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 $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
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, Balance	$ 5,320,990
Issuance of common stock warrants		8,768,143
Change in fair value of common stock warrants	(4,845,165)	(3,447,153)
Fair value of common stock warrant liability at the ending, Balance	$ 475,825	$ 5,320,990