Fair Value Measurements - Fair Value Hierarchy (Details) - Recurring - Measured at fair value - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash and cash equivalents, money market funds with less than 90 days maturity	$ 10,708	$ 14,670
Total Assets	10,708	14,670
Liabilities
Warrant liability	838	1,073
Derivative liability	3,199	3,136
Total Liabilities	4,037	4,209
Quoted prices in active markets (Level 1)
Assets
Cash and cash equivalents, money market funds with less than 90 days maturity	10,708	14,670
Total Assets	10,708	14,670
Liabilities
Warrant liability	0	0
Derivative liability	0	0
Total Liabilities	0	0
Quoted prices in inactive markets (Level 2)
Assets
Cash and cash equivalents, money market funds with less than 90 days maturity	0	0
Total Assets	0	0
Liabilities
Warrant liability	0	0
Derivative liability	0	0
Total Liabilities	0	0
Significant unobservable inputs (Level 3)
Assets
Cash and cash equivalents, money market funds with less than 90 days maturity	0	0
Total Assets	0	0
Liabilities
Warrant liability	838	1,073
Derivative liability	3,199	3,136
Total Liabilities	$ 4,037	$ 4,209