Net loss per share - Schedule of Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator
Net loss	$ (21,860)	$ (24,027)	$ (8,729)
Accretion for liquidation preference on preferred stock		(1,133)	(5,351)
Accretion for share issuance costs on preferred stock		(20)	(123)
Tax expense on preferred stock		(100)	(738)
Reversal of tax payable on preferred stock due to conversion of preferred stock		1,898
Extinguishment of remaining share issuance costs due to conversion of preferred stock		(439)
Total loss attributable to common stockholders	$ (21,860)	$ (23,821)	$ (14,941)
Denominator
Weighted average shares used to compute basic and diluted net loss per common share	12,637,839	8,667,387	301,745
Net loss per share attributable to common stockholders - basic and diluted	$ (1.73)	$ (2.75)	$ (49.52)