Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 87,316	$ 41,283	$ 28,520	$ 20,865	$ 1,535				$ 177,984	$ 1,535
Operating expenses	75,949	65,459	50,656	65,837	39,579	$ 36,182	$ 36,188	$ 26,046	257,901	137,995	$ 61,513
Loss from operations	11,367	(24,176)	(22,136)	(44,972)	(38,044)	(36,182)	(36,188)	(26,046)	(79,917)	(136,460)	(61,513)
Other income (expense), net	914	1,042	618	429	182	(126)	409	248
Net (loss)/income	$ 12,281	$ (23,134)	$ (21,518)	$ (44,543)	$ (37,862)	$ (36,308)	$ (35,779)	$ (25,798)	$ (76,914)	$ (135,747)	$ (60,716)
Net (loss)/income per share:
basic	$ 0.26	$ (0.49)	$ (0.53)	$ (1.15)
diluted	$ 0.25	$ (0.49)	$ (0.53)	$ (1.15)
Weighted-average number of common shares:
basic	47,353,166	46,938,618	40,819,957	38,759,221
diluted	49,719,548	46,938,618	40,819,957	38,759,221
Net loss per share —basic and diluted					$ (0.99)	$ (0.96)	$ (0.95)	$ (0.70)	$ (1.77)	$ (3.60)	$ (2.29)