Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilites) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 833	$ 1,016
Derivative Asset, Fair Value, Gross Liability	(707)	(869)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	126	147
Derivative Liability, Fair Value, Gross Liability	(936)	(1,020)
Derivative Liability, Fair Value, Gross Asset	698	869
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(238)	(151)
Natural Gas Commodity Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	396	645
Derivative Asset, Fair Value, Gross Liability	(382)	(605)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	14	40
Derivative Liability, Fair Value, Gross Liability	(503)	(661)
Derivative Liability, Fair Value, Gross Asset	395	605
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(108)	(56)
Electricity Commodity Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	400	360
Derivative Asset, Fair Value, Gross Liability	(291)	(258)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	109	102
Derivative Liability, Fair Value, Gross Liability	(398)	(351)
Derivative Liability, Fair Value, Gross Asset	269	258
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(129)	(93)
Other Commodity Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Liability, Fair Value, Gross Liability	(34)	(7)
Derivative Liability, Fair Value, Gross Asset	34	6
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	(1)
Other Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	37	11
Derivative Asset, Fair Value, Gross Liability	(34)	(6)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	3	5
Derivative Liability, Fair Value, Gross Liability	(1)	(1)
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ (1)	$ (1)