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Concentrations of Credit Risk - Narrative (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Concentration Risk [Line Items]    
Interest bearing balances held at banks $ 1,700.0 $ 1,900.0
Credit risk to banks 5,900.0 4,700.0
Federal funds sold and securities purchased under agreements to resell 2,654.1 451.0
Securities purchased under agreements to resell 2,654.1 426.0
Total Loans 41,948.3 43,390.6
Personal    
Concentration Risk [Line Items]    
Total Loans 21,517.3 23,111.8
Commercial    
Concentration Risk [Line Items]    
Total Loans 20,431.0 20,278.8
Residential Real Estate | Personal    
Concentration Risk [Line Items]    
Total Loans $ 6,077.3 6,109.9
Lower limit of generally required loan to collateral value for residential real estate loans 65.00%  
Upper limit of generally required loan to collateral value for residential real estate loans 80.00%  
Legally binding undrawn commitments to extend credit $ 515.2 $ 679.5
Residential Real Estate | Personal | Financing Receivable, Excluding Accrued Interest, Before Allowance for Credit Loss | Product Concentration Risk    
Concentration Risk [Line Items]    
Concentration risk, percentage 16.00% 15.00%
Commercial Real Estate | Commercial    
Concentration Risk [Line Items]    
Total Loans $ 5,272.2 $ 5,314.2
Commercial Real Estate | Commercial | Financing Receivable, Excluding Accrued Interest, Before Allowance for Credit Loss | Product Concentration Risk    
Concentration Risk [Line Items]    
Concentration risk, percentage 13.00% 13.00%
Commercial Real Estate | Letter of Credit    
Concentration Risk [Line Items]    
Legally binding undrawn commitments to extend credit $ 268.7 $ 376.5
Commercial Real Estate | Standby Letters of Credit    
Concentration Risk [Line Items]    
Legally binding undrawn commitments to extend credit $ 79.9 $ 81.9