Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Collaboration and license revenue	$ 1,750	$ 1,750	$ 3,500	$ 3,500
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 35,693	$ 24,999	$ 58,835	$ 40,553
General and administrative	11,432	5,911	18,440	10,879
Total operating expenses	47,125	30,910	77,275	51,432
Loss from operations	(45,375)	(29,160)	(73,775)	(47,932)
Non-operating income (expense):
Interest and other income, net	301	1,482	948	3,016
Gain on deemed sale from equity method investee	131		613
Share of loss from equity method investee	(131)	(412)	(613)	(844)
Total non-operating income, net	301	1,070	948	2,172
Net loss	(45,074)	(28,090)	(72,827)	(45,760)
Other comprehensive income (loss)	(144)	84	80	220
Comprehensive loss	$ (45,218)	$ (28,006)	$ (72,747)	$ (45,540)
Net loss per share, basic and diluted	$ (0.93)	$ (0.64)	$ (1.57)	$ (1.05)
Weighted-average number of shares used to compute basic and diluted net loss per share	48,556,843	43,797,718	46,419,724	43,653,325