Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Collaboration and license revenue	$ 15,000	$ 8,353
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 78,481	$ 49,646
General and administrative	25,228	13,566
Total operating expenses	103,709	63,212
Loss from operations	(88,709)	(54,859)
Non-operating income (expense):
Interest and other income, net	5,201	4,922
Gain on deemed sale from equity method investee	0	1,229
Share of loss from equity method investee	(1,202)	(886)
Total non-operating income, net	3,999	5,265
Net loss	(84,710)	(49,594)
Other comprehensive income (loss)	171	(65)
Comprehensive loss	$ (84,539)	$ (49,659)
Net loss per share, basic and diluted	$ (1.93)	$ (1.43)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,825,991	34,618,237