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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details)
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Stock Options    
Stock-Based Compensation    
Vesting period of awards 4 years  
Assumptions used to estimate fair value of awards    
Dividend yield (as a percent) 0.00% 0.00%
Expected volatility range, minimum (as a percent) 78.10% 79.30%
Expected volatility range, maximum (as a percent) 79.30% 98.00%
Risk-free interest rate range, minimum (as a percent) 0.31% 1.50%
Risk-free interest rate range, maximum (as a percent) 1.62% 2.56%
Expected term range 5 years 3 months  
Stock Options | Minimum    
Assumptions used to estimate fair value of awards    
Expected term range   5 years 3 months
Stock Options | Maximum    
Assumptions used to estimate fair value of awards    
Expected term range   6 years 5 months 8 days
Employee Stock Purchase Plan    
Assumptions used to estimate fair value of awards    
Dividend yield (as a percent) 0.00% 0.00%
Expected volatility range, minimum (as a percent) 47.80% 64.60%
Expected volatility range, maximum (as a percent) 81.80% 165.30%
Risk-free interest rate range, minimum (as a percent) 0.16% 1.94%
Risk-free interest rate range, maximum (as a percent) 1.57% 2.63%
Employee Stock Purchase Plan | Minimum    
Assumptions used to estimate fair value of awards    
Expected term range 6 months 6 months
Employee Stock Purchase Plan | Maximum    
Assumptions used to estimate fair value of awards    
Expected term range 12 months 12 months