Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 10,433	$ 7,469
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 23,709	$ 22,493
General and administrative	10,533	7,406
Total operating expenses	34,242	29,899
Operating loss	(23,809)	(22,430)
Interest income	1,869	1,074
Net loss	$ (21,940)	$ (21,356)
Net loss per share, basic and diluted	$ (0.49)	$ (0.51)
Weighted average shares outstanding, basic and diluted	45,234,326	42,042,586
Other comprehensive loss:
Unrealized gain on marketable securities	$ 87
Comprehensive loss	$ (21,853)	$ (21,356)