Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	[1]	$ 289,590	$ 3,124	$ 40,997
Operating expenses:
Research and development	[2]	179,362	113,773	69,800
General and administrative		63,488	48,294	35,845
Total operating expenses		242,850	162,067	105,645
Income (loss) from operations		46,740	(158,943)	(64,648)
Other income (expense):
Loss from equity method investment		(5,467)	(4,275)	(1,763)
Other income (expense), net		26,033	(1,210)	(197)
Total other income (expense), net		20,566	(5,485)	(1,960)
Net income (loss) before income taxes		67,306	(164,428)	(66,608)
Provision for income taxes		(448)	(553)	(1,749)
Net income (loss)		66,858	(164,981)	(68,357)
Foreign currency translation adjustment		15	(22)	40
Comprehensive income (loss)		$ 66,873	$ (165,003)	$ (68,317)
Net income (loss) per share attributable to common shareholders— basic		$ 1.23	$ (3.44)	$ (1.71)
Weighted-average common shares outstanding used in net income (loss) per share attributable to common shareholders—basic		54,392,304	47,964,368	40,057,365
Net income (loss) per share attributable to common shareholders— diluted		$ 1.17	$ (3.44)	$ (1.71)
Weighted-average common shares outstanding used in net income (loss) per share attributable to common shareholders—diluted		56,932,798	47,964,368	40,057,365

[1] Including the following amounts of revenue from a related party, see Note 7

[2] Including the following amounts of research and development from a related party, see Note 7