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Loans and Allowance for Loan Losses - Summary of Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment and Based on Impairment Method (Parenthetical) (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]      
Allowances for loan losses $ 7,631 $ 6,361 $ 4,134
Paycheck Protection Program Loans      
Financing Receivable Allowance For Credit Losses [Line Items]      
Allowances for loan losses $ 0 $ 0