Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 0	$ 0	$ 209,977
Operating expenses:
Research and development	39,644	41,737	66,289
General and administrative	21,628	23,612	35,683
Total operating expenses	61,272	65,349	101,972
Income (loss) from operations	(61,272)	(65,349)	108,005
Interest income	2,151	2,389	1,522
Gain on extinguishment of Royalty Purchase Liability	0	125,000	0
Other income (expense)	151	(16)	(34)
Income (loss) before income tax benefit	(58,970)	62,024	109,493
Income tax benefit	(111)	(1,063)	(4,712)
Net income (loss)	$ (58,859)	$ 63,087	$ 114,205
Net income (loss) per common share:
Basic (in dollars per share)	$ (1.39)	$ 1.70	$ 3.18
Dilutive (in dollars per share)	$ (1.39)	$ 1.70	$ 3.17
Weighted average common shares outstanding:
Basic (in shares)	42,224	37,061	35,919
Dilutive (in shares)	42,224	37,088	36,007