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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details)
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
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) 77.70% 78.10%
Expected volatility range, maximum (as a percent) 78.30% 79.30%
Risk-free interest rate range, minimum (as a percent) 0.51% 0.31%
Risk-free interest rate range, maximum (as a percent) 0.94% 1.62%
Expected term 5 years 6 months 5 years 3 months
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) 50.70% 47.80%
Expected volatility range, maximum (as a percent) 70.70% 81.80%
Risk-free interest rate range, minimum (as a percent) 0.09% 0.16%
Risk-free interest rate range, maximum (as a percent) 0.16% 1.57%
Employee Stock Purchase Plan | Minimum    
Assumptions used to estimate fair value of awards    
Expected term 6 months 6 months
Employee Stock Purchase Plan | Maximum    
Assumptions used to estimate fair value of awards    
Expected term 12 months 12 months