Concentrations of Credit Risk - Additional Information (Detail) - USD ($) $ in Billions	Dec. 31, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Credit risk to banks	$ 15.8	$ 16.0
Credit risk to banks - non-interest bearing balances held at banks	2.7	1.1
Residential Real Estate	$ 8.9	$ 9.8
Residential real estate loans as percentage of total U.S. loans	28.00%	32.00%
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	75.00%
Legally binding undrawn commitments to extend credit	$ 1.4	$ 1.6
Florida
Concentration Risk [Line Items]
Residential Real Estate	2.3
Greater Chicago Area
Concentration Risk [Line Items]
Residential Real Estate	2.0
California
Concentration Risk [Line Items]
Residential Real Estate	$ 1.5