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STOCK-BASED COMPENSATION - NARRATIVE (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Jun. 30, 2021
Dec. 31, 2017
2017 LTIP        
STOCK-BASED COMPENSATION        
Shares reserved for future issuance (in shares)       2,467,430
2017 LTIP | Performance stock units        
STOCK-BASED COMPENSATION        
Distribution of shares to recipients (in shares)   0    
2017 LTIP | Performance stock units | Minimum        
STOCK-BASED COMPENSATION        
Ratio at which award holders get common stock of the company 0      
2017 LTIP | Performance stock units | Maximum        
STOCK-BASED COMPENSATION        
Ratio at which award holders get common stock of the company 2      
2021 LTIP        
STOCK-BASED COMPENSATION        
Shares reserved for future issuance (in shares)     700,000  
LTIP        
STOCK-BASED COMPENSATION        
Granted (in shares) 0      
Exercised in period, aggregate intrinsic value $ 0.2      
LTIP | Restricted stock units        
STOCK-BASED COMPENSATION        
Number of shares released upon vesting (in shares) 1      
Vesting period 3 years      
Granted (in shares) 178,567      
Granted, fair value $ 6.1      
LTIP | Restricted stock units | 2019        
STOCK-BASED COMPENSATION        
Vesting percent of shares 33.00%      
LTIP | Restricted stock units | 2020        
STOCK-BASED COMPENSATION        
Vesting percent of shares 33.00%      
LTIP | Restricted stock units | 2021        
STOCK-BASED COMPENSATION        
Vesting percent of shares 33.00%      
LTIP | Performance stock units        
STOCK-BASED COMPENSATION        
Vesting period 3 years      
Granted (in shares) 64,258      
Granted, fair value $ 4.4      
Number of trading days 30 days      
LTIP | Performance stock units | Minimum        
STOCK-BASED COMPENSATION        
Ratio at which award holders get common stock of the company 0      
LTIP | Performance stock units | Maximum        
STOCK-BASED COMPENSATION        
Ratio at which award holders get common stock of the company 2      
LTIP | Stock options        
STOCK-BASED COMPENSATION        
Expiration period 10 years      
Granted (in shares) 0