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Fair value of financial instruments - Additional Information (Details) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Disclosure of Fair value of financial instruments [Line Items]    
Interest receivable Loans $ 7  
Unearned interest and deferred fees 0  
Lease liabilities excluded 18,700,000 $ 19,900,000
Loan Commitments    
Disclosure of Fair value of financial instruments [Line Items]    
Interest receivable Loans 27,300,000 41,700,000
Allowance for loans losses 41,100,000 99,300,000
Unearned interest and deferred fees 5,800,000 12,100,000
Securities at amortized cost    
Disclosure of Fair value of financial instruments [Line Items]    
Interest receivable Loans 1,900,000 800,000
Allowance for loans losses $ 400,000 $ 100,000