Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 11,118	$ 7,677	$ 21,551	$ 15,146
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 25,460	$ 23,467	$ 49,169	$ 45,960
General and administrative	13,118	7,805	23,651	15,211
Total operating expenses	38,578	31,272	72,820	61,171
Operating loss	(27,460)	(23,595)	(51,269)	(46,025)
Interest income	1,777	1,376	3,646	2,450
Net loss	$ (25,683)	$ (22,219)	$ (47,623)	$ (43,575)
Net loss per share, basic and diluted	$ (0.56)	$ (0.52)	$ (1.05)	$ (1.03)
Weighted average shares outstanding, basic and diluted	45,814	42,836	45,526	42,441
Other comprehensive income:
Unrealized gain on marketable securities	$ 196		$ 283
Comprehensive loss	$ (25,487)	$ (22,219)	$ (47,340)	$ (43,575)