,080	$ (1,070)	$ (2,025,308)	$ 130,735
Balance (in shares) at Sep. 30, 2019	32,733

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (246,523)	$ (221,237)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	42,809	38,415
(Accretion) amortization of (discount) premium on investment debt securities	(595)	266
Amortization of deferred financing costs	1,529	1,144
Depreciation	2,839	3,551
Non-cash operating lease cost	3,974
Gain on lease termination	(1,995)
Loss on the disposal of fixed assets	2,682	1,331
Accretion of debt discount	15,051	10,412
Unrealized gain on investments		(119)
Changes in operating assets:
Prepaid expenses and other current assets	(3,320)	(856)
Accounts receivable	(7,128)	(5,579)
Inventory	(2,240)	(3,995)
Security deposits	1,018	7,138
Other assets	(19,489)
Changes in operating liabilities:
Accounts payable, accrued expenses and other current liabilities	33,990	(11,719)
Operating lease liabilities	(5,019)
Long-term other liabilities	9,516	(1,300)
Interest payable	(2,012)	(3,738)
Deferred revenue	(1,216)	(1,616)
Net cash used in operating activities	(176,129)	(187,902)
Cash flows from investing activities:
Purchases of investment debt securities	(560,733)	(378,261)
Sales and maturities of investment debt securities	306,898	293,307
Purchases of equipment, leasehold improvements, and furniture and fixtures	(1,041)	(45)
Net cash used by investing activities	(254,876)	(84,999)
Cash flows from financing activities:
Proceeds from issuance of 2026 Convertible Notes, net of issuance costs	223,424
Proceeds from issuance of common stock, net of issuance costs	227,180	261,362
Proceeds from exercise of options, net	2,511	3,201
Payments of employee withholding taxes related to stock-based awards	(1,476)	(1,458)
Net cash provided by financing activities	451,639	263,105
Effect of exchange rate changes	(505)	(835)
Net increase (decrease) in cash and cash equivalents	20,129	(10,631)
Cash and cash equivalents - beginning of period	43,248	70,013
Cash and cash equivalents - end of period	$ 63,377	$ 59,382

Overview of Business

Overview of Business	9 Months Ended
	Sep. 30, 2019
Overview of Business [Abstract]
Overview of Business	1. Overview of Business Intercept Pharmaceuticals, Inc. (the “Company”) is a biopharmaceutical company focused on the development and commercialization of novel therapeutics to treat progressive non-viral liver diseases, including primary biliary cholangitis (“PBC”) and nonalcoholic steatohepatitis (“NASH”). The Company currently has one marketed product, Ocaliva (obeticholic acid or “OCA”). Founded in 2002 in New York, the Company has operations in the United States, Europe and Canada.

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2019
Overview of Business [Abstract]
Basis of Presentation	2. Basis of Presentation The Company’s financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”). All intercompany balances and transactions have been eliminated in consolidation. Certain information that is normally required by U.S. GAAP has been condensed or omitted in accordance with rules and regulations of the Securities and Exchange Commission (“SEC”). Operating results for the three and nine months ended September 30, 2019 are not necessarily indicative of the results that may be expected for any future period or for the year ending December 31, 2019. In the opinion of management, these unaudited condensed consolidated financial statements include all normal and recurring adjustments considered necessary for a fair presentation of these interim unaudited condensed consolidated financial statements. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2018, included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 filed with the SEC. Use of Estimates The preparation of these unaudited condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the reported amounts of assets and liabilities, the disclosure of assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from these estimates.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Summary of Significant Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of Significant Accounting Policies The Company’s significant accounting policies are described in Note 2 of Notes to Consolidated Financial Statements included in its Annual Report on Form 10-K for the year ended December 31, 2018 filed with the SEC. Other than the adoption of Accounting Standards Codification (“ASC”) Topic 842, Leases Leases The Company determines if an arrangement is a lease at inception and records right-of-use (“ROU”) assets and lease liabilities on the condensed consolidated balance sheets at lease commencement based on the present value of remaining lease payments over the lease term. The Company only considers payments that are fixed and determinable at the time of commencement. Operating leases are included in other assets, accounts payable, accrued expenses and other liabilities and long-term other liabilities on the condensed consolidated balance sheets. Operating lease liabilities are recognized based on the present value of the future minimum lease payments discounted by the Company’s incremental borrowing rate. The Company measures ROU assets based on the corresponding lease liability adjusted for (i) payments made to the lessor at or before the commencement date, (ii) initial direct costs incurred and (iii) tenant incentives under the lease. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that it will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. For short-term leases, the Company records rent expense in its consolidated statements of operations on a straight-line basis over the lease term and records variable lease payments as incurred. Additional information and disclosures are contained in Note 8 — Operating Leases below. Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) established ASC 842, by issuing Accounting Standards Update (“ASU”) No. 2016-02, which requires lessees to recognize leases on-balance sheet and disclose key information about leasing arrangements. ASC 842 was subsequently amended by ASU No. 2018-01, “Land Easement Practical Expedient for Transition to Topic 842”; ASU No. 2018-10, “Codification Improvements to Topic 842, Leases”; and ASU No. 2018-11, “Targeted Improvements”. The new standard establishes a ROU model that requires a lessee to recognize a ROU asset and lease liability on the balance sheet for all leases with a term longer than 12 months. Leases will be classified as finance or operating, with classification affecting the pattern and classification of expense recognition in the income statement. A modified retrospective transition approach is required, applying the new standard to all leases existing at the date of initial application. An entity may choose to use either (i) its effective date or (ii) the beginning of the earliest comparative period presented in the financial statements as its date of initial application. The Company adopted the new standard on January 1, 2019 using the effective date as the date of initial application. The new standard provides a number of optional practical expedients in transition. The Company elected the “package of practical expedients”, which permits the Company to not reassess under the new standard the Company’s prior conclusions about lease identification, lease classification and initial direct costs. The new standard also provides practical expedients for an entity’s ongoing accounting. The Company elected the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, the Company will not recognize ROU assets or lease liabilities, and this includes not recognizing ROU assets or lease liabilities for existing short-term leases of those assets in transition. The Company also elected the practical expedient to not separate lease and non-lease components for all of the Company’s leases. Upon adoption, the Company recognized additional operating liabilities of $25.4 million, with corresponding ROU assets of $19.6 million based on the present value of the remaining minimum rental payments under current leasing standards for existing operating leases. In July 2017, the FASB issued ASU No. 2017-11, “Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480) and Derivatives and Hedging (Topic 815): I. Accounting for Certain Financial Instruments with Down Round Features; II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception” (“ASU 2017-11”). Part I of this update addresses the complexity of accounting for certain financial instruments with down round features. Down round features are features of certain equity-linked instruments (or embedded features) that result in the strike price being reduced on the basis of the pricing of future equity offerings. Current accounting guidance creates cost and complexity for entities that issue financial instruments (such as warrants and convertible instruments) with down round features that require fair value measurement of the entire instrument or conversion option. Part II of this update addresses the difficulty of navigating ASC Topic 480, Distinguishing Liabilities from Equity In June 2018, the FASB issued ASU No. 2018-07, “Improvements to Nonemployee Share-Based Payment Accounting” (“ASU 2018-07”), which simplifies the accounting for share-based payments granted to nonemployees for goods and services. Under this ASU, most of the guidance on such payments to nonemployees would be aligned with the requirements for share-based payments granted to employees. The changes take effect for public companies for fiscal years starting after December 15, 2018, including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than an entity’s adoption date of ASC Topic 606, Revenue from Contracts with Customers Recent Accounting Pronouncements to be Adopted In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments” (“ASU 2016-13”), which replaces the incurred loss impairment methodology under current U.S. GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 was subsequently updated by ASU No. 2019-04, “Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments”, to clarify that entities should include recoveries when estimating the allowance for credit losses. The Company will be required to use a forward-looking expected credit loss model for accounts receivables, loans and other financial instruments. Credit losses relating to available-for-sale debt securities will also be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 and must be adopted using a modified retrospective approach, with certain exceptions. The Company is currently evaluating the impact of this standard on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework — Changes to the Disclosure Requirements for Fair Value Measurement” (“ASU 2018-13”), which makes a number of changes meant to add, modify or remove certain disclosure requirements associated with the movement amongst or hierarchy associated with Level 1, Level 2 and Level 3 fair value measurements. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted upon issuance of the update. The Company plans to adopt ASU 2018-13 effective January 1, 2020 and does not expect the adoption of this guidance to have a material impact on the Company’s consolidated financial statements and related disclosures.

Significant Agreements

Significant Agreements	9 Months Ended
	Sep. 30, 2019
Significant Agreements [Abstract]
Significant Agreements	4. Significant Agreements Sumitomo Dainippon Pharma Co., Ltd. In March 2011, the Company entered into an exclusive license agreement (the “Original Sumitomo Agreement”) with Sumitomo Dainippon Pharma Co., Ltd. (“Sumitomo Dainippon”), pursuant to which the Company granted to Sumitomo Dainippon an exclusive license to research, develop and commercialize OCA for the treatment of PBC and NASH in Japan and China (excluding Taiwan) and an option to research, develop and commercialize OCA in certain countries outside of such territories (the “Country Option”). The Company received an upfront payment from Sumitomo Dainippon of $15.0 million under the terms of the Original Sumitomo Agreement. In May 2014, Sumitomo Dainippon exercised the Country Option in part to add Korea as part of its licensed territories and paid the Company a $1.0 million upfront fee in connection therewith. In February 2018, the Company and Sumitomo Dainippon entered into Amendment No. 3 (the “Sumitomo Amendment”) to the Original Sumitomo Agreement (as amended, the “Sumitomo Agreement”). Pursuant to the Sumitomo Amendment, (i) Sumitomo Dainippon agreed to return the rights to develop and commercialize OCA in Japan and Korea and waived its rights to the Country Option, (ii) the Company agreed to forego any further milestone or royalty payments relating to the development and commercialization of OCA in Japan and Korea and (iii) certain milestone payment obligations with respect to the development and commercialization of OCA were adjusted. In addition, the Company and Sumitomo Dainippon agreed that if certain clinical development milestones in China are not met by December 31, 2020, Sumitomo Dainippon may choose either to make a milestone payment to the Company or terminate the Sumitomo Agreement. Sumitomo Dainippon may also terminate the Sumitomo Agreement in its entirety or on an indication-by-indication basis at any time upon 90 days’ written notice. As of September 30, 2019, the Company had achieved $6.0 million of development milestones under the Sumitomo Agreement. The Company may be eligible to receive additional milestone payments under the Sumitomo Agreement in an aggregate amount of up to approximately $23.0 million based on the occurrence of certain clinical trial and regulatory-related events and tiered royalty payments up to the mid-twenties in percentage terms based on net sales of OCA products in China (excluding Taiwan). Sumitomo Dainippon is responsible for the costs of developing and commercializing OCA in its territory. The Company has concluded that Sumitomo Dainippon does not represent a customer of the Company, and therefore the Sumitomo Agreement is outside of the scope of ASC 606. The Company has accounted, and continues to account, for the Sumitomo Agreement under the legacy accounting guidance. The Company’s substantive performance obligations under this agreement include an exclusive license to its technology, technical and scientific support to the development plan and participation on a joint steering committee. The Company determined that these performance obligations represent a single unit of accounting, since, initially, the license does not have stand-alone value to Sumitomo Dainippon without the Company’s technical expertise and steering committee participation during the development of OCA. The development period is currently estimated as continuing through June 2020 and, as such, the $15.0 million upfront payment is being recognized ratably over this period. The Company recognized licensing revenue of $0.4 million and $0.4 million for the three months ended September 30, 2019 and 2018, respectively, and $1.2 million and $1.6 million for the nine months ended September 30, 2019 and 2018, respectively, under the Sumitomo Agreement. Included in licensing revenue for the nine months ended September 30, 2018 is $0.4 million related to the accelerated recognition, as a result of the Sumitomo Amendment, of the remaining portion of deferred revenue associated with the $1.0 million upfront payment that the Company received under the Original Sumitomo Agreement in connection with Sumitomo Dainippon’s exercise of the Country Option with respect to Korea. The Company recognizes milestone payments when the associated milestones are achieved. As of September 30, 2019, and December 31, 2018, the Company had recorded deferred revenues of $1.2 million and $2.4 million, respectively, under this agreement.

