Financial and Other Derivative Instruments Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	$ 693	$ 833
Net Amounts of Assets Presented in the Statements of Financial Position	97	126
Fair Value Gross Amounts of Recognized (Liabilities)	(787)	(936)
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	(153)	(238)
Natural Gas Commodity Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	254	396
Net Amounts of Assets Presented in the Statements of Financial Position	29	14
Fair Value Gross Amounts of Recognized (Liabilities)	(342)	(503)
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	(74)	(108)
Electricity Commodity Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	386	400
Net Amounts of Assets Presented in the Statements of Financial Position	64	109
Fair Value Gross Amounts of Recognized (Liabilities)	(394)	(398)
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	(77)	(129)
Other Commodity Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	52	37
Net Amounts of Assets Presented in the Statements of Financial Position	4	3
Fair Value Gross Amounts of Recognized (Liabilities)	(48)	(34)
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	0	0
Other Derivative Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	1	0
Net Amounts of Assets Presented in the Statements of Financial Position	0	0
Fair Value Gross Amounts of Recognized (Liabilities)	(3)	(1)
Net Amounts of (Liabilities) Presented in the Statements of Financial Position	(2)	(1)
Fair Value, Master Netting Arrangements [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	(596)	(707)
Fair Value Gross Amounts of Recognized (Liabilities)	634	698
Fair Value, Master Netting Arrangements [Member] | Natural Gas Commodity Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	(225)	(382)
Fair Value Gross Amounts of Recognized (Liabilities)	268	395
Fair Value, Master Netting Arrangements [Member] | Electricity Commodity Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	(322)	(291)
Fair Value Gross Amounts of Recognized (Liabilities)	317	269
Fair Value, Master Netting Arrangements [Member] | Other Commodity Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	(48)	(34)
Fair Value Gross Amounts of Recognized (Liabilities)	48	34
Fair Value, Master Netting Arrangements [Member] | Other Derivative Contract [Member]
Offsetting Assets [Line Items]
Fair Value, Gross Amount of Recognized Asset	(1)	0
Fair Value Gross Amounts of Recognized (Liabilities)	$ 1	$ 0