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Derivative Instruments- Fair Value of Derivative Contracts Falling under Early Termination Provisions in Net Liability Position (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
A3/A-
 
Credit Derivatives [Line Items]  
Potential close-out exposures (fair value) $ 824 [1]
Baa2/BBB
 
Credit Derivatives [Line Items]  
Potential close-out exposures (fair value) 993 [1]
Bal/BB+
 
Credit Derivatives [Line Items]  
Potential close-out exposures (fair value) $ 2,010 [1]
[1] The change between rating categories is incremental, not cumulative.