Financial and Other Derivative Instruments Financial and Other Derivative Instruments (Details 1) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 843	$ 1,016
Gross Amounts Offset in the Statements of Financial Position	(745)	(869)
Net Amounts of Assets Presented in the Statements of Financial Position	98	147
Fair Value Gross Amounts of Recognized Liabilities	(815)	(1,020)
Gross Amounts Offset in the Statement of Financial Position	736	869
Net Amounts of Liabilities Presented in the Statements of Financial Position	(79)	(151)
Natural Gas Commodity Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	483	645
Gross Amounts Offset in the Statements of Financial Position	(474)	(605)
Net Amounts of Assets Presented in the Statements of Financial Position	9	40
Fair Value Gross Amounts of Recognized Liabilities	(471)	(661)
Gross Amounts Offset in the Statement of Financial Position	445	605
Net Amounts of Liabilities Presented in the Statements of Financial Position	(26)	(56)
Electricity [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	298	360
Gross Amounts Offset in the Statements of Financial Position	(216)	(258)
Net Amounts of Assets Presented in the Statements of Financial Position	82	102
Fair Value Gross Amounts of Recognized Liabilities	(287)	(351)
Gross Amounts Offset in the Statement of Financial Position	236	258
Net Amounts of Liabilities Presented in the Statements of Financial Position	(51)	(93)
Other Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Derivative Asset, Fair Value, Gross Asset	62	11
Gross Amounts Offset in the Statements of Financial Position	(55)	(6)
Net Amounts of Assets Presented in the Statements of Financial Position	7	5
Fair Value Gross Amounts of Recognized Liabilities	(57)	(7)
Gross Amounts Offset in the Statement of Financial Position	55	6
Net Amounts of Liabilities Presented in the Statements of Financial Position	(2)	(1)
Interest rate contracts [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Liabilities	0	(1)
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statements of Financial Position	$ 0	$ (1)