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STOCK BASED COMPENSATION - Valuation assumptions (Details) - Stock Options that Contain Service, Performance and Market Conditions
9 Months Ended
Sep. 30, 2019
$ / shares
Valuation Assumptions  
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