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Warrants - Calculated Aggregated Fair Values and Assumptions (Details) - Warrant Liability - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Class of Warrant or Right [Line Items]    
Calculated aggregate value $ 1,073 $ 6,034
Weighted average exercise price per share of warrant (usd per share) $ 6.02 $ 5.98
Closing price per share of common stock (usd per share) $ 0.64 $ 1.89
Volatility 92.20% 85.60%
Contractual remaining term 3 years 4 months 4 years 3 months
Risk-free interest rate 2.00% 1.75%
Dividend yield 0.00% 0.00%