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Derivative Financial Instruments - Fair Value of Derivative Instruments that are Designated and their Location on Balance Sheet (Detail) (Interest rate contracts [Member], USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Other assets [Member]
   
Derivatives, Fair Value [Line Items]    
Fair value hedge assets $ 9 $ 4
Interest taxes and other liabilities [Member]
   
Derivatives, Fair Value [Line Items]    
Fair value hedge liabilities $ 933 $ 1,134