Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Information (unaudited)
Collaboration revenue	$ 4,782	$ 3,448	$ 1,597	$ 41,678	$ 1,684	$ 39,575			$ 51,505	$ 41,259
Research and development expenses	33,917	40,479	32,059	24,557	22,196	17,105	$ 34,707	$ 14,377	131,012	88,385	$ 33,215
General and administrative expenses	12,066	10,412	11,959	9,584	10,656	8,812	7,570	6,349	44,021	33,387	14,210
Income (loss) from operations	(41,201)	(47,443)	(42,421)	7,537	(31,168)	13,658	(42,277)	(20,726)	(123,528)	(80,513)	(47,425)
Net income (loss) attributable to common stockholders	$ (35,649)	$ (39,573)	$ (37,131)	$ 6,636	$ (31,652)	$ 8,552	$ (72,990)	$ (20,682)	$ (105,717)	$ (116,772)	$ (57,036)
Basic earnings (loss) per common share (in dollars per share)	$ (1.02)	$ (1.14)	$ (1.08)	$ 0.19	$ (0.94)	$ 0.26	$ (2.19)	$ (0.64)
Diluted earnings (loss) per common share (in dollars per share)	$ (1.02)	$ (1.14)	$ (1.08)	$ 0.19	$ (0.94)	$ 0.25	$ (2.19)	$ (0.64)