Financial and Other Derivative Instruments Financial and Other Derivative Instruments (Netting Offsets of Derivative Assets and Liabilities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 926	$ 833
Gross Amounts Offset in the Statements of Financial Position	(830)	(707)
Net Amounts of Assets Presented in the Statements of Financial Position	96	126
Fair Value Gross Amounts of Recognized Liabilities	(984)	(936)
Gross Amounts Offset in the Statement of Financial Position	828	698
Net Amounts of Liabilities Presented in the Statements of Financial Position	(156)	(238)
Natural Gas Commodity Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	383	396
Gross Amounts Offset in the Statements of Financial Position	(367)	(382)
Net Amounts of Assets Presented in the Statements of Financial Position	16	14
Fair Value Gross Amounts of Recognized Liabilities	(416)	(503)
Gross Amounts Offset in the Statement of Financial Position	369	395
Net Amounts of Liabilities Presented in the Statements of Financial Position	(47)	(108)
Electricity Commodity Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	503	400
Gross Amounts Offset in the Statements of Financial Position	(422)	(291)
Net Amounts of Assets Presented in the Statements of Financial Position	81	109
Fair Value Gross Amounts of Recognized Liabilities	(524)	(398)
Gross Amounts Offset in the Statement of Financial Position	417	269
Net Amounts of Liabilities Presented in the Statements of Financial Position	(107)	(129)
Other Commodity Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	40	37
Gross Amounts Offset in the Statements of Financial Position	(41)	(34)
Net Amounts of Assets Presented in the Statements of Financial Position	(1)	3
Fair Value Gross Amounts of Recognized Liabilities	(40)	(34)
Gross Amounts Offset in the Statement of Financial Position	41	34
Net Amounts of Liabilities Presented in the Statements of Financial Position	1	0
Other Derivative Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Liabilities	(4)	(1)
Gross Amounts Offset in the Statement of Financial Position	1	0
Net Amounts of Liabilities Presented in the Statements of Financial Position	$ (3)	$ (1)