Quarterly report pursuant to Section 13 or 15(d)

Fair Value of Warrants - Assumptions Used in Computing Fair Value of Derivative Warrants (Details)

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Fair Value of Warrants - Assumptions Used in Computing Fair Value of Derivative Warrants (Details) - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
2017 Debt            
Class of Warrant or Right [Line Items]            
Exercise price (in dollars per share) $ 2.82       $ 2.82  
Expected life (years) 6 years 8 months 23 days       7 years  
Expected volatility 76.39%       74.61%  
Risk-free interest rate 2.14%       2.22%  
Expected dividend yield 0.00%       0.00%  
2016 Offering            
Class of Warrant or Right [Line Items]            
Exercise price (in dollars per share) $ 2.25 $ 2.25 $ 2.25 $ 2.25 $ 2.25 $ 2.25
Expected life (years) 4 years 6 months 29 days 5 years 22 days 4 years 6 months 4 days 5 years 6 months 4 years 9 months 15 days 5 years 6 months
Expected volatility 76.39% 72.82% 77.11% 75.57% 76.29% 75.57%
Risk-free interest rate 1.89% 1.93% 1.80% 1.40% 1.94% 1.40%
Expected dividend yield 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%