Fair Value (Reconciliations of beginning and ending balances for assets measured at fair value on recurring basis) (Details) (Fair Value Measurements, Recurring Basis [Member], Common stock warrant liability, USD $)	9 Months Ended
	Sep. 30, 2013
Fair Value Measurements, Recurring Basis [Member] | 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	16,171,061
Issuance of common stock warrants	2,451,028
Exercise of common stock warrants	6,202,350
Fair value of common stock warrant liability at September 30, 2013	$ 12,895,564