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Derivative Instruments - Additional Information (Detail) (USD $)
Jun. 30, 2013
Dec. 31, 2012
Derivative [Line Items]    
Gross derivative liability $ 1,129,000,000 $ 1,419,000,000
Aggregate fair value of derivative instruments with credit-risk-related contingent features for which a net liability position existed, net of collateral 62,000,000 94,000,000
Five Billion Dollar Facility [Member]
   
Derivative [Line Items]    
Five billion dollar facility 5,000,000,000  
Permitted to Offset Gross Derivative Asset with Financial Institutions [Member]
   
Derivative [Line Items]    
Gross derivative liability 437,000,000 339,000,000
Securing most derivative counterparties [Member] | Five Billion Dollar Facility [Member]
   
Derivative [Line Items]    
Five billion dollar facility $ 5,000,000,000  
Commodity [Member] | Natural Gas [Member]
   
Derivative [Line Items]    
Fixed-price physical transactions 8,000,000 10,000,000
Derivative transactions 8,000,000 10,000,000