Reconciliations of the beginning and ending balances for assets measured at fair value on a recurring basis (Details) (Fair Value Measurement Using Significant Unobservable Inputs, USD $)	Fair Value Measurement Using Significant Unobservable Inputs USD ($)
Fair value of common stock warrant liability at Dec. 31, 2012
Change in fair value of common stock warrants	$ (4,845,165)
Change in fair value of common stock warrants	37,101,818
Issuance of common stock warrants	2,451,028
Exercise of common stock warrants	(11,198,822)
Fair value of common stock warrant liability at Dec. 31, 2013	$ 28,829,849