Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 43,103	$ 30,434	$ 26,117
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 108,413	$ 89,115	$ 67,647
General and administrative	41,058	32,189	28,025
Total operating expenses	149,471	121,304	95,672
Operating loss	(106,368)	(90,870)	(69,555)
Interest income	6,835	5,527	2,012
Net loss	$ (99,533)	$ (85,343)	$ (67,543)
Net loss per share, basic and diluted	$ (2.11)	$ (1.98)	$ (1.88)
Weighted average shares outstanding, basic and diluted	47,247	43,069	36,006
Other comprehensive income (loss):
Unrealized gain (loss) on marketable securities	$ 289	$ (28)
Comprehensive loss	$ (99,244)	$ (85,371)	$ (67,543)