Unaudited Quarterly Results - Schedule of Results of Operations on Quarterly Basis (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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 Data [Abstract]
Collaboration revenue	$ 10,936	$ 10,616	$ 11,118	$ 10,433	$ 7,880	$ 7,408	$ 7,677	$ 7,469	$ 43,103	$ 30,434	$ 26,117
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:
Research and development	$ 31,731	$ 27,513	$ 25,460	$ 23,709	$ 19,918	$ 23,237	$ 23,467	$ 22,493	$ 108,413	$ 89,115	$ 67,647
General and administrative	8,976	8,431	13,118	10,533	8,708	8,270	7,805	7,406	41,058	32,189	28,025
Total operating expenses	40,707	35,944	38,578	34,242	28,626	31,507	31,272	29,899	149,471	121,304	95,672
Operating loss	(29,771)	(25,328)	(27,460)	(23,809)	(20,746)	(24,099)	(23,595)	(22,430)	(106,368)	(90,870)	(69,555)
Interest income	1,495	1,694	1,777	1,869	1,680	1,397	1,376	1,074	6,835	5,527	2,012
Net loss	$ (28,276)	$ (23,634)	$ (25,683)	$ (21,940)	$ (19,066)	$ (22,702)	$ (22,219)	$ (21,356)	$ (99,533)	$ (85,343)	$ (67,543)
Net loss per share, basic and diluted	$ (0.57)	$ (0.49)	$ (0.56)	$ (0.49)	$ (0.43)	$ (0.53)	$ (0.52)	$ (0.51)	$ (2.11)	$ (1.98)	$ (1.88)
Weighted average shares outstanding, basic and diluted	49,350	48,554	45,814	45,234	44,215	43,161	42,836	42,043	47,247	43,069	36,006