Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 16,263	$ 11,118	$ 29,179	$ 21,551
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 37,771	$ 25,460	$ 72,421	$ 49,169
General and administrative	11,526	13,118	22,840	23,651
Total operating expenses	49,297	38,578	95,261	72,820
Operating loss	(33,034)	(27,460)	(66,082)	(51,269)
Interest income	641	1,777	1,883	3,646
Net loss	$ (32,393)	$ (25,683)	$ (64,199)	$ (47,623)
Net loss per share, basic and diluted	$ (0.61)	$ (0.56)	$ (1.24)	$ (1.05)
Weighted average shares outstanding, basic and diluted	53,369	45,814	51,938	45,526
Other comprehensive (loss) income:
Unrealized (loss) gain on marketable securities	$ (218)	$ 196	$ (106)	$ 283
Comprehensive loss	$ (32,611)	$ (25,487)	$ (64,305)	$ (47,340)