Cash, Cash Equivalents and Inve

Cash, Cash Equivalents and Investment Debt Securities	9 Months Ended
	Sep. 30, 2019
Cash, Cash Equivalents, and Investment Debt Securities [Abstract]
Cash, Cash Equivalents, and Investment Debt Securities	5. Cash, Cash Equivalents and Investment Debt Securities The following table summarizes the Company’s cash, cash equivalents and investment debt securities as of September 30, 2019 and December 31, 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of September 30, 2019 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ ​ ​ ​ Unrealized ​ Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value ​ ​ (in thousands) Cash and cash equivalents: ​ ​ ​ ​ Cash and money market funds ​ $ 63,377 ​ $ — ​ $ — ​ $ 63,377 Total cash and cash equivalents ​ ​ 63,377 ​ ​ — ​ ​ — ​ ​ 63,377 Investment debt securities: ​ ​ ​ ​ Commercial paper ​ 70,009 ​ 62 ​ (8) ​ 70,063 Corporate debt securities ​ 574,046 ​ 984 ​ (95) ​ 574,935 U.S. government and agency securities ​ 3,992 ​ 4 ​ — ​ 3,996 Total investment debt securities ​ 648,047 ​ 1,050 ​ (103) ​ 648,994 Total cash, cash equivalents and investment debt securities ​ $ 711,424 ​ $ 1,050 ​ $ (103) ​ $ 712,371 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2018 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ ​ ​ ​ Unrealized ​ Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value ​ ​ (in thousands) Cash and cash equivalents: ​ ​ ​ ​ Cash and money market funds ​ $ 43,248 ​ $ — ​ $ — ​ $ 43,248 Investment debt securities: ​ ​ ​ ​ Commercial paper ​ 34,353 ​ — ​ (26) ​ 34,327 Corporate debt securities ​ 349,854 ​ 27 ​ (704) ​ 349,177 U.S. government and agency securities ​ 9,410 ​ 5 ​ (7) ​ 9,408 Total investment debt securities ​ 393,617 ​ 32 ​ (737) ​ 392,912 Total cash, cash equivalents and investment debt securities ​ $ 436,865 ​ $ 32 ​ $ (737) ​ $ 436,160 ​ As of September 30, 2019, the Company held a total of five positions that were in a continuous unrealized loss position for twelve months or longer. The Company has determined that the unrealized losses are deemed to be temporary impairments as of September 30, 2019. The Company believes that the unrealized losses generally are caused by increases in the risk premiums required by market participants rather than an adverse change in cash flows or a fundamental weakness in the credit quality of the issuer or underlying assets. Because the Company has the ability and intent to hold these investments until a recovery of fair value, which may be maturity, it does not consider the investments to be other-than-temporarily impaired at September 30, 2019.

Fixed Assets, Net

Fixed Assets, Net	9 Months Ended
	Sep. 30, 2019
Fixed Assets, Net [Abstract]
Fixed Assets, Net	6. Fixed Assets, Net Fixed assets are stated at cost and depreciated or amortized using the straight-line method based on useful lives as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Useful lives ​ ​ ​ ​ ​ ​ ​ (Years) September 30, 2019 December 31, 2018 ​ ​ ​ ​ (in thousands) Office equipment and software 3 ​ $ 4,343 ​ $ 3,986 Leasehold improvements Over life of lease ​ 10,444 ​ 14,464 Furniture and fixtures 7 ​ 3,974 ​ 3,907 Subtotal ​ ​ ​ 18,761 ​ 22,357 Less: accumulated depreciation ​ ​ ​ (12,788) ​ (11,946) Fixed assets, net ​ ​ ​ $ 5,973 ​ $ 10,411 ​

Inventory, Net

Inventory, Net	9 Months Ended
	Sep. 30, 2019
Inventory, Net [Abstract]
Inventory, Net	7. Inventory, Net Inventories are stated at the lower of cost or market. Inventories consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) Work-in-process ​ $ 9,289 ​ $ 7,019 Finished goods ​ 59 ​ 89 Inventory, net ​ $ 9,348 ​ $ 7,108 ​

Operating Leases

Operating Leases	9 Months Ended
	Sep. 30, 2019
Operating Leases [Abstract]
Operating Leases	8. Operating Leases The Company leases various office spaces under non-cancelable operating leases with original lease periods expiring between the fourth quarter of 2019 and 2024. The Company subleases one of its office spaces to a third party. The Company also enters into leases for equipment. A number of the Company’s leases include one or more options to renew, with renewal terms that can extend the lease term. The exercise of lease renewal options is typically at the sole discretion of the Company; therefore, all renewals to extend the lease terms are not included in the ROU assets and lease liabilities as they are not reasonably certain of exercise. The Company regularly evaluates the renewal options and when they are reasonably certain of exercise, includes the renewal period in the lease term. These operating leases do not contain material variable rent payments, residual value guarantees, covenants, or other restrictions. The Company has elected the practical expedient to exclude short-term leases from its ROU assets and lease liabilities; therefore leases with an initial term of 12 months or less are not recorded on the balance sheet. The Company recognizes lease expense for these leases on a straight-line basis over the lease term. The Company elected the practical expedient not to separate non-lease components from all leases. As the Company’s leases do not provide an implicit rate, the Company uses an incremental borrowing rate based on the information available at the lease commencement date in determining the present value of the lease payments. The Company’s incremental borrowing rate is the estimated rate that would be required to pay for a collateralized borrowing equal to the total lease payment over the lease term. The Company estimates its incremental borrowing rate based on an analysis of publicly traded debt securities of companies with credit and financial profiles similar to its own. Operating lease assets and liabilities are classified on the condensed consolidated balance sheet as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Leases ​ Classification ​ September 30, 2019 Assets ​ ​ ​ (in thousands) Operating lease assets ​ Other assets ​ $ 9,484 Total leased assets ​ ​ ​ $ 9,484 ​ ​ ​ ​ ​ ​ Liabilities ​ ​ ​ ​ ​ Current ​ ​ ​ ​ ​ Operating lease liabilities ​ Accounts payable, accrued expenses and other liabilities ​ $ 6,215 Noncurrent ​ ​ ​ ​ ​ Operating lease liabilities ​ Long-term other liabilities ​ ​ 6,028 Total lease liabilities ​ ​ ​ $ 12,243 ​ Operating lease costs for the three and nine-month periods ended September 30, 2019 are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended Lease Cost ​ Classification ​ September 30, 2019 ​ September 30, 2019 ​ ​ ​ ​ (in thousands) ​ (in thousands) Operating lease cost ​ Selling, general and administrative expenses ​ $ 1,456 ​ $ 4,603 Short-term lease cost ​ Selling, general and administrative expenses ​ ​ 392 ​ ​ 1,746 Variable lease cost ​ Selling, general and administrative expenses ​ ​ 230 ​ ​ 626 Sublease income ​ Other income, net ​ ​ (182) ​ ​ (546) Net lease cost ​ ​ ​ $ 1,896 ​ $ 6,429 ​ The weighted-average remaining term of the Company’s operating leases was 2.5 years and the weighted-average discount rate used to measure the present value of the Company’s operating lease liabilities was 5.0% as of September 30, 2019. Maturities of the Company’s operating lease liabilities, which do not include short-term leases, as of September 30, 2019 are as follows: ​ ​ ​ ​ Maturity of Lease Liabilities ​ Operating leases ​ ​ (in thousands) 2019 ​ $ 1,889 2020 ​ ​ 6,142 2021 ​ ​ 2,828 2022 ​ ​ 896 2023 ​ ​ 896 Thereafter ​ ​ 374 Total lease payments ​ ​ 13,025 Less: Present value discount ​ ​ (782) Total operating lease liabilities ​ $ 12,243 ​ Cash payments included in the measurement of the Company’s lease liabilities were $5.5 million for the nine months ended September 30, 2019.

Accounts Payable, Accrued Expen

Accounts Payable, Accrued Expenses and Other Liabilities	9 Months Ended
	Sep. 30, 2019
Accounts Payable, Accrued Expenses and Other Liabilities [Abstract]
Accounts Payable, Accrued Expenses and Other Liabilities	9. Accounts Payable, Accrued Expenses and Other Liabilities Accounts payable, accrued expenses and other liabilities consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) Accounts payable ​ $ 19,485 ​ $ 11,765 Accrued employee compensation ​ 19,343 ​ 20,335 Accrued contracted services ​ 66,231 ​ 54,681 Other liabilities ​ ​ 22,021 ​ ​ 18,328 Operating lease liabilities ​ ​ 6,215 ​ ​ — Accounts payable, accrued expenses and other liabilities ​ $ 133,295 ​ $ 105,109 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Research & Development Tax Credit The Company benefits from the U.K. Small and Medium-sized Enterprise R&D Tax Credit scheme, or the SME scheme, under which it can obtain a refundable credit of up to 33.4% of eligible research and development expenses incurred by the Company in the U.K.. Eligible expenses generally include employment costs for research staff, consumables, software and certain internal overhead costs incurred as part of research projects. The Company submitted a claim seeking to obtain tax credits for qualifying R&D expenses incurred in the years ended December 31, 2015 and 2016. In September 2019, the Company received a partial payment of $10.5 million from Her Majesty’s Revenue and Customs, the U.K.’s government tax authority. Given the claim review has not been finalized, the credit received is recorded as a deferred liability within Accounts payable, accrued expenses, and other liabilities.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2019
Fair Value Measurements [Abstract]
Fair Value Measurements	10. Fair Value Measurements The carrying amounts of the Company’s receivables and payables approximate their fair value due to their short maturities. Accounting principles provide guidance for using fair value to measure assets and liabilities. The guidance includes a three-level hierarchy of valuation techniques used to measure fair value, defined as follows: ● Unadjusted Quoted Prices — The fair value of an asset or liability is based on unadjusted quoted prices in active markets for identical assets or liabilities (Level 1). ● Pricing Models with Significant Observable Inputs — The fair value of an asset or liability is based on information derived from either an active market quoted price, which may require further adjustment based on the attributes of the financial asset or liability being measured, or an inactive market transaction (Level 2). ● Pricing Models with Significant Unobservable Inputs — The fair value of an asset or liability is primarily based on internally derived assumptions surrounding the timing and amount of expected cash flows for the financial instrument. Therefore, these assumptions are unobservable in either an active or inactive market (Level 3). The Company considers an active market as one in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing information on an ongoing basis. Conversely, the Company views an inactive market as one in which there are few transactions for the asset or liability, the prices are not current, or price quotations vary substantially either over time or among market makers. Where appropriate, non-performance risk, or that of a counterparty, is considered in determining the fair values of liabilities and assets, respectively. The Company’s money market funds are classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices. Investment debt securities are classified as Level 2 instruments based on market pricing and other observable inputs. Financial assets carried at fair value are classified in the tables below in one of the three categories described above: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements Using ​ Total Level 1 Level 2 Level 3 ​ ​ (in thousands) September 30, 2019 ​ ​ ​ ​ Assets ​ ​ ​ ​ Cash and cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash and cash equivalents) ​ $ 31,691 ​ $ 31,691 ​ $ — ​ $ — Available-for-sale debt securities: ​ ​ ​ ​ Commercial paper ​ 70,063 ​ — ​ 70,063 ​ — Corporate debt securities ​ 574,935 ​ — ​ 574,935 ​ — U.S. government and agency securities ​ 3,996 ​ — ​ 3,996 ​ — Total financial assets ​ $ 680,685 ​ $ 31,691 ​ $ 648,994 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2018 ​ ​ ​ ​ Assets ​ ​ ​ ​ Money market funds (included in cash and cash equivalents) ​ $ 11,647 ​ $ 11,647 ​ $ — ​ $ — Available-for-sale debt securities: ​ ​ ​ ​ Commercial paper ​ 34,327 ​ — ​ 34,327 ​ — Corporate debt securities ​ 349,177 ​ — ​ 349,177 ​ — U.S. government and agency securities ​ 9,408 ​ — ​ 9,408 ​ — Total financial assets ​ $ 404,559 ​ $ 11,647 ​ $ 392,912 ​ $ — ​ The estimated fair value of the available-for-sale debt securities (commercial paper, corporate debt securities and U.S. government and agency securities), by contractual maturity, are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value as of ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) Due in one year or less ​ $ 486,971 ​ $ 319,717 Due after one year through two years ​ 162,023 ​ 73,195 Total investments in debt securities ​ $ 648,994 ​ $ 392,912 ​ Actual maturities may differ from contractual maturities because issuers may have the right to call or prepay obligations without call or prepayment penalties.

