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STOCK BASED COMPENSATION - Valuation assumptions (Details) - $ / shares
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Stock options - Service conditions only    
Valuation Assumptions    
Expected term/Time to expiration (in years) 6 years 1 month 6 days 6 years
Volatility 68.77% 68.44%
Risk-free interest rate 0.59% 2.47%
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