Fair Value Measurements - Reconciliation (Details) - Recurring basis - Significant Other Unobservable Inputs (Level 3) - Warrants - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Reconciliations of the beginning and ending balances for liabilities measured at fair value on a recurring basis using significant unobservable inputs (i.e. Level 3)
Balance at the beginning of the period	$ 9,418	$ 28,830
Change in fair value of common stock warrants	(3,661)	52,260
Issuance of common stock warrants		11,773
Exercise of common stock warrants	(22)	(83,445)
Balance at the end of the period	$ 5,735	$ 9,418