Long-Term Debt

Long-Term Debt	9 Months Ended
	Sep. 30, 2019
Long-Term Debt [Abstract]
Long-Term Debt	11. Long-Term Debt Debt, net of discounts and deferred financing costs, consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) 2023 Convertible Notes ​ $ 460,000 ​ $ 460,000 2026 Convertible Notes ​ ​ 230,000 ​ ​ — Long-term debt, gross ​ ​ 690,000 ​ ​ 460,000 Less: Unamortized debt discounts and fees ​ ​ 164,661 ​ ​ 88,750 Long-term debt, net ​ ​ 525,339 ​ ​ 371,250 ​ 2019 Offering ​ On May 14, 2019, the Company issued and sold $230.0 million aggregate principal amount of 2.00% Convertible Senior Notes due 2026 (the “2026 Convertible Notes”). The Company received net proceeds from the sale of the 2026 Convertible Notes of $223.4 million, after deducting underwriting discounts, commissions and estimated offering expenses of approximately $6.6 million. The 2026 Convertible Notes were issued pursuant to a Second Supplemental Indenture, dated as of May 14, 2019 (the “Second Supplemental Indenture”), which supplements the Indenture (the “Base Indenture”), as supplemented by a First Supplemental Indenture (the “First Supplemental Indenture” and collectively with the Base Indenture and the Second Supplemental Indenture, the “Indenture”), each dated as of July 6, 2016, by and between the Company and U.S. Bank National Association, as trustee. The 2026 Convertible Notes are senior unsecured obligations of the Company, bear interest at a fixed rate of 2.00% per annum (payable semi-annually on May 15 and November 15 of each year, beginning on November 15, 2019) and will mature on May 15, 2026, unless earlier repurchased, redeemed or converted. Holders may convert their 2026 Convertible Notes at their option at any time prior to the close of business on the business day immediately preceding February 15, 2026 only under the following circumstances: (i) during any calendar quarter (and only during such calendar quarter) commencing after the calendar quarter ended on September 30, 2019, if the last reported sale price of the Company’s common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (ii) during the five business day period after any five consecutive trading day period in which the trading price (as defined in the Indenture) per $1,000 principal amount of 2026 Convertible Notes for each trading day of such five consecutive trading day period was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate on each such trading day; (iii) if the Company calls any or all of the 2026 Convertible Notes for redemption, at any time prior to the close of business on the scheduled trading day immediately preceding the redemption date; or (iv) upon the occurrence of specified corporate events. On or after February 15, 2026 until the close of business on the business day immediately preceding the maturity date, holders may convert their 2026 Convertible Notes at any time, regardless of the foregoing circumstances. Upon conversion of the 2026 Convertible Notes, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock (and cash in lieu of any fractional shares) or a combination of cash and shares of the Company’s common stock, at the Company’s election. The initial conversion rate of the 2026 Convertible Notes is 9.2123 shares of the Company’s common stock per $1,000 principal amount of 2026 Convertible Notes, which is equivalent to an initial conversion price of approximately $108.55 per share of the Company’s common stock. The conversion rate is subject to adjustment in some events but will not be adjusted for any accrued and unpaid interest. In addition, following certain corporate events that occur prior to the maturity date, the Company will increase the conversion rate for a holder who elects to convert its 2026 Convertible Notes in connection with such a corporate event in certain circumstances. The Company may not redeem the 2026 Convertible Notes prior to May 20, 2023. The Company may redeem for cash all or any portion of the 2026 Convertible Notes, at the Company’s option, on or after May 20, 2023, if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive) during any 30 consecutive trading day period (including the last trading day of such period) ending on, and including, the trading day immediately preceding the date on which the Company provides notice of redemption at a redemption price equal to 100% of the principal amount of the 2026 Convertible Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. No sinking fund is provided for the 2026 Convertible Notes. If the Company undergoes a fundamental change (as defined in the Indenture), holders may require the Company to repurchase for cash all or any portion of their 2026 Convertible Notes at a fundamental change repurchase price equal to 100% of the principal amount of the 2026 Convertible Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date. The Indenture provides for customary events of default. In accordance with ASC Subtopic 470-20, “Debt with Conversion and Other Options” (“ASC 470-20”), the Company used an effective interest rate of 9.9% to determine the liability component of the 2026 Convertible Notes. This resulted in the recognition of $137.5 million as the liability component of the 2026 Convertible Notes and the recognition of the residual $85.9 million as the debt discount with a corresponding increase to additional paid-in capital for the equity component of the 2026 Convertible Notes. The underwriting discount and estimated offering expenses totaling $6.6 million were allocated between the debt and equity issuance costs in proportion to the allocation of the liability and equity components of the 2026 Convertible Notes. Accordingly, equity issuance costs of $2.5 million were recorded as an offset to additional paid-in capital and total debt issuance costs of $4.1 million were recorded on the issuance date and are reflected in the unaudited condensed consolidated balance sheet as a direct deduction from the carrying value of the associated debt liability. The debt discount and debt issuance costs will be amortized as non-cash interest expense through May 15, 2026. The fair value of the 2026 Convertible Notes was approximately $213.9 million at September 30, 2019 and was determined using Level 2 inputs based on quoted market values. 2016 Offerings On July 6, 2016, the Company issued and sold $460.0 million aggregate principal amount of 3.25% Convertible Senior Notes due 2023 (the “2023 Convertible Notes”, and together with the 2026 Convertible Notes, the “Convertible Notes”). The Company received net proceeds from the sale of the 2023 Convertible Notes of $447.6 million, after deducting underwriting discounts, commissions and estimated offering expenses of approximately $12.4 million. The Company used approximately $38.4 million of such net proceeds to fund the cost of the Capped Call Transactions (as defined below) that were entered into in connection with the issuance of the 2023 Convertible Notes. The 2023 Convertible Notes were issued pursuant to the Base Indenture, as supplemented by the First Supplemental Indenture. The 2023 Convertible Notes are senior unsecured obligations of the Company, bear interest at a fixed rate of 3.25% per year (payable semi-annually on January 1 and July 1 of each year, beginning on January 1, 2017) and will mature on July 1, 2023, unless earlier repurchased, redeemed or converted. Holders may convert their 2023 Convertible Notes at their option at any time prior to the close of business on the business day immediately preceding January 1, 2023 only under the following circumstances: (i) during any calendar quarter commencing after the calendar quarter ended on September 30, 2016, if the last reported sale price of the Company’s common stock for at least 20 trading days (whether or not consecutive) during the period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (ii) during the five business day period after any five consecutive trading day period in which the trading price (as defined in the Indenture) per $1,000 principal amount of 2023 Convertible Notes for each trading day of such five consecutive trading day period was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate on each such trading day; (iii) if the Company calls any or all of the 2023 Convertible Notes for redemption, at any time prior to the close of business on the scheduled trading day immediately preceding the redemption date; or (iv) upon the occurrence of specified corporate events. On or after January 1, 2023 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2023 Convertible Notes at any time, regardless of the foregoing circumstances. Upon conversion of the 2023 Convertible Notes, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock (and cash in lieu of any fractional shares) or a combination of cash and shares of the Company’s common stock, at the Company’s election. The initial conversion rate of the 2023 Convertible Notes is 5.0358 shares of the Company’s common stock per $1,000 principal amount of 2023 Convertible Notes, which is equivalent to an initial conversion price of approximately $198.58 per share of the Company’s common stock. The conversion rate is subject to adjustment upon the occurrence of certain events but will not be adjusted for any accrued and unpaid interest. If the Company undergoes a fundamental change (as defined in the Indenture), holders may require the Company to repurchase for cash all or any portion of their 2023 Convertible Notes at a fundamental change repurchase price equal to 100% of the principal amount of the 2023 Convertible Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the fundamental change repurchase date. In addition, if certain make-whole fundamental changes occur, the Company will, in certain circumstances, increase the conversion rate for any 2023 Convertible Notes converted in connection with such make-whole fundamental change. The Company may not redeem the 2023 Convertible Notes prior to July 6, 2021. The Company may redeem for cash all or part of the 2023 Convertible Notes, at its option, on or after July 6, 2021, if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive) during any 30 consecutive trading day period (including the last trading day of such period) ending on, and including, the trading day immediately preceding the date on which the Company provides notice of redemption at a redemption price equal to 100% of the principal amount of the 2023 Convertible Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. The Indenture provides for customary events of default. On June 30, 2016, in connection with the pricing of the 2023 Convertible Notes, the Company entered into privately-negotiated capped call transactions (the “Base Capped Call Transactions”) with each of Royal Bank of Canada, UBS AG, London Branch, and Credit Suisse Capital LLC (the “Option Counterparties”). On July 1, 2016, in connection with the underwriters’ exercise of their over-allotment option in full, the Company entered into additional capped call transactions (the “Additional Capped Call Transactions” and, together with the Base Capped Call Transactions, the “Capped Call Transactions”) with the Option Counterparties. The Capped Call Transactions are expected generally to reduce the potential dilution with respect to the Company’s common stock and/or offset the cash payments the Company would be required to make in excess of the principal amount of converted 2023 Convertible Notes, as the case may be, upon conversion of the 2023 Convertible Notes in the event that the market price per share of the Company’s common stock, as measured under the terms of the Capped Call Transactions, is greater than the strike price of the Capped Call Transactions, which initially corresponds to the conversion price of the 2023 Convertible Notes and is subject to anti-dilution adjustments substantially similar to those applicable to the conversion rate of the 2023 Convertible Notes. The cap price of the Capped Call Transactions is initially $262.2725 per share, and is subject to certain adjustments under the terms of the Capped Call Transactions. If, however, the market price per share of the Company’s common stock, as measured under the terms of the Capped Call Transactions, exceeds the cap price of the Capped Call Transactions, there would nevertheless be dilution and/or there would not be an offset of such potential cash payments, in each case, upon conversion of the Convertible Notes to the extent that such market price exceeds the cap price of the Capped Call Transactions. In accordance with ASC 470-20, the Company used an effective interest rate of 8.4% to determine the liability component of the 2023 Convertible Notes. This resulted in the recognition of $334.4 million as the liability component of the 2023 Convertible Notes and the recognition of the residual $113.1 million as the debt discount with a corresponding increase to additional paid-in capital for the equity component of the 2023 Convertible Notes. The fair value of the 2023 Convertible Notes was approximately $400.7 million and $410.9 million at September 30, 2019 and December 31, 2018, respectively, and was determined using Level 2 inputs based on quoted market values. ​ Interest Expense on Convertible Notes Interest expense was $11.8 million and $7.7 million for the three months ended September 30, 2019 and 2018, respectively, and $29.5 million and $22.8 million for the nine months ended September 30, 2019 and 2018, respectively, related to the Convertible Notes. Accrued interest on the Convertible Notes was approximately $5.5 million and $7.5 million as of September 30, 2019 and December 31, 2018, respectively. The Company recorded debt issuance costs of $19.0 million, which are being amortized using the effective interest method. As of September 30, 2019, $13.8 million of debt issuance costs are recorded on the unaudited condensed consolidated balance sheet in Long-term debt, in accordance with ASU No. 2015-03, “Interest – Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs.” As of September 30, 2019, $230.0 million aggregate principal amount of the 2026 Convertible Notes and $460.0 million aggregate principal amount of the 2023 Convertible Notes was outstanding, for a total of $690.0 million aggregate principal amount outstanding.

