Net Loss Per Share - Reconciliation of Numerator and Denominator Used in Computing Basic and Diluted Net Loss per Share (Detail) - USD ($) 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	$ (8,816)	$ (9,467)	$ (12,404)	$ (14,894)	$ (12,380)	$ (12,148)	$ (14,088)	$ (11,422)	$ (45,581)	$ (50,038)	$ (29,281)
Accretion of mandatorily redeemable convertible preferred stock									0	0	(4,653)
Net loss attributable to common stockholders									$ (45,581)	$ (50,038)	$ (33,934)
Denominator:
Weighted-average common shares outstanding-basic and diluted									19,027	17,839	7,643