Fair Value Measurements - Reconciliation of Warrant Liability (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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	$ 1,073	$ 6,034
Exercise of warrants		(41)
Change in fair value of liability	(921)	(4,920)
Ending Balance	152	1,073
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	3,136	1,735
Change in fair value of liability	(1,662)	1,407
Issuance of convertible notes		(6)
Ending Balance	$ 1,474	$ 3,136