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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net loss	$ (6,452,342)	$ (3,684,826)	$ (10,540,477)	$ (6,512,416)
Accretion of stock issuance costs	(10,969)	(186,206)	(35,046)	(208,687)
Series E and D, preferred stock dividends	(367,350)	(828,125)	(1,165,932)	(1,568,762)
Net loss attributable to common stockholders	$ (6,870,807)	$ (4,784,874)	$ (11,869,071)	$ (8,426,959)
Denominator:
Weighted-average common shares outstanding -basic and diluted	3,973,055	96,440	2,163,295	96,426
Series E Warrants
Numerator:
Accretion of Series E warrant discount	$ 40,146	$ 85,717	$ 127,616	$ 137,094