Summary of Significant Accounting Policies - Summary of Numerators and Denominators in Calculation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Numerators:
Net loss	$ (8,811)	$ (8,832)	$ (9,014)	$ (7,852)	$ (5,845)	$ (6,487)	$ (5,478)	$ (4,853)	$ (34,509)	$ (22,663)	$ (22,110)
Denominators:
Weighted average shares used to compute basic net loss per share									133,163	118,523	111,666
Effect of dilutive securities:
Dilution from stock options									0	0	0
Dilution from ESPP									0	0	0
Dilutive common shares									0	0	0
Weighted average shares used to compute basic net loss per share									133,163	118,523	111,666
Net loss per share:
Basic									$ (0.26)	$ (0.19)	$ (0.20)
Diluted	$ (0.06)	$ (0.06)	$ (0.07)	$ (0.06)	$ (0.05)	$ (0.05)	$ (0.05)	$ (0.04)	$ (0.26)	$ (0.19)	$ (0.20)