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Stock Based Compensation - Stock Options Awards (Details) - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Granted in period (in shares) 0            
Employee Stock Option              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Expiration period 10 years            
Award vesting period 3 years            
Book value target (in dollars per share)     $ 11.52 $ 10.79 $ 9.97 $ 10.14 $ 8.96
Valuation Input              
Historical volatility, assumption   27.60%          
Risk-free rate   1.51%          
Dividend yield   2.20%          
Expected term (years)   7 years          
Vesting - first tranche | Employee Stock Option              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award vesting period 3 years            
Vesting - second tranche | Employee Stock Option              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award vesting period 4 years            
Vesting - third tranche | Employee Stock Option              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Award vesting period 5 years