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Derivatives and Other Financial Instruments - Schedule of Derivative Contract Assets and Liabilities that are Measured and Recognized at Fair Value on Recurring Basis (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair value asset derivatives $ 596 $ 94
Fair value liability derivatives 679 737
Level 1 [Member]
   
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair value asset derivatives 27 57
Fair value liability derivatives 15 64
Level 2 [Member]
   
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair value asset derivatives 45 47
Fair value liability derivatives 26 44
Level 3 [Member]
   
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair value asset derivatives 550 12
Fair value liability derivatives 638 630
Margin Held [Member]
   
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Margin held (26) (22)
Less Margin Posted [Member]
   
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Margin posted   $ (1)