Derivative Instruments and Hedging Activities (Balance Sheet Disclosures) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-Term	$ 45	$ 65
Derivative Assets, Long-Term	59	70
Derivative Liabilities, Short-Term	173	266
Derivative Liabilities, Long-Term	796	805
Economic Hedges
Derivative, Fair Value, Net [Abstract]
Aggregate fair value of all derivative liabilities with credit-risk-related contingent features	96	216
Additional collateral, aggregate fair value	52
Collateral already posted, fair value	0	0
Commodity contracts
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-Term	45	65
Derivative Assets, Long-Term	59	70
Derivative Assets	104	135
Derivative Liabilities, Short-Term	173	266
Derivative Liabilities, Long-Term	796	805
Derivative Liabilities	969	1,071
Net Liability	865	936
Commodity contracts | Economic Hedges
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-Term	69	86
Derivative Assets, Long-Term	75	85
Derivative Assets	144	171
Derivative Liabilities, Short-Term	245	303
Derivative Liabilities, Long-Term	830	856
Derivative Liabilities	1,075	1,159
Net Liability	931	988
Commodity contracts | Netting and Collateral
Derivative, Fair Value, Net [Abstract]
Derivative Assets, Short-Term	(24)	(21)
Derivative Assets, Long-Term	(16)	(15)
Derivative Assets	(40)	(36)
Derivative Liabilities, Short-Term	(72)	(37)
Derivative Liabilities, Long-Term	(34)	(51)
Derivative Liabilities	(106)	(88)
Net Liability	$ (66)	$ (52)