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Fair Value Measurements - Reconciliation of Warrant Liability (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Common Stock      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Issuance of shares from cashless warrant exercises (shares) 11,584    
Warrants      
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]      
Beginning Balance $ 11,286 $ 15,216 $ 14,515
Exercise of warrants (82)   (352)
Cancellation of warrants (276)   (41)
Change in fair value of warrant liability (2,653) (3,930) 1,094
Ending Balance $ 8,275 $ 11,286 $ 15,216