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Derivative Instruments- Impact of Derivative Instruments on Balance Sheet (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Abstract]    
Master netting agreements, cash collateral received $ 1,189 $ 1,452
Master netting agreements, cash collateral paid $ 156 $ 131