Selected Quarterly Financial Data (Unaudited) - Summary (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
License and collaboration revenues	$ 526	$ 988	$ 577	$ 1,244	$ 3,459	$ 0	$ 0	$ 9,467	$ 3,335	$ 12,926	$ 0
Loss from operations	(74,646)	(57,059)	(47,641)	(42,758)	(29,672)	(28,939)	(28,670)	(15,346)	(222,104)	(102,627)	(71,535)
Net loss	$ (72,376)	$ (54,273)	$ (45,695)	$ (40,912)	$ (28,272)	$ (27,568)	$ (27,869)	$ (14,709)	$ (213,256)	$ (98,418)	$ (70,430)
Loss per share, basic and diluted (USD per share)	$ (1.83)	$ (1.38)	$ (1.26)	$ (1.17)	$ (0.87)	$ (0.85)	$ (0.94)	$ (0.51)	$ (5.69)	$ (3.19)	$ (2.78)