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Carrying Amounts and Estimated Fair Values for Certain Financial Instruments (Parenthetical) (Detail) (Residential Mortgage Loans, USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Residential Mortgage Loans
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Loans measured at fair value on a recurring basis $ 83 $ 76