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Fair Value Measurements (Summary Of Difference Between Aggregate Fair Value And Aggregate Unpaid Principal Balance For Mortgage Loans Held For Sale Measured At Fair Value) (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Aggregate Fair Value $ 950 $ 844
Aggregate Unpaid Principal 908 815
Aggregate Fair Value Less Aggregate Unpaid Principal $ 42 $ 29