Product Revenue, Net

Product Revenue, Net	9 Months Ended
	Sep. 30, 2019
Product Revenue, Net [Abstract]
Product Revenue, Net	12. Product Revenue, Net The Company recognized net sales of Ocaliva of $61.5 million and $46.6 million for the three months ended September 30, 2019 and 2018, respectively, and $179.3 million and $124.9 million for the nine months ended September 30, 2019 and 2018, respectively. The table below summarizes consolidated product revenue, net by region: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2019 2018 2019 2018 ​ ​ (in thousands) ​ (in thousands) Product revenue, net: ​ ​ ​ ​ U.S. ​ $ 45,232 ​ $ 36,684 ​ $ 133,914 ​ $ 99,697 ex-U.S. ​ 16,313 ​ 9,897 ​ 45,372 ​ 25,211 Total product revenue, net ​ $ 61,545 ​ $ 46,581 ​ $ 179,286 ​ $ 124,908 ​

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2019
Stockholders' Equity [Abstract]
Stockholders' Equity	13. Stockholders’ Equity 2019 Public Offering and Concurrent Private Placement On May 14, 2019, the Company issued and sold (i) 2,760,000 shares of common stock in a registered public offering (including 360,000 shares issued and sold upon the exercise in full of the underwriters’ option to purchase additional shares), at a price to the public of $83.50 per share (the “2019 Public Offering”) and (ii) 119,760 shares of common stock (the “2019 Private Placement Shares”) in a concurrent private placement of common stock (the “2019 Concurrent Private Placement”) exempt from the registration requirements of the Securities Act of 1933, as amended (the “Securities Act”), at a purchase price per share equivalent to the price to the public set in the 2019 Public Offering and pursuant to a securities purchase agreement (the “2019 Securities Purchase Agreement”) that the Company entered into with Samsara BioCapital, L.P. (“Samsara”), one of the Company’s existing stockholders. Pursuant to the 2019 Securities Purchase Agreement, the Company granted to Samsara certain registration rights requiring the Company, upon request of Samsara on or after July 9, 2019 and subject to certain terms and conditions, to register the resale by Samsara of its 2019 Private Placement Shares. Such registration rights expire upon the earlier of (i) May 8, 2020 and (ii) the date that all of the 2019 Private Placement Shares have been sold or can be sold publicly under Rule 144 of the Securities Act on a single day. As of the date of this Quarterly Report on Form 10-Q, Samsara has not exercised any such registration rights. The net proceeds to the Company from the 2019 Public Offering and the 2019 Concurrent Private Placement were approximately $227.2 million, after deducting underwriting discounts, commissions and estimated offering expenses of approximately $13.9 million. 2018 Public Offering and Concurrent Private Placement On April 9, 2018, the Company issued and sold (i) 2,695,313 shares of common stock in a registered public offering (including 351,563 shares issued and sold upon the exercise in full of the underwriters’ option to purchase additional shares), at a price to the public of $64.00 per share (the “2018 Public Offering”) and (ii) 1,562,500 shares of common stock (the “2018 Private Placement Shares”) in a concurrent private placement (the “2018 Concurrent Private Placement”) exempt from the registration requirements of the Securities Act, at a purchase price per share equivalent to the price to the public set in the 2018 Public Offering and pursuant to a securities purchase agreement (the “2018 Securities Purchase Agreement”) that the Company entered into with the purchasers in the 2018 Concurrent Private Placement (the “Private Placement Purchasers”). Pursuant to the 2018 Securities Purchase Agreement, the Company granted to the Private Placement Purchasers certain registration rights which expired on April 4, 2019.

Stock Compensation

Stock Compensation	9 Months Ended
	Sep. 30, 2019
Stock Compensation [Abstract]
Stock Compensation	14. Stock Compensation The Company’s 2012 Equity Incentive Plan (“2012 Plan”) became effective upon the pricing of its initial public offering in October 2012. At the same time, the Company’s 2003 Stock Incentive Plan (“2003 Plan”) was terminated and 555,843 shares available under the 2003 Plan were added to the 2012 Plan. On January 1, 2019, the number of shares available for issuance under the 2012 Plan increased by 1,187,599 shares, as a result of the automatic increase provisions thereof. The estimated fair value of the stock options granted in the nine months ended September 30, 2019 was determined utilizing a Black-Scholes option-pricing model at the date of grant. The fair value of the restricted stock units (“RSUs”) granted in the nine months ended September 30, 2019 was determined utilizing the closing price of the Company’s common stock on the date of grant. The fair value of the performance restricted stock units (“PRSUs”) granted in the nine months ended September 30, 2019 was determined utilizing the Monte Carlo simulation method. The following table summarizes stock option activity during the nine months ended September 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Average ​ ​ ​ ​ ​ Number ​ Weighted ​ Remaining ​ Aggregate ​ ​ of Options ​ Average ​ Contractual ​ Intrinsic Value ​ (in thousands) Exercise Price Term (years) (in thousands) Outstanding at December 31, 2018 1,873 ​ $ 97.63 7.5 ​ $ 45,381 Granted 484 ​ $ 105.55 — ​ $ — Exercised (69) ​ $ 36.59 — ​ $ — Cancelled/forfeited (148) ​ $ 92.40 — ​ $ — Expired (54) ​ $ 165.26 — ​ $ — Outstanding at September 30, 2019 2,086 ​ $ 97.93 7.4 ​ $ 11,747 Expected to vest 985 ​ $ 91.42 8.7 ​ $ 2,533 Exercisable 1,101 ​ $ 103.75 6.3 ​ $ 9,214 ​ The aggregate intrinsic value of options is calculated as the difference between the exercise price of the underlying options and the deemed fair value of the Company’s common stock for those options that had exercise prices lower than the deemed fair value of the Company’s common stock. As of September 30, 2019, the total compensation cost related to non-vested option awards not yet recognized is approximately $54.9 million with a weighted average remaining vesting period of 1.2 years. The Company estimated the fair value of stock options granted in the periods presented utilizing a Black-Scholes option-pricing model utilizing the following assumptions: ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ 2019 2018 ​ Volatility 87.0 - 89.9 % 58.9 - 75.8 % Expected term (in years) 5.5 - 6.0 6.0 Risk-free rate 1.4 - 2.9 % 1.8 - 2.9 % Expected dividend yield — % — % ​ Effective January 1, 2019, the Company changed its expected volatility assumption to be estimated based on the historical stock price volatility of the Company over the expected term given the availability of sufficient historical trading data. In prior years, the expected volatility was estimated based on historical volatility information of publicly-traded peer companies. The following table summarizes the aggregate RSU, restricted stock award (“RSA”), PRSU and performance restricted share award (“PRSA”) activity during the nine months ended September 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ Number of ​ Average Grant Date ​ Awards Fair Value ​ ​ (in thousands) ​ ​ ​ Non-vested awards at December 31, 2018 773 ​ $ 76.10 Granted 354 ​ $ 110.05 Vested ​ (261) ​ $ 78.71 Forfeited (83) ​ $ 85.67 Non-vested awards at September 30, 2019 783 ​ $ 89.57 ​ As of September 30, 2019, there is approximately $53.4 million of total unrecognized compensation expense related to unvested RSUs, RSAs, PRSUs and PRSAs, which is expected to be recognized over a weighted average vesting period of 1.4 years. During the nine months ended September 30, 2019, the Company granted a total of 57,800 PRSUs to certain of the Company’s executive officers. The performance criterion for such PRSUs is based on the Total Shareholder Return (“TSR”) of the Company’s common stock relative to the TSR of the companies comprising the S&P Biotechnology Select Industry Index (the “TSR Peer Group”) over a 3-year performance period and is accounted for as a market condition under ASC Topic 718, Compensation – Stock Compensation The Company accounts for all forfeitures when they occur. Ultimately, the actual expense recognized over the vesting period will be for only those shares that vest and are not forfeited. The Company has in the past, and may in the future, grant performance-based awards with vesting terms based on the achievement of specified goals. To the extent such awards do not contain a market condition, the Company recognizes no expense until achievement of the performance requirement is deemed probable. Stock-based compensation expense has been reported in the Company’s condensed consolidated statements of operations as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2019 2018 2019 2018 ​ ​ ​ (in thousands) (in thousands) Selling, general and administrative ​ $ 10,104 ​ $ 9,436 ​ $ 33,029 ​ $ 28,875 Research and development ​ 3,026 ​ 2,558 ​ 9,780 ​ 9,540 Total stock-based compensation ​ $ 13,130 ​ $ 11,994 ​ $ 42,809 ​ $ 38,415 ​

