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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data [Abstract]
Collaboration revenue	$ 7,880	$ 7,408	$ 7,677	$ 7,469	$ 6,668	$ 7,317	$ 5,917	$ 6,215	$ 30,434	$ 26,117	$ 16,479
Operating expenses:
Research and development	19,918	23,237	23,467	22,493	21,170	17,481	15,565	13,431	89,115	67,647	31,840
General and administrative	8,708	8,270	7,805	7,406	10,213	5,711	6,369	5,732	32,189	28,025	16,798
Total operating expenses	28,626	31,507	31,272	29,899	31,383	23,192	21,934	19,163	121,304	95,672	48,638
Operating loss	(20,746)	(24,099)	(23,595)	(22,430)	(24,715)	(15,875)	(16,017)	(12,948)	(90,870)	(69,555)	(32,159)
Interest income	1,680	1,397	1,376	1,074	752	519	424	317	5,527	2,012	525
Net loss	$ (19,066)	$ (22,702)	$ (22,219)	$ (21,356)	$ (23,963)	$ (15,356)	$ (15,593)	$ (12,631)	$ (85,343)	$ (67,543)	$ (31,634)
Net loss per share, basic and diluted	$ (0.43)	$ (0.53)	$ (0.52)	$ (0.51)	$ (0.61)	$ (0.44)	$ (0.45)	$ (0.36)	$ (1.98)	$ (1.88)	$ (1.42)
Weighted average shares outstanding, basic and diluted	44,215	43,161	42,836	42,043	39,155	35,189	34,916	34,723	43,069	36,006	22,222