Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Statement of Operations Pro-Forma Amounts (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues:
License and milestone fees	$ 11,540	$ 18,730
Non-cash royalty revenue related to the sale of future royalties	7,190	7,613
Research and development support	383	1,478
Clinical Material Revenue	702	678
Total revenues	19,815	28,499
Operating Expenses:
Research and development	44,831	32,888
General and administrative	9,995	8,119
Restructuring charge	1,731	386
Total operating expenses	56,557	41,393
Loss from operations	(36,742)	(12,894)
Investment income, net	662	115
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(3,046)	(3,575)
Interest expense on convertible senior notes	(24)	(1,125)
Other income, net	537	134
Net loss	$ (38,613)	$ (17,345)
Basic and diluted net loss per common share (in dollar per share)	$ (0.30)	$ (0.20)
Basic and diluted weighted average common shares outstanding (in shares)	130,619	87,160
Total comprehensive loss	$ (38,613)	$ (17,345)
Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
License and milestone fees	9,830
Non-cash royalty revenue related to the sale of future royalties	8,875
Research and development support	383
Clinical Material Revenue	702
Total revenues	19,790
Operating Expenses:
Research and development	44,831
General and administrative	9,995
Restructuring charge	1,731
Total operating expenses	56,557
Loss from operations	(36,767)
Investment income, net	662
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(3,046)
Interest expense on convertible senior notes	(24)
Other income, net	537
Net loss	$ (38,638)
Basic and diluted net loss per common share (in dollar per share)	$ (0.30)