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STOCK-BASED COMPENSATION (Restricted Stock Activity) (Details) - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Weighted- Average Grant-Date Fair Value Per Share    
Total shares excluded from net loss per share attributable to common stockholders 7,087,528 6,101,177
Non-vested restricted stock unit awards    
Number of Restricted Share Units    
Outstanding at beginning of year (in shares) 1,921,594  
Granted (in shares) 743,803  
Vested (in shares) (116,997)  
Canceled (in shares) 0  
Weighted- Average Grant-Date Fair Value Per Share    
Outstanding at beginning of year (in dollars per share) $ 4.78  
Granted (in dollars per share) 3.25  
Vested (in dollars per share) 5.50  
Canceled (in dollars per share) 0.00  
Outstanding at end of year (in dollars per share) $ 4.30