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STOCK BASED COMPENSATION - Valuation assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Valuation Assumptions    
Dividend yield (as a percent) 0.00%  
Stock options - Service conditions only    
Valuation Assumptions    
Expected term/Time to expiration (in years) 5 years 8 months 12 days 5 years 10 months 24 days
Volatility 72.54% 71.78%
Risk-free interest rate 2.17% 2.71%
Dividend yield (as a percent) 0.00% 0.00%
Stock options - Service, performance and market conditions    
Valuation Assumptions    
Expected term/Time to expiration (in years) 10 years  
Volatility 68.54%  
Risk-free interest rate 2.64%  
Dividend yield (as a percent) 0.00%  
Cost of equity 24.00%  
Fair value of underlying common stock (as of valuation date) $ 5.85