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Fair Value Measurements - Reconciliations of beginning and ending balances for assets measured at fair value on recurring basis (Details 1) (USD $)
3 Months Ended
Mar. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Exercise of common stock warrants $ 483,512
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 2,131,314
Issuance of common stock warrants 2,451,028
Exercise of common stock warrants (483,512)
Fair value of common stock warrant liability at March 31, 2013 $ 4,574,655