Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Collaboration revenue	$ 12,916	$ 10,433
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 34,650	$ 23,709
General and administrative	11,314	10,533
Total operating expenses	45,964	34,242
Operating loss	(33,048)	(23,809)
Interest income	1,242	1,869
Net loss	$ (31,806)	$ (21,940)
Net loss per share, basic and diluted	$ (0.63)	$ (0.49)
Weighted average shares outstanding, basic and diluted	50,491	45,234
Other comprehensive loss:
Unrealized gain on marketable securities	$ 112	$ 87
Comprehensive loss	$ (31,694)	$ (21,853)