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Concentrations of Credit Risk - Narrative (Detail) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Concentration Risk [Line Items]    
Interest bearing balances held at banks $ 1,900.0 $ 1,900.0
Credit risk to banks 4,700.0 4,600.0
Federal funds sold and securities purchased under agreements to resell 800.0  
Securities purchased under agreements to resell 784.7 1,070.3
Residential real estate $ 6,327.1 $ 6,413.5
Residential real estate loans as percentage of total U.S. loans 14.00% 16.00%
Residential Real Estate    
Concentration Risk [Line Items]    
Legally binding undrawn commitments to extend credit $ 804.2 $ 783.4
Residential Real Estate | Personal    
Concentration Risk [Line Items]    
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%  
Commercial Real Estate | Letter of Credit    
Concentration Risk [Line Items]    
Legally binding undrawn commitments to extend credit $ 413.0 449.3
Commercial Real Estate | Standby Letters of Credit    
Concentration Risk [Line Items]    
Legally binding undrawn commitments to extend credit $ 82.1 $ 69.7