Fair Value Measurements - Summary of basis used to measure certain financial assets at fair value on recurring basis (Details) (Fair Value, Measurements, Recurring, Common stock warrants, USD $)	Dec. 31, 2012	Dec. 31, 2011
Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liability	$ 475,825	$ 5,320,990
Quoted Prices in Active Markets for Identical Items (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liability
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liability
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Common stock warrant liability	$ 475,825	$ 5,320,990