Unaudited Selected Quarterly Financial Data - Selected Quarterly Financial Data (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 Data [Abstract]
Revenue	$ 3,517	$ 3,743	$ 3,157	$ 3,608	$ 5,167	$ 4,743	$ 4,441	$ 4,773	$ 14,025	$ 19,124	$ 19,401
Net loss	$ (8,811)	$ (8,832)	$ (9,014)	$ (7,852)	$ (5,845)	$ (6,487)	$ (5,478)	$ (4,853)	$ (34,509)	$ (22,663)	$ (22,110)
Basic net loss per share	$ (0.06)	$ (0.06)	$ (0.07)	$ (0.06)	$ (0.05)	$ (0.05)	$ (0.05)	$ (0.04)
Diluted net loss per share	$ (0.06)	$ (0.06)	$ (0.07)	$ (0.06)	$ (0.05)	$ (0.05)	$ (0.05)	$ (0.04)	$ (0.26)	$ (0.19)	$ (0.20)