Significant Accounting Policies - Reconciliation of Common Stock Warrant Liability Measured at Fair Value on Recurring Basis (Details 5) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Change in the fair value of common stock warrant liability, net	$ (660)	$ (6,789)	$ (6,924)	$ (8,204)
Adjustment for warrants exercised/extinguished			(7,262)
Recurring basis | Fair Value Measurements Using Significant Unobservable Inputs | Common stock warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of common stock warrant liability at December 31, 2014			2,491
Issuance of common stock warrants			12,246
Change in the fair value of common stock warrant liability, net			(6,924)
Adjustment for warrants exercised/extinguished			(7,262)
Fair value of common stock warrant liability at September 30, 2015	$ 551		$ 551