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Letters of Credit (Detail) - USD ($)
$ in Millions
Sep. 30, 2015
Dec. 31, 2014
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Letters of credit $ 3,323 $ 3,974
Pass    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Letters of credit 2,894 3,483
Risk Level, Special Mention    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Letters of credit 107 147
Risk Level, Substandard    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Letters of credit 265 299
Risk Level, Doubtful    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Letters of credit $ 57 $ 45