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Concentrations of Credit Risk - Narrative (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Concentration Risk [Line Items]    
Interest bearing balances held at banks $ 4,400.0 $ 4,900.0
Federal funds sold and securities purchased under agreements to resell 1,600.0  
Credit risk to banks 4,300.0 4,300.0
Federal funds sold 0.0 5.0
Residential real estate $ 6,000.0 $ 6,000.0
Residential real estate loans as percentage of total U.S. loans 19.00% 20.00%
Legally binding undrawn commitments to extend credit $ 676.1 $ 714.2
Balance at December 31 1,596.5 $ 707.8
Florida    
Concentration Risk [Line Items]    
Residential real estate 1,600.0  
California    
Concentration Risk [Line Items]    
Residential real estate 1,300.0  
Greater Chicago Area    
Concentration Risk [Line Items]    
Residential real estate $ 894.9