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Netting Arrangements for Certain Financial Instruments - Information on Company's Accounting Netting Adjustments and Items Not Offset in Consolidated Balance Sheet But Available for Offset in Event of Default (Parenthetical) (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]    
Derivative assets not subject to netting arrangements $ 160 $ 260
Derivative liabilities not subject to netting arrangements $ 8 $ 5