Selected Quarterly Financial Data (Unaudited) (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]
Collaboration revenue	$ 4,672	$ 13,604	$ 4,197	$ 4,205	$ 125,676	$ 1,195	$ 1,648	$ 641	$ 26,678	$ 129,160	$ 0
Income (loss) from operations	(57,758)	(50,155)	(62,763)	(42,508)	74,722	(37,964)	(57,382)	(25,748)	(213,184)	(46,372)	(90,140)
Net loss	$ (54,025)	$ (46,260)	$ (58,337)	$ (38,992)	$ 77,533	$ (35,371)	$ (54,724)	$ (23,678)	$ (197,614)	$ (36,240)	$ (88,185)
Net income (loss) per share, basic (usd per share)					$ 0.82	$ (0.38)	$ (0.59)	$ (0.26)
Net income (loss) per share, diluted (usd per share)					$ 0.79	$ (0.38)	$ (0.59)	$ (0.26)
Net income (loss) per share, basic and diluted (usd per share)	$ (0.56)	$ (0.48)	$ (0.61)	$ (0.41)					$ (2.07)	$ (0.39)	$ (5.89)
Weighted average common shares outstanding (in shares)	96,078,950								95,608,208	92,621,991	14,964,144
Weighted average shares outstanding, basic (in shares)					94,299,096
Total number of shares outstanding, diluted (in shares)					97,746,224