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Derivative Instruments and Hedging Activities (Balance Sheet Disclosures) (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-Term $ 37 $ 65
Derivative Assets, Long-Term 74 70
Derivative Liabilities, Short-Term 128 266
Derivative Liabilities, Long-Term 983 805
Economic Hedges
    
Derivative, Fair Value, Net [Abstract]    
Aggregate fair value of all derivative liabilities with credit-risk-related contingent features 113 216
Additional collateral, aggregate fair value 25  
Collateral already posted, fair value 0 0
Commodity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-Term 37 65
Derivative Assets, Long-Term 74 70
Derivative Assets 111 135
Derivative Liabilities, Short-Term 128 266
Derivative Liabilities, Long-Term 983 805
Derivative Liabilities 1,111 1,071
Net Liability 1,000 936
Commodity contracts | Economic Hedges
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-Term 55 86
Derivative Assets, Long-Term 80 85
Derivative Assets 135 171
Derivative Liabilities, Short-Term 177 303
Derivative Liabilities, Long-Term 1,002 856
Derivative Liabilities 1,179 1,159
Net Liability 1,044 988
Commodity contracts | Netting and Collateral
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-Term (18) (21)
Derivative Assets, Long-Term (6) (15)
Derivative Assets (24) (36)
Derivative Liabilities, Short-Term (49) (37)
Derivative Liabilities, Long-Term (19) (51)
Derivative Liabilities (68) (88)
Net Liability $ (44) $ (52)