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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 $)
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Fair Value Measurement Using Significant Unobservable Inputs
USD ($)
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| 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 |