Selected Quarterly Financial Data (Unaudited) - Summary (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
License and collaboration revenues	$ 1,707	$ 11,424	$ 0	$ 267	$ 526	$ 988	$ 577	$ 1,244	$ 13,398	$ 3,335	$ 12,926
Loss from operations	(106,336)	(88,678)	(84,862)	(89,487)	(74,646)	(57,059)	(47,641)	(42,758)	(369,363)	(222,104)	(102,627)
Net loss	$ (101,087)	$ (87,336)	$ (82,859)	$ (86,655)	$ (72,376)	$ (54,273)	$ (45,695)	$ (40,912)	$ (357,937)	$ (213,256)	$ (98,418)
Basic and diluted net loss per share (USD per share)	$ (2.08)	$ (1.96)	$ (1.89)	$ (2.02)	$ (1.83)	$ (1.38)	$ (1.26)	$ (1.17)	$ (7.96)	$ (5.69)	$ (3.19)