Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 5,322	$ 1,668	$ 28,198	$ 15,721	$ 4,782	$ 3,448	$ 1,597	$ 41,678	$ 50,909	$ 51,505	$ 41,259
Research and development expenses	59,409	50,854	48,262	37,770	33,917	40,479	32,059	24,557	196,295	131,012	88,385
General and administrative expenses	12,968	12,024	10,489	14,696	12,066	10,412	11,959	9,584	50,178	44,021	33,387
Loss from operations	(67,055)	(61,210)	(30,553)	(36,745)	(41,201)	(47,443)	(42,421)	7,537	$ (195,564)	$ (123,528)	$ (80,513)
Net income (loss) attributable to common stockholders	$ (66,283)	$ (60,891)	$ (29,945)	$ (36,301)	$ (35,649)	$ (39,573)	$ (37,131)	$ 6,636
Basic and diluted (in dollars per share)	$ (1.86)	$ (1.71)	$ (0.85)	$ (1.03)					$ (5.45)	$ (3.06)	$ (3.51)
Basic earnings (loss) per common share (in dollars per share)					$ (1.02)	$ (1.14)	$ (1.08)	$ 0.19
Diluted earnings (loss) per common share (in dollars per share)					$ (1.02)	$ (1.14)	$ (1.08)	$ 0.19