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CONVERTIBLE SENIOR NOTES - Embedded Derivatives (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Embedded derivatives      
Embedded derivatives      
Share price (in dollars per share) $ 0.33 $ 0.25 $ 0.20
Risk-Free Interest Rate (as a percent) 1.89% 1.47% 1.54%
Estimated Stock Volatility (as a percent) 40.00% 40.00% 40.00%
Default Intensity (as a percent) 2.00% 2.00% 2.00%
Recovery Rate (as a percent) 30.00% 30.00% 30.00%
Mandatory Redemption, Probability of Event (as a percent) 80.00%    
Conversion Option, Probability of Event (as a percent) 10.00%    
Note Reaches Maturity, Probability of Event (as a percent) 10.00%    
Convertible Senior Notes.      
Embedded derivatives      
Carrying value of Notes $ 5,700,000 $ 5,500,000  
Unamortized debt discount 200,000 400,000  
Fair value of notes payable 6,700,000 7,100,000  
Convertible Senior Notes. | Embedded derivatives      
Embedded derivatives      
Fair value of embedded derivatives 57,000 100,000  
Net gain on changes in fair value of embedded derivatives 100,000    
Non-convertible Promissory Note      
Embedded derivatives      
Carrying value of Notes 1,300,000 1,300,000  
Fair value of notes payable $ 1,000,000 $ 1,000,000