Fair Value Measurements - Reconciliations of beginning and ending balances for assets measured at fair value on recurring basis (Details 1) (Fair Value, Measurements, Recurring, Significant Unobservable Inputs (Level 3), Common stock warrant liability, USD $)	6 Months Ended
	Jun. 30, 2013
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Common stock warrant liability
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning of period - January 1, 2013	$ 475,825
Change in fair value of common stock warrants	7,964,632
Issuance of common stock warrants	2,451,028
Exercise of common stock warrants	(6,202,350)
Fair value of common stock warrant liability at June 30, 2013	$ 4,689,135