Fair Value Fair Value (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ (1,020,000,000)	$ (2,811,000,000)
Derivative Asset, Fair Value, Gross Asset	1,016,000,000	2,805,000,000
Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Gross Liability	(93,000,000)	(140,000,000)
Derivative Asset, Fair Value, Gross Asset	102,000,000	257,000,000
Natural Gas [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Gross Liability	(56,000,000)	(104,000,000)
Derivative Asset, Fair Value, Gross Asset	40,000,000	33,000,000
Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Maximum Weighted Average Range for Unobservable Inputs	16
Weighted Average of Unobservable Fair Value Inputs	3
Minimum Weighted Average Range for Unobservable Inputs	(2)
Derivative Liability, Fair Value, Gross Liability	(111,000,000)	(192,000,000)
Derivative Asset, Fair Value, Gross Asset	134,000,000	224,000,000
Fair Value, Inputs, Level 3 [Member] | Natural Gas [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Maximum Weighted Average Range for Unobservable Inputs	1.95
Weighted Average of Unobservable Fair Value Inputs	0.03
Minimum Weighted Average Range for Unobservable Inputs	(0.63)
Derivative Liability, Fair Value, Gross Liability	(62,000,000)	(14,000,000)
Derivative Asset, Fair Value, Gross Asset	$ 24,000,000	$ 20,000,000