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies [Abstract]
Commitments and Contingencies	15. Commitments and Contingencies Legal Proceedings The Company is involved in various disputes, governmental inquiries and investigations, legal proceedings and litigation in the course of its business, including the matters described below and, from time to time, intellectual property, employment and other litigation. These matters, which could result in damages, fines or other administrative, civil or criminal remedies, liabilities or penalties, are often complex and the outcome of such matters is often uncertain. The Company may from time to time enter into settlements to resolve such matters. On September 27, 2017, a purported shareholder class action, initially styled DeSmet v. Intercept Pharmaceuticals, Inc., et al, was filed in the United States District Court for the Southern District of New York, naming the Company and certain of its officers as defendants. The Court appointed lead plaintiffs in the lawsuit on June 1, 2018, and the lead plaintiffs filed an amended complaint on July 31, 2018, captioned Hou Liu and Amy Fu v. Intercept Pharmaceuticals, Inc., et al., naming the Company and certain of its current and former officers as defendants. The lead plaintiffs claim to be suing on behalf of anyone who purchased or otherwise acquired the Company’s common stock between June 9, 2016 and September 20, 2017. This lawsuit alleges that material misrepresentations and/or omissions of material fact were made in the Company’s public disclosures during the period from June 9, 2016 to September 20, 2017, in violation of Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and Rule 10b-5 promulgated thereunder. The alleged improper disclosures relate to statements regarding Ocaliva dosing, use and pharmacovigilance-related matters, as well as the Company’s operations, financial performance and prospects. The plaintiffs seek unspecified monetary damages on behalf of the putative class, an award of costs and expenses, including attorney’s fees, and rescissory damages. On September 14, 2018, the Company filed a motion to dismiss the amended complaint. Separately, on January 5, 2018, a follow-on derivative suit, styled Davis v. Pruzanski et al., was filed in New York state court by shareholder Gregg Davis based on substantially the same allegations as those set forth in the securities case. On December 1, 2017, a purported shareholder demand was made on the Company based on substantially the same allegations as those set forth in the securities case. While the Company believes that it has a number of valid defenses to the claims described above and intends to vigorously defend itself, the matters are in the early stages of litigation and no assessment can be made as to the likely outcome of the matters or whether they will be material to the Company. Accordingly, an estimate of the potential loss, or range of loss, if any, to the Company relating to the matters is not possible at this time. In May 2018, the Company received a subpoena from the SEC requesting information in connection with the Company’s patient assistance program and certain of the Company’s commercial activities. The SEC’s letter enclosing the subpoena states that the investigation and the subpoena do not mean that the Company or anyone else has broken the law, or that the SEC has a negative opinion of any person, entity or security. The Company is cooperating fully with the SEC in this matter. At this time, the Company is unable to predict whether any proceeding may be instituted in connection with the subpoena, or the outcome of any such proceeding, if instituted.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2019
Net loss per common and potential common share:
Net Loss Per Share	16. Net Loss Per Share Basic loss per share is computed by dividing net loss attributable to common stockholders (numerator) by the weighted average number of common shares outstanding (denominator) during the period. For the three and nine-month periods ended September 30, 2019 and 2018, as the Company was in a net loss position, the diluted loss per share computations for such periods did not assume the conversion of the Convertible Notes, exercise of stock options or vesting of RSUs or PRSUs as they would have had an anti-dilutive effect on basic and diluted loss per share. The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding for the three and nine-month periods ended of September 30, 2019 and 2018, as the inclusion thereof would have been anti-dilutive: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2019 2018 2019 2018 ​ ​ (in thousands) ​ (in thousands) Convertible Notes ​ 4,435 ​ 2,316 ​ 4,435 ​ 2,316 Options 2,086 1,997 2,086 1,997 Restricted stock units 597 447 597 447 Total 7,118 4,760 7,118 4,760 ​

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	​ 17. Subsequent Events Sumitomo Agreement ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Basis of Presentation (Policy)

Basis of Presentation (Policy)	9 Months Ended
	Sep. 30, 2019
Overview of Business [Abstract]
Use of Estimates	Use of Estimates The preparation of these unaudited condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the reported amounts of assets and liabilities, the disclosure of assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from these estimates.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Summary of Significant Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of these unaudited condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and judgments that affect the reported amounts of assets and liabilities, the disclosure of assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from these estimates.
Fair Value of Financial Instruments	The carrying amounts of the Company’s receivables and payables approximate their fair value due to their short maturities. Accounting principles provide guidance for using fair value to measure assets and liabilities. The guidance includes a three-level hierarchy of valuation techniques used to measure fair value, defined as follows: ● Unadjusted Quoted Prices — The fair value of an asset or liability is based on unadjusted quoted prices in active markets for identical assets or liabilities (Level 1). ● Pricing Models with Significant Observable Inputs — The fair value of an asset or liability is based on information derived from either an active market quoted price, which may require further adjustment based on the attributes of the financial asset or liability being measured, or an inactive market transaction (Level 2). ● Pricing Models with Significant Unobservable Inputs — The fair value of an asset or liability is primarily based on internally derived assumptions surrounding the timing and amount of expected cash flows for the financial instrument. Therefore, these assumptions are unobservable in either an active or inactive market (Level 3). The Company considers an active market as one in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing information on an ongoing basis. Conversely, the Company views an inactive market as one in which there are few transactions for the asset or liability, the prices are not current, or price quotations vary substantially either over time or among market makers. Where appropriate, non-performance risk, or that of a counterparty, is considered in determining the fair values of liabilities and assets, respectively. The Company’s money market funds are classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices. Investment debt securities are classified as Level 2 instruments based on market pricing and other observable inputs.
Leases	Leases The Company determines if an arrangement is a lease at inception and records right-of-use (“ROU”) assets and lease liabilities on the condensed consolidated balance sheets at lease commencement based on the present value of remaining lease payments over the lease term. The Company only considers payments that are fixed and determinable at the time of commencement. Operating leases are included in other assets, accounts payable, accrued expenses and other liabilities and long-term other liabilities on the condensed consolidated balance sheets. Operating lease liabilities are recognized based on the present value of the future minimum lease payments discounted by the Company’s incremental borrowing rate. The Company measures ROU assets based on the corresponding lease liability adjusted for (i) payments made to the lessor at or before the commencement date, (ii) initial direct costs incurred and (iii) tenant incentives under the lease. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that it will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. For short-term leases, the Company records rent expense in its consolidated statements of operations on a straight-line basis over the lease term and records variable lease payments as incurred. Additional information and disclosures are contained in Note 8 — Operating Leases below.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) established ASC 842, by issuing Accounting Standards Update (“ASU”) No. 2016-02, which requires lessees to recognize leases on-balance sheet and disclose key information about leasing arrangements. ASC 842 was subsequently amended by ASU No. 2018-01, “Land Easement Practical Expedient for Transition to Topic 842”; ASU No. 2018-10, “Codification Improvements to Topic 842, Leases”; and ASU No. 2018-11, “Targeted Improvements”. The new standard establishes a ROU model that requires a lessee to recognize a ROU asset and lease liability on the balance sheet for all leases with a term longer than 12 months. Leases will be classified as finance or operating, with classification affecting the pattern and classification of expense recognition in the income statement. A modified retrospective transition approach is required, applying the new standard to all leases existing at the date of initial application. An entity may choose to use either (i) its effective date or (ii) the beginning of the earliest comparative period presented in the financial statements as its date of initial application. The Company adopted the new standard on January 1, 2019 using the effective date as the date of initial application. The new standard provides a number of optional practical expedients in transition. The Company elected the “package of practical expedients”, which permits the Company to not reassess under the new standard the Company’s prior conclusions about lease identification, lease classification and initial direct costs. The new standard also provides practical expedients for an entity’s ongoing accounting. The Company elected the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, the Company will not recognize ROU assets or lease liabilities, and this includes not recognizing ROU assets or lease liabilities for existing short-term leases of those assets in transition. The Company also elected the practical expedient to not separate lease and non-lease components for all of the Company’s leases. Upon adoption, the Company recognized additional operating liabilities of $25.4 million, with corresponding ROU assets of $19.6 million based on the present value of the remaining minimum rental payments under current leasing standards for existing operating leases. In July 2017, the FASB issued ASU No. 2017-11, “Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480) and Derivatives and Hedging (Topic 815): I. Accounting for Certain Financial Instruments with Down Round Features; II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception” (“ASU 2017-11”). Part I of this update addresses the complexity of accounting for certain financial instruments with down round features. Down round features are features of certain equity-linked instruments (or embedded features) that result in the strike price being reduced on the basis of the pricing of future equity offerings. Current accounting guidance creates cost and complexity for entities that issue financial instruments (such as warrants and convertible instruments) with down round features that require fair value measurement of the entire instrument or conversion option. Part II of this update addresses the difficulty of navigating ASC Topic 480, Distinguishing Liabilities from Equity In June 2018, the FASB issued ASU No. 2018-07, “Improvements to Nonemployee Share-Based Payment Accounting” (“ASU 2018-07”), which simplifies the accounting for share-based payments granted to nonemployees for goods and services. Under this ASU, most of the guidance on such payments to nonemployees would be aligned with the requirements for share-based payments granted to employees. The changes take effect for public companies for fiscal years starting after December 15, 2018, including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than an entity’s adoption date of ASC Topic 606, Revenue from Contracts with Customers Recent Accounting Pronouncements to be Adopted In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments-Credit Losses: Measurement of Credit Losses on Financial Instruments” (“ASU 2016-13”), which replaces the incurred loss impairment methodology under current U.S. GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 was subsequently updated by ASU No. 2019-04, “Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments”, to clarify that entities should include recoveries when estimating the allowance for credit losses. The Company will be required to use a forward-looking expected credit loss model for accounts receivables, loans and other financial instruments. Credit losses relating to available-for-sale debt securities will also be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019 and must be adopted using a modified retrospective approach, with certain exceptions. The Company is currently evaluating the impact of this standard on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU No. 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework — Changes to the Disclosure Requirements for Fair Value Measurement” (“ASU 2018-13”), which makes a number of changes meant to add, modify or remove certain disclosure requirements associated with the movement amongst or hierarchy associated with Level 1, Level 2 and Level 3 fair value measurements. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted upon issuance of the update. The Company plans to adopt ASU 2018-13 effective January 1, 2020 and does not expect the adoption of this guidance to have a material impact on the Company’s consolidated financial statements and related disclosures.

Fair Value Measurements (Policy

Fair Value Measurements (Policy)	9 Months Ended
	Sep. 30, 2019
Fair Value Measurements [Abstract]
Fair Value of Financial Instruments	The carrying amounts of the Company’s receivables and payables approximate their fair value due to their short maturities. Accounting principles provide guidance for using fair value to measure assets and liabilities. The guidance includes a three-level hierarchy of valuation techniques used to measure fair value, defined as follows: ● Unadjusted Quoted Prices — The fair value of an asset or liability is based on unadjusted quoted prices in active markets for identical assets or liabilities (Level 1). ● Pricing Models with Significant Observable Inputs — The fair value of an asset or liability is based on information derived from either an active market quoted price, which may require further adjustment based on the attributes of the financial asset or liability being measured, or an inactive market transaction (Level 2). ● Pricing Models with Significant Unobservable Inputs — The fair value of an asset or liability is primarily based on internally derived assumptions surrounding the timing and amount of expected cash flows for the financial instrument. Therefore, these assumptions are unobservable in either an active or inactive market (Level 3). The Company considers an active market as one in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing information on an ongoing basis. Conversely, the Company views an inactive market as one in which there are few transactions for the asset or liability, the prices are not current, or price quotations vary substantially either over time or among market makers. Where appropriate, non-performance risk, or that of a counterparty, is considered in determining the fair values of liabilities and assets, respectively. The Company’s money market funds are classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices. Investment debt securities are classified as Level 2 instruments based on market pricing and other observable inputs.

