Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Collaboration and license revenue	$ 64,530	$ 1,750	$ 68,030	$ 5,250
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 51,801	$ 17,241	$ 110,636	$ 57,795
General and administrative	11,177	7,758	29,617	18,637
Total operating expenses	62,978	24,999	140,253	76,432
Income (loss) from operations	1,552	(23,249)	(72,223)	(71,182)
Non-operating income (expense):
Interest and other income, net	270	1,254	1,218	4,272
Gain on deemed sale from equity method investee			613
Share of loss from equity method investee		(357)	(613)	(1,202)
Total non-operating income, net	270	897	1,218	3,070
Net income (loss)	1,822	(22,352)	(71,005)	(68,112)
Other comprehensive income (loss)	(63)	(59)	17	160
Comprehensive income (loss)	$ 1,759	$ (22,411)	$ (70,988)	$ (67,952)
Net income (loss) per share, basic	$ 0.03	$ (0.51)	$ (1.37)	$ (1.56)
Weighted-average number of shares used to compute basic net income (loss) per share	62,599,193	43,939,281	51,852,247	43,750,154
Net income (loss) per share, diluted	$ 0.03	$ (0.51)	$ (1.37)	$ (1.56)
Weighted-average number of shares used to compute diluted net income (loss) per share	65,145,707	43,939,281	51,852,247	43,750,154