Net Loss Per Share - Schedule 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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (8,549)	$ (8,829)	$ (9,310)	$ (8,165)	$ (12,090)	$ (12,109)	$ (14,630)	$ (17,324)	$ (34,853)	$ (56,153)	$ (75,371)
Denominator:
Weighted-average common shares outstanding (shares)									75,365	71,251	67,858
Less: Weighted-average unvested common shares subject to repurchase or forfeiture (shares)									148	500	477
Weighted-average shares used to compute net loss per share, basic and diluted (shares)									75,217	70,751	67,381
Net loss per share, basic and diluted (dollars per shares)									$ (0.46)	$ (0.79)	$ (1.12)