Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Pro Forma Statement of Operations and Comprehensive Loss (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended														6 Months Ended		9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016
Revenues:
Total revenues	$ 13,416	$ 10,928	$ 9,287	$ 19,815	$ 39,448	$ 8,480	$ 39,020	$ 28,499	$ 13,846	$ 7,660	$ 7,408	$ 19,714	$ 18,029	$ 14,851	$ 21,506	$ 32,880		$ 53,446	$ 115,447	$ 48,628	$ 60,002
Operating Expenses:
Research and development	43,681	47,243	38,701	44,831	39,843	31,689	35,319	32,888	33,657	32,909	38,652	36,094	38,199	35,132	66,566	73,331		174,456	139,739	141,312	148,077
General and administrative	9,752	8,347	8,652	9,995	9,048	7,908	8,836	8,119	8,536	9,459	9,298	11,235	8,054	8,329	17,995	16,383		36,746	33,911	38,528	36,916
Restructuring charge	406	870	686	1,731	393			386	301	4,130					4,431		$ 2,347	3,693	779	4,431
Total operating expenses	53,839	56,460	48,039	56,557	49,284	39,597	44,155	41,393	42,494	46,498	47,950	47,329	46,253	43,461	88,992	89,714		214,895	174,429	184,271	184,993
Loss from operations	(40,423)	(45,532)	(38,752)	(36,742)	(9,836)	(31,117)	(5,135)	(12,894)	(28,648)	(38,838)	(40,542)	(27,615)	(28,224)	(28,610)	(67,486)	(56,834)		(161,449)	(58,982)	(135,643)	(124,991)
Investment income, net															259	111		4,227	1,146	473	325
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(2,428)	(2,546)	(2,611)	(3,046)	(3,221)	(3,385)	(3,501)	(3,575)	(3,647)	(5,018)	(4,956)	(4,972)	(5,059)	(5,143)	(8,665)	(10,202)		(10,631)	(13,682)	(18,593)	(20,130)
Interest expense on convertible senior notes	(25)	(23)	(23)	(24)	(28)	(762)	(1,125)	(1,125)	(1,099)	(1,150)					(2,249)			(95)	(3,040)	(2,387)	(138)
Other (expense) income, net															(742)	(42)		(895)	1,461	(583)	117
Net loss	$ (41,799)	$ (46,807)	$ (41,624)	$ (38,613)	$ (13,118)	$ (56,682)	$ (8,867)	$ (17,345)	$ (34,152)	$ (44,731)	$ (45,922)	$ (31,928)	$ (33,227)	$ (33,740)	$ (78,883)	$ (66,967)		$ (168,843)	$ (96,012)	$ (156,733)	$ (144,817)
Basic and diluted net loss per common share (in dollar per share)	$ (0.28)	$ (0.32)	$ (0.31)	$ (0.30)	$ (0.11)	$ (0.61)	$ (0.10)	$ (0.20)	$ (0.39)	$ (0.51)	$ (0.53)	$ (0.37)	$ (0.38)	$ (0.39)	$ (0.91)	$ (0.77)		$ (1.21)	$ (0.98)	$ (1.80)	$ (1.67)
Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
Total revenues																		$ 54,582
Operating Expenses:
Research and development																		174,456
General and administrative																		36,746
Restructuring charge																		3,693
Total operating expenses																		214,895
Loss from operations																		(160,313)
Investment income, net																		4,227
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes																		(10,631)
Interest expense on convertible senior notes																		(95)
Other (expense) income, net																		(895)
Net loss																		$ (167,707)
Basic and diluted net loss per common share (in dollar per share)																		$ (1.20)
Adjustments due to new guidance | ASU 2014-09
Revenues:
Total revenues																		$ 5,000
License and milestone fees
Revenues:
Total revenues	$ 1,747	$ 672	$ 1,321	$ 11,540	$ 29,580	$ 79	$ 31,080	$ 18,730	$ 5,076	$ 76	$ 76	$ 10,077	$ 10,692	$ 6,070	$ 5,152	$ 16,762		15,280	$ 79,469	$ 15,305	$ 26,915
License and milestone fees | Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
Total revenues																		16,765
License and milestone fees | Adjustments due to new guidance | ASU 2014-09
Operating Expenses:
Potential milestone payment	5,000																$ 5,000	5,000
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	9,281	8,441	7,242	7,190	7,587	6,503	6,439	7,613	6,710	6,184	5,944	7,380	6,291	5,684	12,894	11,975		32,154	28,142	26,218	25,299
Non-cash royalty revenue related to the sale of future royalties | Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
Total revenues																		31,805
Research and development support
Revenues:
Total revenues	218	388	388	383	452	650	902	1,478	1,427	1,354	1,335	1,059	848	772	2,781	1,620		1,377	3,482	5,175	4,014
Research and development support | Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
Total revenues																		1,377
Clinical materials revenue
Revenues:
Total revenues	$ 2,170	$ 1,427	$ 336	$ 702	$ 1,829	$ 1,248	$ 599	$ 678	$ 633	$ 46	$ 53	$ 1,198	$ 3	$ 2,325	$ 679	$ 2,328		4,635	$ 4,354	$ 1,930	$ 3,579
Clinical materials revenue | Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
Total revenues																		$ 4,635