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 and license revenue		$ 10,640	$ 4,582
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 53,761	$ 52,989	$ 29,007
General and administrative	22,481	12,186	5,168
Total operating expenses	76,242	65,175	34,175
Loss from operations	(76,242)	(54,535)	(29,593)
Interest income and other expense, net	2,539	3,090	517
Net loss	(73,703)	(51,445)	(29,076)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	120	(45)	(25)
Comprehensive loss	$ (73,583)	$ (51,490)	$ (29,101)
Net loss attributable to common stockholders per share, basic and diluted	$ (3.09)	$ (2.37)	$ (5.69)
Weighted-average number of common shares used to compute basic and diluted net loss per share	23,818,691	21,668,689	5,108,121