Net loss per share - Schedule of Computation of Basic and Diluted Earnings Per Share (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator
Net loss	$ (6,284,080)	$ (3,367,487)	$ (15,936,517)	$ (5,189,256)
Accretion for liquidation preference on preferred stock		(1,397,867)	(1,132,893)	(3,953,595)
Accretion for share issuance costs on preferred stock		(9,602)	(19,551)	(96,039)
Tax expense on preferred stock		54,726	(100,291)	(421,974)
Reversal of tax payable on preferred stock due to conversion of preferred stock			1,897,702
Extinguishment of remaining share issuance costs due to conversion of preferred stock			(439,227)
Total loss attributable to common stockholders	$ (6,284,080)	$ (4,720,230)	$ (15,730,777)	$ (9,660,864)
Denominator
Weighted average shares used to compute basic and diluted net loss per common share	10,695,042	301,745	7,984,052	301,745
Net loss per share attributable to common stockholders - basic and diluted	$ (0.59)	$ (15.64)	$ (1.97)	$ (32.02)