Net loss per share - Schedule of Computation of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator
Net loss	$ (5,035)	$ (6,284)	$ (16,197)	$ (15,937)
Accretion for liquidation preference on preferred stock				(1,133)
Accretion for share issuance costs on preferred stock				(20)
Tax expense on preferred stock				(100)
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	$ (5,035)	$ (6,284)	$ (16,197)	$ (15,731)
Denominator
Weighted average shares used to compute basic and diluted net loss per common share	11,841,147	10,695,042	11,096,369	7,984,052
Net loss per share attributable to common stockholders - basic and diluted	$ (0.43)	$ (0.59)	$ (1.46)	$ (1.97)