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Derivatives and Other Financial Instruments - Schedule of Derivative Contract Assets and Liabilities Measured and Recognized at Fair Value on Recurring Basis (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2012
Assets:      
Fair value asset derivatives $ 249 $ 390 $ 596
Liabilities:      
Margin posted (6) (18)  
Fair value liability derivatives 446 479  
Level 1 [Member]
      
Assets:      
Fair value asset derivatives 8 6  
Liabilities:      
Fair value liability derivatives 15 66  
Level 2 [Member]
      
Assets:      
Fair value asset derivatives 27 32  
Liabilities:      
Fair value liability derivatives        
Level 3 [Member]
      
Assets:      
Fair value asset derivatives 217 355  
Liabilities:      
Fair value liability derivatives 437 431  
Margin Held [Member]
      
Assets:      
Margin held (3) (3)  
Margin Posted [Member]
      
Liabilities:      
Margin posted $ (6) $ (18)