Cash, Cash Equivalents and In_2

Cash, Cash Equivalents and Investment Debt Securities (Tables)	9 Months Ended
	Sep. 30, 2019
Cash, Cash Equivalents, and Investment Debt Securities [Abstract]
Cash, Cash Equivalents and Investment Debt Securities	The following table summarizes the Company’s cash, cash equivalents and investment debt securities as of September 30, 2019 and December 31, 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of September 30, 2019 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ ​ ​ ​ Unrealized ​ Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value ​ ​ (in thousands) Cash and cash equivalents: ​ ​ ​ ​ Cash and money market funds ​ $ 63,377 ​ $ — ​ $ — ​ $ 63,377 Total cash and cash equivalents ​ ​ 63,377 ​ ​ — ​ ​ — ​ ​ 63,377 Investment debt securities: ​ ​ ​ ​ Commercial paper ​ 70,009 ​ 62 ​ (8) ​ 70,063 Corporate debt securities ​ 574,046 ​ 984 ​ (95) ​ 574,935 U.S. government and agency securities ​ 3,992 ​ 4 ​ — ​ 3,996 Total investment debt securities ​ 648,047 ​ 1,050 ​ (103) ​ 648,994 Total cash, cash equivalents and investment debt securities ​ $ 711,424 ​ $ 1,050 ​ $ (103) ​ $ 712,371 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2018 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ ​ ​ ​ Unrealized ​ Unrealized ​ ​ ​ ​ Amortized Cost Gains Losses Fair Value ​ ​ (in thousands) Cash and cash equivalents: ​ ​ ​ ​ Cash and money market funds ​ $ 43,248 ​ $ — ​ $ — ​ $ 43,248 Investment debt securities: ​ ​ ​ ​ Commercial paper ​ 34,353 ​ — ​ (26) ​ 34,327 Corporate debt securities ​ 349,854 ​ 27 ​ (704) ​ 349,177 U.S. government and agency securities ​ 9,410 ​ 5 ​ (7) ​ 9,408 Total investment debt securities ​ 393,617 ​ 32 ​ (737) ​ 392,912 Total cash, cash equivalents and investment debt securities ​ $ 436,865 ​ $ 32 ​ $ (737) ​ $ 436,160

Fixed Assets, Net (Tables)

Fixed Assets, Net (Tables)	9 Months Ended
	Sep. 30, 2019
Fixed Assets, Net [Abstract]
Fixed Assets, Net	Fixed assets are stated at cost and depreciated or amortized using the straight-line method based on useful lives as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Useful lives ​ ​ ​ ​ ​ ​ ​ (Years) September 30, 2019 December 31, 2018 ​ ​ ​ ​ (in thousands) Office equipment and software 3 ​ $ 4,343 ​ $ 3,986 Leasehold improvements Over life of lease ​ 10,444 ​ 14,464 Furniture and fixtures 7 ​ 3,974 ​ 3,907 Subtotal ​ ​ ​ 18,761 ​ 22,357 Less: accumulated depreciation ​ ​ ​ (12,788) ​ (11,946) Fixed assets, net ​ ​ ​ $ 5,973 ​ $ 10,411

Inventory, Net (Tables)

Inventory, Net (Tables)	9 Months Ended
	Sep. 30, 2019
Inventory, Net [Abstract]
Schedule of Inventory	Inventories are stated at the lower of cost or market. Inventories consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) Work-in-process ​ $ 9,289 ​ $ 7,019 Finished goods ​ 59 ​ 89 Inventory, net ​ $ 9,348 ​ $ 7,108

Operating Leases (Tables)

Operating Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Operating Leases [Abstract]
Schedule of operating lease assets and liabilities	Operating lease assets and liabilities are classified on the condensed consolidated balance sheet as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Leases ​ Classification ​ September 30, 2019 Assets ​ ​ ​ (in thousands) Operating lease assets ​ Other assets ​ $ 9,484 Total leased assets ​ ​ ​ $ 9,484 ​ ​ ​ ​ ​ ​ Liabilities ​ ​ ​ ​ ​ Current ​ ​ ​ ​ ​ Operating lease liabilities ​ Accounts payable, accrued expenses and other liabilities ​ $ 6,215 Noncurrent ​ ​ ​ ​ ​ Operating lease liabilities ​ Long-term other liabilities ​ ​ 6,028 Total lease liabilities ​ ​ ​ $ 12,243
Schedule of operating lease costs	Operating lease costs for the three and nine-month periods ended September 30, 2019 are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended Lease Cost ​ Classification ​ September 30, 2019 ​ September 30, 2019 ​ ​ ​ ​ (in thousands) ​ (in thousands) Operating lease cost ​ Selling, general and administrative expenses ​ $ 1,456 ​ $ 4,603 Short-term lease cost ​ Selling, general and administrative expenses ​ ​ 392 ​ ​ 1,746 Variable lease cost ​ Selling, general and administrative expenses ​ ​ 230 ​ ​ 626 Sublease income ​ Other income, net ​ ​ (182) ​ ​ (546) Net lease cost ​ ​ ​ $ 1,896 ​ $ 6,429 ​
Schedule of maturities of the company's operating lease liabilities	Maturities of the Company’s operating lease liabilities, which do not include short-term leases, as of September 30, 2019 are as follows: ​ ​ ​ ​ Maturity of Lease Liabilities ​ Operating leases ​ ​ (in thousands) 2019 ​ $ 1,889 2020 ​ ​ 6,142 2021 ​ ​ 2,828 2022 ​ ​ 896 2023 ​ ​ 896 Thereafter ​ ​ 374 Total lease payments ​ ​ 13,025 Less: Present value discount ​ ​ (782) Total operating lease liabilities ​ $ 12,243

Accounts Payable, Accrued Exp_2

Accounts Payable, Accrued Expenses and Other Liabilities (Tables)	9 Months Ended
	Sep. 30, 2019
Accounts Payable, Accrued Expenses and Other Liabilities [Abstract]
Schedule of Accounts Payable and Accrued Liabilities	Accounts payable, accrued expenses and other liabilities consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) Accounts payable ​ $ 19,485 ​ $ 11,765 Accrued employee compensation ​ 19,343 ​ 20,335 Accrued contracted services ​ 66,231 ​ 54,681 Other liabilities ​ ​ 22,021 ​ ​ 18,328 Operating lease liabilities ​ ​ 6,215 ​ ​ — Accounts payable, accrued expenses and other liabilities ​ $ 133,295 ​ $ 105,109

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Measurements [Abstract]
Fair Value, Marketable Securities Measured on Recurring and Nonrecurring Basis	Financial assets carried at fair value are classified in the tables below in one of the three categories described above: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements Using ​ Total Level 1 Level 2 Level 3 ​ ​ (in thousands) September 30, 2019 ​ ​ ​ ​ Assets ​ ​ ​ ​ Cash and cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash and cash equivalents) ​ $ 31,691 ​ $ 31,691 ​ $ — ​ $ — Available-for-sale debt securities: ​ ​ ​ ​ Commercial paper ​ 70,063 ​ — ​ 70,063 ​ — Corporate debt securities ​ 574,935 ​ — ​ 574,935 ​ — U.S. government and agency securities ​ 3,996 ​ — ​ 3,996 ​ — Total financial assets ​ $ 680,685 ​ $ 31,691 ​ $ 648,994 ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2018 ​ ​ ​ ​ Assets ​ ​ ​ ​ Money market funds (included in cash and cash equivalents) ​ $ 11,647 ​ $ 11,647 ​ $ — ​ $ — Available-for-sale debt securities: ​ ​ ​ ​ Commercial paper ​ 34,327 ​ — ​ 34,327 ​ — Corporate debt securities ​ 349,177 ​ — ​ 349,177 ​ — U.S. government and agency securities ​ 9,408 ​ — ​ 9,408 ​ — Total financial assets ​ $ 404,559 ​ $ 11,647 ​ $ 392,912 ​ $ —
Schedule of Available for Sale Securities Debt Maturities	The estimated fair value of the available-for-sale debt securities (commercial paper, corporate debt securities and U.S. government and agency securities), by contractual maturity, are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value as of ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) Due in one year or less ​ $ 486,971 ​ $ 319,717 Due after one year through two years ​ 162,023 ​ 73,195 Total investments in debt securities ​ $ 648,994 ​ $ 392,912 ​

Long-Term Debt (Tables)

Long-Term Debt (Tables)	9 Months Ended
	Sep. 30, 2019
Long-Term Debt [Abstract]
Schedule of Long-term Debt Instruments	Debt, net of discounts and deferred financing costs, consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 December 31, 2018 ​ ​ (in thousands) 2023 Convertible Notes ​ $ 460,000 ​ $ 460,000 2026 Convertible Notes ​ ​ 230,000 ​ ​ — Long-term debt, gross ​ ​ 690,000 ​ ​ 460,000 Less: Unamortized debt discounts and fees ​ ​ 164,661 ​ ​ 88,750 Long-term debt, net ​ ​ 525,339 ​ ​ 371,250

Product Revenue, Net (Tables)

Product Revenue, Net (Tables)	9 Months Ended
	Sep. 30, 2019
Product Revenue, Net [Abstract]
Product Revenues	The table below summarizes consolidated product revenue, net by region: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2019 2018 2019 2018 ​ ​ (in thousands) ​ (in thousands) Product revenue, net: ​ ​ ​ ​ U.S. ​ $ 45,232 ​ $ 36,684 ​ $ 133,914 ​ $ 99,697 ex-U.S. ​ 16,313 ​ 9,897 ​ 45,372 ​ 25,211 Total product revenue, net ​ $ 61,545 ​ $ 46,581 ​ $ 179,286 ​ $ 124,908

Stock Compensation (Tables)

Stock Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Stock Compensation [Abstract]
Schedule of Share Based Compensation Stock Options Activities	The following table summarizes stock option activity during the nine months ended September 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Average ​ ​ ​ ​ ​ Number ​ Weighted ​ Remaining ​ Aggregate ​ ​ of Options ​ Average ​ Contractual ​ Intrinsic Value ​ (in thousands) Exercise Price Term (years) (in thousands) Outstanding at December 31, 2018 1,873 ​ $ 97.63 7.5 ​ $ 45,381 Granted 484 ​ $ 105.55 — ​ $ — Exercised (69) ​ $ 36.59 — ​ $ — Cancelled/forfeited (148) ​ $ 92.40 — ​ $ — Expired (54) ​ $ 165.26 — ​ $ — Outstanding at September 30, 2019 2,086 ​ $ 97.93 7.4 ​ $ 11,747 Expected to vest 985 ​ $ 91.42 8.7 ​ $ 2,533 Exercisable 1,101 ​ $ 103.75 6.3 ​ $ 9,214
Schedule of Share Based Compensation Arrangement By Share Based Payment Award Grants in Period Fair Value Assumptions	The Company estimated the fair value of stock options granted in the periods presented utilizing a Black-Scholes option-pricing model utilizing the following assumptions: ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ 2019 2018 ​ Volatility 87.0 - 89.9 % 58.9 - 75.8 % Expected term (in years) 5.5 - 6.0 6.0 Risk-free rate 1.4 - 2.9 % 1.8 - 2.9 % Expected dividend yield — % — %
Schedule of Share-based Compensation, Restricted Stock Units Award Activity	The following table summarizes the aggregate RSU, restricted stock award (“RSA”), PRSU and performance restricted share award (“PRSA”) activity during the nine months ended September 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ Number of ​ Average Grant Date ​ Awards Fair Value ​ ​ (in thousands) ​ ​ ​ Non-vested awards at December 31, 2018 773 ​ $ 76.10 Granted 354 ​ $ 110.05 Vested ​ (261) ​ $ 78.71 Forfeited (83) ​ $ 85.67 Non-vested awards at September 30, 2019 783 ​ $ 89.57 ​
Schedule of Stock Based Compensation Expense	Stock-based compensation expense has been reported in the Company’s condensed consolidated statements of operations as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2019 2018 2019 2018 ​ ​ ​ (in thousands) (in thousands) Selling, general and administrative ​ $ 10,104 ​ $ 9,436 ​ $ 33,029 ​ $ 28,875 Research and development ​ 3,026 ​ 2,558 ​ 9,780 ​ 9,540 Total stock-based compensation ​ $ 13,130 ​ $ 11,994 ​ $ 42,809 ​ $ 38,415

