Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Collaboration revenue - related party	$ 15,360	$ 59,417	$ 12,826
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 52,118	$ 52,492	$ 47,783
General and administrative	20,608	16,076	11,033
Total operating expenses	72,726	68,568	58,816
Loss from operations	(57,366)	(9,151)	(45,990)
Interest and other income, net	2,577	2,554	613
Net loss	(54,789)	(6,597)	(45,377)
Accretion of redeemable convertible preferred stock to redemption value		(11)	(40)
Net loss attributable to common stockholders	$ (54,789)	$ (6,608)	$ (45,417)
Net loss per share attributable to common stockholders—basic and diluted	$ (1.97)	$ (0.33)	$ (18.35)
Weighted average common shares outstanding—basic and diluted	27,854,912	19,990,773	2,474,800
Comprehensive loss:
Net loss	$ (54,789)	$ (6,597)	$ (45,377)
Other comprehensive loss:
Unrealized gain on marketable securities, net of tax	222	127	107
Comprehensive loss	$ (54,567)	$ (6,470)	$ (45,270)