Selected Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Collaboration and license revenue							$ 10,143	$ 497		$ 10,640	$ 4,582
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses	$ 21,931	$ 17,426	$ 17,216	$ 19,669	$ 18,440	$ 13,702	$ 19,316	$ 13,717	$ 76,242	$ 65,175	$ 34,175
Other income (expense), net	433	571	739	796	846	855	826	563
Net loss	$ (21,498)	$ (16,855)	$ (16,477)	$ (18,873)	$ (17,594)	$ (12,847)	$ (8,347)	$ (12,657)	$ (73,703)	$ (51,445)	$ (29,076)
Basic and diluted net loss per common share	$ (0.89)	$ (0.70)	$ (0.69)	$ (0.81)	$ (0.76)	$ (0.56)	$ (0.36)	$ (0.72)	$ (3.09)	$ (2.37)	$ (5.69)