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Letters of Credit (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2011
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]      
Letters of credit $ 4,581 $ 4,744 $ 5,096
Pass
     
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]      
Letters of credit 4,081 4,338 4,605
Risk Level Special Mention [Member]
     
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]      
Letters of credit 220 149 193
Risk Level, Substandard
     
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]      
Letters of credit 280 254 288
Risk Level, Doubtful
     
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]      
Letters of credit   2 9
Unlikely to be Collected Financing Receivable
     
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]      
Letters of credit   $ 1 $ 1