Fair Value Measurements - Reconciliation of Warrant Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
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	$ 8,275	$ 11,286
Exercise of warrants		(82)
Cancellation of warrants		(276)
Issuance of warrants	9,643
Expiration of warrants	(1,910)
Change in fair value of warrant liability	(9,974)	(2,653)
Ending Balance	6,034	8,275
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0
Issuance of convertible notes	1,273
Change in fair value of warrant liability	462
Ending Balance	$ 1,735	$ 0