Fair Value Measurements - Reconciliation of Warrant Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Common Stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Issuance of shares from cashless warrant exercises (shares)	6,941
Issuance of shares from conversion of convertible notes (in shares)	24,911
Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 6,034	$ 8,275
Issuance of warrants		9,643
Expiration of warrants		(1,910)
Change in fair value of warrant liability	(4,920)	(9,974)
Change in fair value of derivative liability	4,920	9,974
Exercise of warrants	(41)
Ending Balance	1,073	6,034
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1,735	0
Change in fair value of warrant liability	(1,407)	(462)
Change in fair value of derivative liability	1,407	462
Issuance of convertible notes		1,273
Derivative liability to equity upon note conversion	(6)
Ending Balance	$ 3,136	$ 1,735