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Letters of Credit (Detail) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Line Of Credit $ 3,055 $ 3,974
Pass    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Line Of Credit 2,606 3,483
Special Mention    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Line Of Credit 130 147
Substandard    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Line Of Credit 258 299
Doubtful    
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]    
Line Of Credit $ 61 $ 45