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Net loss per common and potential common share:
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding for the three and nine-month periods ended of September 30, 2019 and 2018, as the inclusion thereof would have been anti-dilutive: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2019 2018 2019 2018 ​ ​ (in thousands) ​ (in thousands) Convertible Notes ​ 4,435 ​ 2,316 ​ 4,435 ​ 2,316 Options 2,086 1,997 2,086 1,997 Restricted stock units 597 447 597 447 Total 7,118 4,760 7,118 4,760

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Product revenue, net	$ 61,950	$ 46,986	$ 180,502	$ 126,524
Accounts receivable, net	32,822		32,822			$ 25,694
Deferred revenue, current	1,216		1,216			$ 1,621
Operating lease liabilities	12,243		12,243
Right of use asset	9,484		$ 9,484
Accounting Standards Update 2016-02 [Member]
Operating lease liabilities					$ 25,400
Right of use asset					$ 19,600
Stock Options [Member]
Expected term (in years)				6 years
Stock Options [Member] | Minimum [Member]
Expected term (in years)			5 years 6 months
Stock Options [Member] | Maximum [Member]
Expected term (in years)			6 years
Product [Member]
Product revenue, net	$ 61,545	$ 46,581	$ 179,286	$ 124,908

Significant Agreements (Narrati

Significant Agreements (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2014	Mar. 31, 2011	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Significant Agreements [Line Items]
License arrangement, deferred revenue, recognized						$ 400
Revenue			$ 61,950	$ 46,986	$ 180,502	126,524
License [Member]
Significant Agreements [Line Items]
Revenue			405	405	1,216	1,616
Product [Member]
Significant Agreements [Line Items]
Revenue			61,545	46,581	179,286	124,908
Dainippon Sumitomo Pharma Co Ltd [Member]
Significant Agreements [Line Items]
Development milestone reached					6,000
Intellectual Property [Member] | License [Member]
Significant Agreements [Line Items]
Revenue			400	$ 400	1,200	$ 1,600
Up-front Payment [Member] | Dainippon Sumitomo Pharma Co Ltd [Member]
Significant Agreements [Line Items]
License agreement payment		$ 15,000
Milestone Payment [Member] | Dainippon Sumitomo Pharma Co Ltd [Member]
Significant Agreements [Line Items]
License arrangement, deferred revenue, recognized					1,200		$ 2,400
Additional Development Receivable [Member] | Dainippon Sumitomo Pharma Co Ltd [Member]
Significant Agreements [Line Items]
Contingent arrangement receivable amount			$ 23,000		$ 23,000
Korea [Member] | Up-front Payment [Member] | Dainippon Sumitomo Pharma Co Ltd [Member]
Significant Agreements [Line Items]
License agreement payment	$ 1,000

Cash, Cash Equivalents and In_3

Cash, Cash Equivalents and Investment Debt Securities (Cash, Cash Equivalents and Investment Debt Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash and cash equivalents:
Cash and Cash Equivalents, Amortized Cost	$ 63,377
Cash and cash equivalents, Fair Value	63,377
Investment Debt Securities Abstract
Total investments, Amortized Cost	648,047	$ 393,617
Total cash and cash equivalents and investment debt securities, Amortized Cost	711,424	436,865
Total investments, Gross Unrealized Gains	1,050	32
Total cash and cash equivalents and investment debt securities, Gross Unrealized Gains	1,050	32
Total investments, Gross Unrealized Losses	(103)	(737)
Total cash and cash equivalents and investment debt securities, Gross Unrealized Losses	(103)	(737)
Total investments, Fair Value	648,994	392,912
Total Investments, Fair Value	648,994
Total cash and cash equivalents and investment debt securities, Fair Value	712,371	436,160
Commercial Paper [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	70,009	34,353
Total investments, Gross Unrealized Gains	62
Total investments, Gross Unrealized Losses	(8)	(26)
Total investments, Fair Value	70,063	34,327
Corporate Debt Securities [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	574,046	349,854
Total investments, Gross Unrealized Gains	984	27
Total investments, Gross Unrealized Losses	(95)	(704)
Total investments, Fair Value	574,935	349,177
US Government Agencies Debt Securities [Member]
Investment Debt Securities Abstract
Total investments, Amortized Cost	3,992	9,410
Total investments, Gross Unrealized Gains	4	5
Total investments, Gross Unrealized Losses		(7)
Total investments, Fair Value	3,996	9,408
Cash and Money Market Funds [Member]
Cash and cash equivalents:
Cash and Cash Equivalents, Amortized Cost	63,377	43,248
Cash and cash equivalents, Fair Value	$ 63,377	$ 43,248

Cash, Cash Equivalents, and Inv

Cash, Cash Equivalents, and Investment Debt Securities (Narrative) (Details)	Sep. 30, 2019security
Cash, Cash Equivalents, and Investment Debt Securities [Abstract]
Number of positions that were in a continuous unrealized loss position for more than twelve months	5

Fixed Assets, Net (Fixed Assets

Fixed Assets, Net (Fixed Assets Stated at Cost) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fixed assets	$ 18,761	$ 22,357
Less: accumulated depreciation	(12,788)	(11,946)
Fixed assets, net	5,973	10,411
Office Equipment [Member]
Fixed assets	$ 4,343	3,986
Property, Plant and Equipment, Useful Life	3 years
Leasehold Improvements [Member]
Fixed assets	$ 10,444	14,464
Property, Plant and Equipment, Estimated Useful Lives	Over life of lease
Furniture and Fixtures [Member]
Fixed assets	$ 3,974	$ 3,907
Property, Plant and Equipment, Useful Life	7 years

Inventory (Schedule of Inventor

Inventory (Schedule of Inventory) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Inventory, Net [Abstract]
Work-in-process	$ 9,289	$ 7,019
Finished goods	59	89
Inventory, net	$ 9,348	$ 7,108

Operating Leases (Details)

Operating Leases (Details)	9 Months Ended
	Sep. 30, 2019
Operating Leases [Abstract]
Operating lease, existence of option to extend	true

Operating Leases (Assets and Li

Operating Leases (Assets and Liabilities) (Details) $ in Thousands	Sep. 30, 2019USD ($)
Operating Leases [Abstract]
Operating lease assets	$ 9,484
Other assets	us-gaap:OtherAssetsNoncurrent
Total leased assets	$ 9,484
Operating lease liabilities, current	$ 6,215
Accounts payable, accrued expenses and other liabilities	us-gaap:AccountsPayableAndOtherAccruedLiabilities
Operating lease liabilities, Noncurrent	$ 6,028
Long-term other liabilities	us-gaap:LongTermDebtNoncurrent
Total lease liabilities	$ 12,243

Operating Leases (Lease Cost, T

Operating Leases (Lease Cost, Term and Discount Rate) (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2019USD ($)
Lease Cost
Net lease cost	$ 1,896	$ 6,429
Operating leases, weighted-average remaining term	2 years 6 months	2 years 6 months
Operating leases, weighted-average discount rate	5.00%	5.00%
Selling, General and Administrative Expenses [Member]
Lease Cost
Operating lease cost	$ 1,456	$ 4,603
Short-term lease cost	392	1,746
Variable lease cost	230	626
Other income, net
Lease Cost
Sublease income	$ (182)	$ (546)

Operating Leases (Maturities of

Operating Leases (Maturities of Operating Lease Liabilities and lease payments) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019USD ($)
Maturity of Lease Liabilities
2019	$ 1,889
2020	6,142
2021	2,828
2022	896
2023	896
Thereafter	374
Total lease payments	13,025
Less: Present value discount	(782)
Operating lease liabilities	12,243
Cash payments included in the measurement of the Company's lease liabilities	$ 5,500

Accounts Payable, Accrued Exp_3

Accounts Payable, Accrued Expenses and Other Liabilities (Narrative) (Details) - Foreign Tax Authority [Member] - Her Majesty's Revenue and Customs (HMRC) [Member] - USD ($) $ in Millions	1 Months Ended	24 Months Ended
	Sep. 30, 2019	Dec. 31, 2016
Percentage of credit eligible from tax authority		33.40%
Investment tax credit	$ 10.5

Accounts Payable, Accrued Exp_4

Accounts Payable, Accrued Expenses and Other Liabilities (Schedule of Accounts Payable and Accrued Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts Payable, Accrued Expenses and Other Liabilities [Abstract]
Accounts payable	$ 19,485	$ 11,765
Accrued employee compensation	19,343	20,335
Accrued contracted services	66,231	54,681
Other liabilities	22,021	18,328
Operating lease liabilities	6,215
Accounts payable, accrued expenses and other liabilities	$ 133,295	$ 105,109

Fair Value Measurements (Fair V

Fair Value Measurements (Fair Value, Marketable Securities Measured on Recurring and Nonrecurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Cash and cash equivalents, fair value disclosure	$ 63,377
Available-For-Sale Securities, Fair Value Disclosure	648,994	$ 392,912
Total financial assets	680,685	404,559
Fair Value, Inputs, Level 1 [Member]
Total financial assets	31,691	11,647
Fair Value, Inputs, Level 2 [Member]
Total financial assets	648,994	392,912
Money Market Funds [Member]
Cash and cash equivalents, fair value disclosure	31,691	11,647
Money Market Funds [Member] | Fair Value, Inputs, Level 1 [Member]
Cash and cash equivalents, fair value disclosure	31,691	11,647
Commercial Paper [Member]
Available-For-Sale Securities, Fair Value Disclosure	70,063	34,327
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member]
Available-For-Sale Securities, Fair Value Disclosure	70,063	34,327
Corporate Debt Securities [Member]
Available-For-Sale Securities, Fair Value Disclosure	574,935	349,177
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-For-Sale Securities, Fair Value Disclosure	574,935	349,177
US Government Agencies Debt Securities [Member]
Available-For-Sale Securities, Fair Value Disclosure	3,996	9,408
US Government Agencies Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Available-For-Sale Securities, Fair Value Disclosure	$ 3,996	$ 9,408

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule of Available for Sale Securities Debt Maturities) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Measurements [Abstract]
Due in one year or less	$ 486,971	$ 319,717
Due after one year through two years	162,023	73,195
Total investments in debt securities	$ 648,994	$ 392,912

