Net Loss per Share - Reconciliation of Numerator and Denominator Used in Computing Basic and Diluted Net Loss per Share (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net loss	$ (5,157,663)	$ (3,307,329)	$ (15,698,139)	$ (9,819,746)
Accretion of stock issuance costs	0	(19,488)	(35,046)	(71,175)
Accretion of Series D and E redeemable convertible preferred stock dividends		(837,225)	(1,165,932)	(2,405,987)
Net loss attributable to common stockholders	$ (5,157,663)	$ (4,258,935)	$ (17,026,733)	$ (12,548,801)
Denominator:
Weighted-average common shares outstanding -basic and diluted	6,829,687	96,486	3,735,852	96,446
Net loss per share attributable to common stockholders, basic and diluted	$ (0.76)	$ (44.14)	$ (4.56)	$ (130.11)
Series E Warrants
Numerator:
Accretion of Series E warrant discount	$ 0	$ (94,893)	$ (127,616)	$ (251,893)