Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 10,616	$ 7,408	$ 32,167	$ 22,554
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 27,513	$ 23,237	$ 76,682	$ 69,197
General and administrative	8,431	8,270	32,082	23,481
Total operating expenses	35,944	31,507	108,764	92,678
Operating loss	(25,328)	(24,099)	(76,597)	(70,124)
Interest income	1,694	1,397	5,340	3,847
Net loss	$ (23,634)	$ (22,702)	$ (71,257)	$ (66,277)
Net loss per share, basic and diluted	$ (0.49)	$ (0.53)	$ (1.53)	$ (1.55)
Weighted average shares outstanding, basic and diluted	48,554	43,161	46,547	42,684
Other comprehensive income:
Unrealized gain on marketable securities	$ 81		$ 364
Comprehensive loss	$ (23,553)	$ (22,702)	$ (70,893)	$ (66,277)