Financial and Other Derivative Instruments Financial and Other Derivative Instruments (Details 1) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	$ 876	$ 1,016
Gross Amounts Offset in the Statements of Financial Position	(759)	(869)
Net Amounts of Assets Presented in the Statements of Financial Position	117	147
Fair Value Gross Amounts of Recognized Liabilities	(870)	(1,020)
Gross Amounts Offset in the Statement of Financial Position	764	869
Net Amounts of Liabilities Presented in the Statements of Financial Position	(106)	(151)
Natural Gas [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	504	645
Gross Amounts Offset in the Statements of Financial Position	(490)	(605)
Net Amounts of Assets Presented in the Statements of Financial Position	14	40
Fair Value Gross Amounts of Recognized Liabilities	(510)	(661)
Gross Amounts Offset in the Statement of Financial Position	469	605
Net Amounts of Liabilities Presented in the Statements of Financial Position	(41)	(56)
Electricity [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	317	360
Gross Amounts Offset in the Statements of Financial Position	(220)	(258)
Net Amounts of Assets Presented in the Statements of Financial Position	97	102
Fair Value Gross Amounts of Recognized Liabilities	(307)	(351)
Gross Amounts Offset in the Statement of Financial Position	247	258
Net Amounts of Liabilities Presented in the Statements of Financial Position	(60)	(93)
Other Contract [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Assets	55	11
Gross Amounts Offset in the Statements of Financial Position	(49)	(6)
Net Amounts of Assets Presented in the Statements of Financial Position	6	5
Fair Value Gross Amounts of Recognized Liabilities	(52)	(7)
Gross Amounts Offset in the Statement of Financial Position	48	6
Net Amounts of Liabilities Presented in the Statements of Financial Position	(4)	(1)
Foreign currency exchange contracts [Member]
Netting Offsets of Derivative Assets and Liabilities [Line Items]
Fair Value Gross Amounts of Recognized Liabilities	(1)	0
Gross Amounts Offset in the Statement of Financial Position	0	0
Net Amounts of Liabilities Presented in the Statements of Financial Position	(1)	0
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)