Condensed Statements of Comprehensive Income - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (5,157,663)	$ (3,307,329)	$ (15,698,139)	$ (9,819,746)
Other comprehensive income, net of tax effect:
Unrealized gain on short and long-term investments	12,993		4,754
Comprehensive loss	(5,144,670)	(3,307,329)	(15,693,385)	(9,819,746)
Less: Accretion of stock issuance costs		(19,488)	(35,046)	(71,175)
Less: Accretion of Series E warrant discount		(94,893)	(127,616)	(251,893)
Less: Accretion of Series D and E redeemable convertible preferred stock dividends		(837,225)	(1,165,932)	(2,405,987)
Comprehensive loss attributable to common stockholders	$ (5,144,670)	$ (4,258,935)	$ (17,021,979)	$ (12,548,801)