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Regulatory capital requirements - Additional Information (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Loans held-in-portfolio $ 29,588,430 $ 27,587,856
PPPL    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Collateral loans 1,000  
SBA loans under PPP program    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Loans held-in-portfolio $ 1,300,000 $ 0
Standardized approach | Basel III    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
CET 1 minimum ratio required ratio 6.50% 6.50%
Tier one risk based capital required for capital adequacy ratio 8.00% 8.00%
Capital required for capital adequacy ratio 10.00% 10.00%
Leverage ratio required 5.00% 5.00%