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
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
License and milestone fees	$ 1,321	$ 31,080	$ 12,861	$ 49,810
Non-cash royalty revenue related to the sale of future royalties	7,242	6,439	14,432	14,052
Research and development support	388	902	771	2,380
Clinical Material Revenue	336	599	1,038	1,277
Total revenues	9,287	39,020	29,102	67,519
Operating Expenses:
Research and development	38,701	35,319	83,532	68,207
General and administrative	8,652	8,836	18,647	16,955
Restructuring charge	686		2,417	386
Total operating expenses	48,039	44,155	104,596	85,548
Loss from operations	(38,752)	(5,135)	(75,494)	(18,029)
Investment income, net	814	143	1,476	258
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(2,611)	(3,501)	(5,657)	(7,076)
Interest expense on convertible senior notes	(23)	(1,125)	(47)	(2,250)
Other (expense) income, net	(1,052)	751	(515)	885
Net loss	$ (41,624)	$ (8,867)	$ (80,237)	$ (26,212)
Basic and diluted net loss per common share (in dollar per share)	$ (0.31)	$ (0.10)	$ (0.61)	$ (0.30)
Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
License and milestone fees	$ 5,330		$ 15,159
Non-cash royalty revenue related to the sale of future royalties	7,222		16,096
Research and development support	388		771
Clinical Material Revenue	336		1,038
Total revenues	13,276		33,064
Operating Expenses:
Research and development	38,701		83,532
General and administrative	8,652		18,647
Restructuring charge	686		2,417
Total operating expenses	48,039		104,596
Loss from operations	(34,763)		(71,532)
Investment income, net	814		1,476
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(2,611)		(5,657)
Interest expense on convertible senior notes	(23)		(47)
Other (expense) income, net	(1,052)		(515)
Net loss	$ (37,635)		$ (76,275)
Basic and diluted net loss per common share (in dollar per share)	$ (0.28)		$ (0.58)
Adjustments due to new guidance | ASU 2014-09
Operating Expenses:
Potential milestone payment	$ 5,000		$ 5,000