Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Collaboration revenue	$ 50,909	$ 51,505	$ 41,259
Operating expenses:
Research and development	196,295	131,012	88,385
General and administrative	50,178	44,021	33,387
Total operating expenses	246,473	175,033	121,772
Loss from operations	(195,564)	(123,528)	(80,513)
Interest income	1,704	971	217
Other income	34	53	0
Loss before income tax (benefit) provision	(193,826)	(122,504)	(80,296)
Income tax (benefit) provision	(406)	(16,787)	36,476
Net loss	$ (193,420)	$ (105,717)	$ (116,772)
Net loss per common share:
Basic and diluted (in dollars per share)	$ (5.45)	$ (3.06)	$ (3.51)
Weighted average common shares outstanding:
Basic and diluted (in shares)	35,486	34,580	33,258