Basic and Diluted Net Loss Per Share - Computation of Company's Basic and Diluted Net Loss Per Share of Common Stock (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator
Net loss	$ (6,089)	$ (5,712)	$ (7,031)	$ (7,309)	$ (6,946)	$ (7,979)	$ (7,771)	$ (6,613)	$ (26,141)	$ (29,309)	$ (32,099)
Denominator
Weighted-average common shares for basic and diluted net loss per share									76,281	72,994	70,069
Basic and diluted net loss per share	$ (0.08)	$ (0.07)	$ (0.09)	$ (0.10)	$ (0.09)	$ (0.11)	$ (0.11)	$ (0.09)	$ (0.34)	$ (0.40)	$ (0.46)