Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Collaboration revenue	$ 6,445	$ 12,916
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 39,276	$ 34,650
General and administrative	13,594	11,314
Total operating expenses	52,870	45,964
Operating loss	(46,425)	(33,048)
Interest income	220	1,242
Net loss	$ (46,205)	$ (31,806)
Net loss per share, basic and diluted	$ (0.69)	$ (0.63)
Weighted average shares outstanding, basic and diluted	67,183	50,491
Other comprehensive (loss) income:
Unrealized (loss) gain on marketable securities	$ (13)	$ 112
Comprehensive loss	$ (46,218)	$ (31,694)