Net Loss per Share - Reconciliation of Numerator and Denominator Used in Computing Basic and Diluted Net Loss per Share (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Net loss	$ (6,851,603)	$ (6,452,342)	$ (13,878,027)	$ (10,540,477)
Accretion of stock issuance costs		(10,969)		(35,046)
Accretion of Series E warrant discount		(40,146)		(127,616)
Series E and D, Convertible Preferred Stock dividends		(367,350)		(1,165,932)
Net loss attributable to common stockholders	$ (6,851,603)	$ (6,870,807)	$ (13,878,027)	$ (11,869,071)
Denominator:
Weighted-average common shares outstanding-basic and diluted	7,018,502	3,973,055	6,952,012	2,163,295
Net loss per share attributable to common stockholders, basic and diluted	$ (0.98)	$ (1.73)	$ (2.00)	$ (5.49)
Series E Warrants
Numerator:
Accretion of Series E warrant discount	$ 0	$ (40,146)	$ 0	$ (127,616)