Selected Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 1,696	$ 125	$ 2,375	$ 393	$ 4,441	$ 1,140	$ 1,363	$ 644	$ 4,589	$ 7,588	$ 6,120
Research and development expense	16,059	12,560	10,680	6,183	4,812	5,664	4,012	2,791	45,482	17,279	4,961
General and administrative expense	5,742	6,200	6,169	5,479	4,232	2,567	3,397	1,716	23,590	11,912	3,851
Total operating expenses	22,165	18,793	17,327	11,644	9,942	8,487	7,659	4,637	69,929	30,725	9,802
Net loss	$ (19,608)	$ (18,154)	$ (14,437)	$ (10,768)	$ (5,178)	$ (7,332)	$ (6,290)	$ (4,011)	$ (62,967)	$ (22,811)	$ (4,003)
Basic and diluted net loss per common share	$ (0.74)	$ (0.69)	$ (0.55)	$ (0.41)	$ (0.20)	$ (1.52)	$ (3.24)	$ (0.94)	$ (2.38)	$ (2.59)	$ (1.82)