Long-Term Debt (Narrative) (Det

Long-Term Debt (Narrative) (Details)	May 14, 2019USD ($)$ / shares	Jul. 06, 2016USD ($)$ / shares	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)item	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2016$ / shares
Proceeds from convertible debt					$ 223,424,000
Payments for capped call transactions and associated costs		$ 38,400,000
Cap price of capped call transaction | $ / shares								$ 262.2725
Long-term debt			$ 525,339,000		525,339,000		$ 371,250,000
Interest expense			11,795,000	$ 7,671,000	29,518,000	$ 22,769,000
Convertible Debt [Member]
Debt issuance costs		19,000,000
Convertible debt, outstanding			$ 690,000,000		$ 690,000,000
Convertible Senior Notes 3.25% [Member] | Convertible Debt [Member]
Debt instrument, face amount		460,000,000
Debt instrument, interest rate, stated percentage			3.25%		3.25%
Proceeds from convertible debt		447,600,000
Debt issuance costs		$ 12,400,000	$ 13,800,000		$ 13,800,000
Debt instrument convertible, percentage of conversion price			130.00%		130.00%
Average percentage of closing sale price of common stock					98.00%
Debt instrument, convertible, conversion ratio		5.0358
Debt instrument, convertible, conversion price | $ / shares		$ 198.58
Percentage of repurchase price is equal to principal amount of convertible notes					100.00%
Debt instrument liability component effective interest rate			8.40%		8.40%
Long-term debt			$ 334,400,000		$ 334,400,000
Debt instrument, unamortized discount, noncurrent			113,100,000		113,100,000
Interest expense			11,800,000	$ 7,700,000	29,500,000	$ 22,800,000
Interest payable, current			5,500,000		5,500,000		7,500,000
Convertible debt, outstanding			460,000,000		460,000,000
Fair value of the Convertible Notes			$ 400,700,000		$ 400,700,000		$ 410,900,000
Convertible Senior Notes 3.25% [Member] | Convertible Debt [Member] | One Hundred Thirty Percent Applicable Conversion Price [Member] | Maximum [Member]
Debt instrument convertible consecutive trading days					30
Convertible Senior Notes 3.25% [Member] | Convertible Debt [Member] | One Hundred Thirty Percent Applicable Conversion Price [Member] | Minimum [Member]
Debt instrument convertible consecutive trading days					20
Convertible Senior Notes 3.25% [Member] | Convertible Debt [Member] | Ninety Eight Percent Applicable Conversion Price [Member]
Debt instrument convertible consecutive trading days | item					5
Convertible Senior Notes 2.00% Due 2026 [Member] | Convertible Debt [Member]
Debt instrument, face amount	$ 230,000,000
Debt instrument, interest rate, stated percentage			2.00%		2.00%
Proceeds from convertible debt	223,400,000
Debt issuance costs	$ 6,600,000
Debt instrument convertible, percentage of conversion price			130.00%		130.00%
Average percentage of closing sale price of common stock					98.00%
Debt instrument, convertible, conversion ratio	9.2123
Debt instrument, convertible, conversion price | $ / shares	$ 108.55
Percentage of repurchase price is equal to principal amount of convertible notes					100.00%
Debt instrument liability component effective interest rate			9.90%		9.90%
Long-term debt			$ 137,500,000		$ 137,500,000
Debt instrument, unamortized discount, noncurrent			85,900,000		85,900,000
Convertible debt, outstanding			230,000,000		230,000,000
Underwriting discounts and estimated offering expenses			6,600,000		6,600,000
Equity issuance cost			2,500,000		2,500,000
Debt issuance costs			4,100,000		4,100,000
Fair value of the Convertible Notes			$ 213,900,000		$ 213,900,000
Convertible Senior Notes 2.00% Due 2026 [Member] | Convertible Debt [Member] | One Hundred Thirty Percent Applicable Conversion Price [Member] | Maximum [Member]
Debt instrument convertible consecutive trading days					30
Convertible Senior Notes 2.00% Due 2026 [Member] | Convertible Debt [Member] | One Hundred Thirty Percent Applicable Conversion Price [Member] | Minimum [Member]
Debt instrument convertible consecutive trading days					20
Convertible Senior Notes 2.00% Due 2026 [Member] | Convertible Debt [Member] | Ninety Eight Percent Applicable Conversion Price [Member]
Debt instrument convertible consecutive trading days | item					5

Long-Term Debt (Schedule of Lon

Long-Term Debt (Schedule of Long-term Debt Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Long-term debt, gross	$ 690,000	$ 460,000
Less: Unamortized debt discounts and fees	164,661	88,750
Long-term Debt, Total	525,339	371,250
Long-term debt outstanding	525,339	371,250
Convertible Senior Notes 3.25% [Member]
Long-term debt, gross	460,000	$ 460,000
Convertible Senior Notes 2.00% Due 2026 [Member]
Long-term debt, gross	$ 230,000

Product Revenue, Net (Narrative

Product Revenue, Net (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Product revenue, net	$ 61,950	$ 46,986	$ 180,502	$ 126,524
Product [Member]
Product revenue, net	$ 61,545	$ 46,581	$ 179,286	$ 124,908

Product Revenue, Net (Schedule

Product Revenue, Net (Schedule of Product Revenue, Net) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Product revenue, net	$ 61,950	$ 46,986	$ 180,502	$ 126,524
Product [Member]
Disaggregation of Revenue [Line Items]
Product revenue, net	61,545	46,581	179,286	124,908
Product [Member] | United States [Member]
Disaggregation of Revenue [Line Items]
Product revenue, net	45,232	36,684	133,914	99,697
Product [Member] | Non-US [Member]
Disaggregation of Revenue [Line Items]
Product revenue, net	$ 16,313	$ 9,897	$ 45,372	$ 25,211

Stockholders' Equity and Prefer

Stockholders' Equity and Preferred Stock (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	May 14, 2019	Apr. 09, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Common stock, authorized			45,000,000		45,000,000
Common stock, par value (in dollars per share)			$ 0.001		$ 0.001
Stock issued during period, shares, new issues	2,760,000	2,695,313
Shares issued, price per share	$ 83.50	$ 64
Proceeds from issuance of common stock			$ 227,180	$ 261,362
Issuance cost			$ 13,900
Private Placement [Member]
Stock issued during period, shares, new issues	119,760	1,562,500
Underwriters [Member]
Stock issued during period, shares, new issues	360,000	351,563

Stock Compensation (Narrative)

Stock Compensation (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2019	Oct. 31, 2012	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019
Granted - Number of Shares							484,000
Granted - Weighted Average Fair Value					$ 110.05
Net proceeds from exercise of stock options			$ 773	$ 2,716	$ 2,511	$ 3,201
Granted - Shares					354,000
2012 Stock Plan [Member]
Additional shares available	1,187,599	555,843
Stock Options [Member]
Share-based compensation not yet recognized			54,900		$ 54,900		$ 54,900
Share-based compensation not yet recognized, period					1 year 2 months 12 days
Restricted and Performance Stock Units and Awards [Member]
Share-based compensation not yet recognized, other than options			$ 53,400		$ 53,400		$ 53,400
Share-based compensation not yet recognized, period					1 year 4 months 24 days
Performance Stock Units and Awards [Member]
Performance period					3 years
Share based compensation expenses					$ 2,000
Performance Stock Units and Awards [Member] | Minimum [Member]
Payout percentage, as percent of target award			0.00%		0.00%		0.00%
Performance Stock Units and Awards [Member] | Maximum [Member]
Payout percentage, as percent of target award			150.00%		150.00%		150.00%
Performance Stock Units (PSUs) [Member]
Shares granted during period for stock based compensation					57,800

Stock Compensation (Schedule of

Stock Compensation (Schedule of Share Based Compensation Stock Options Activities) (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended	21 Months Ended
	Sep. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2019USD ($)$ / sharesshares
Stock Compensation [Abstract]
Beginning Outstanding, Number of Shares | shares	1,873
Granted - Number of Shares | shares			484
Exercised - Number of Shares | shares			(69)
Forfeited - Number of Shares | shares			(148)
Expired - Number of Shares | shares			(54)
Ending Outstanding, Number of Shares | shares	2,086	1,873	2,086
Expected to vest - Number of shares | shares	985		985
Exercisable - Number of Shares | shares	1,101		1,101
Beginning Outstanding, Weighted Average Exercise Price | $ / shares	$ 97.63
Granted - Weighted Average Exercise Price | $ / shares			$ 105.55
Exercised - Weighted Average Exercise Price | $ / shares			36.59
Forfeited - Weighted Average Exercise Price | $ / shares			92.40
Expired - Weighted Average Exercise Price | $ / shares			165.26
Ending Outstanding, Weighted Average Exercise Price | $ / shares	97.93	$ 97.63	97.93
Expected to vest - Weighted Average Exercise Price | $ / shares	91.42		91.42
Exercisable - Weighted Average Exercise Price | $ / shares	$ 103.75		$ 103.75
Options Outstanding - Weighted Average Remaining Life		7 years 6 months	7 years 4 months 24 days
Expected to vest - Weighted Average Remaining Term			8 years 8 months 12 days
Exercisable - Weighted Average Remaining Term			6 years 3 months 18 days
Options Outstanding - Aggregate Intrinsic Value | $	$ 11,747	$ 45,381	$ 11,747
Expected to vest - Aggregate Intrinsic Value | $	2,533		2,533
Exercisable - Aggregate Intrinsic Value | $	$ 9,214		$ 9,214

Stock Compensation (Schedule _2

Stock Compensation (Schedule of Share Based Compensation Arrangement By Share Based Payment Award Grants in Period Fair Value Assumptions) (Details) - Stock Options [Member]	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	87.00%	58.90%
Volatility, maximum	89.90%	75.80%
Expected term (in years)		6 years
Risk-free interest rate, minimum	1.40%	1.80%
Risk-free interest rate, maximum	2.90%	2.90%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 6 months
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years

Stock Compensation (Schedule _3

Stock Compensation (Schedule of Share-based Compensation, Restricted Stock Units and Award Activity) (Details) shares in Thousands	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Stock Compensation [Abstract]
Outstanding, December 31, 2018 | shares	773
Granted - Shares | shares	354
Vested - Shares | shares	(261)
Forfeited - Shares | shares	(83)
Outstanding, September 30, 2019 | shares	783
Outstanding - Weighted Average Fair Value, December 31, 2018 | $ / shares	$ 76.10
Granted - Weighted Average Fair Value | $ / shares	110.05
Vested - Weighted Average Fair Value | $ / shares	78.71
Forfeited - Weighted Average Fair Value | $ / shares	85.67
Outstanding - Weighted Average Fair Value, September 30, 2019 | $ / shares	$ 89.57

Stock Compensation (Schedule _4

Stock Compensation (Schedule of Stock Based Compensation Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated stock-based compensation	$ 13,130	$ 11,994	$ 42,809	$ 38,415
Selling, General and Administrative Expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated stock-based compensation	10,104	9,436	33,029	28,875
Research and Development Expense [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated stock-based compensation	$ 3,026	$ 2,558	$ 9,780	$ 9,540

Net Loss Per Share (Schedule of

Net Loss Per Share (Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share) (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Antidilutive securities excluded from computation of earnings per share, amount	7,118	4,760	7,118	4,760
Convertible Notes [Member]
Antidilutive securities excluded from computation of earnings per share, amount	4,435	2,316	4,435	2,316
Stock Options [Member]
Antidilutive securities excluded from computation of earnings per share, amount	2,086	1,997	2,086	1,997
Restricted Stock Units (RSUs) [Member]
Antidilutive securities excluded from computation of earnings per share, amount	597	447	597	447