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Fair Value (Details 7) (Variable interest entities, USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Variable interest entities
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Contractual principal balance $ 2,954 $ 6,619
Excess of estimated fair value over contractual principal balance 114 201
Carrying value at estimated fair value $ 3,068 $ 6,820