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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net loss	$ (6,488,442)	$ (5,157,663)	$ (20,366,469)	$ (15,698,139)
Accretion of stock issuance costs				(35,046)
Accretion of Series E warrant discount				(127,616)
Series E and D, Convertible Preferred Stock dividends				(1,165,932)
Net loss attributable to common stockholders	$ (6,488,442)	$ (5,157,663)	$ (20,366,469)	$ (17,026,733)
Denominator:
Weighted-average common shares outstanding-basic and diluted	7,053,010	6,829,687	6,985,924	3,735,852
Net loss per share attributable to common stockholders, basic and diluted	$ (0.92)	$ (0.76)	$ (2.92)	$ (4.56)
Series E Warrants
Numerator:
Accretion of Series E warrant discount	$ 0	$ 0	$ 0	$ (127,616)