Financial and Other Derivative Instruments Financial and Other Derivative Instruments (Details 1) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	$ 870	$ 1,016
Gross Amounts Offset in the Statements of Financial Position	(797)	(869)
Net Amounts of Assets Presented in the Statements of Financial Position	73	147
Fair Value Gross Amounts of Recognized Liabilities	(944)	(1,020)
Gross Amounts Offset in the Statement of Financial Position	849	869
Net Amounts of Liabilities Presented in the Statements of Financial Position	(95)	(151)
Natural Gas [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	539	645
Gross Amounts Offset in the Statements of Financial Position	(531)	(605)
Net Amounts of Assets Presented in the Statements of Financial Position	8	40
Fair Value Gross Amounts of Recognized Liabilities	(617)	(661)
Gross Amounts Offset in the Statement of Financial Position	577	605
Net Amounts of Liabilities Presented in the Statements of Financial Position	(40)	(56)
Electricity [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	308	360
Gross Amounts Offset in the Statements of Financial Position	(247)	(258)
Net Amounts of Assets Presented in the Statements of Financial Position	61	102
Fair Value Gross Amounts of Recognized Liabilities	(307)	(351)
Gross Amounts Offset in the Statement of Financial Position	254	258
Net Amounts of Liabilities Presented in the Statements of Financial Position	(53)	(93)
Other Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	23	11
Gross Amounts Offset in the Statements of Financial Position	(19)	(6)
Net Amounts of Assets Presented in the Statements of Financial Position	4	5
Fair Value Gross Amounts of Recognized Liabilities	(19)	(7)
Gross Amounts Offset in the Statement of Financial Position	18	6
Net Amounts of Liabilities Presented in the Statements of Financial Position	(1)	(1)
Interest rate contracts [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Liabilities	(1)	(1)
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statements of Financial Position	$ (1)	$ (1)