Fair Value Fair Value (Details 3) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ (870,000,000)	$ (1,020,000,000)
Derivative Asset, Fair Value, Gross Asset	876,000,000	1,016,000,000
Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Gross Liability	(60,000,000)	(93,000,000)
Derivative Asset, Fair Value, Gross Asset	97,000,000	102,000,000
Natural Gas [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Liability, Fair Value, Gross Liability	(41,000,000)	(56,000,000)
Derivative Asset, Fair Value, Gross Asset	14,000,000	40,000,000
Fair Value, Inputs, Level 3 [Member] | Electricity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Maximum Weighted Average Range for Unobservable Inputs	17	16
Weighted Average of Unobservable Fair Value Inputs	3	3
Minimum Weighted Average Range for Unobservable Inputs	(8)	(2)
Derivative Liability, Fair Value, Gross Liability	(86,000,000)	(111,000,000)
Derivative Asset, Fair Value, Gross Asset	98,000,000	134,000,000
Fair Value, Inputs, Level 3 [Member] | Natural Gas [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Maximum Weighted Average Range for Unobservable Inputs	0.54	1.95
Weighted Average of Unobservable Fair Value Inputs	0.01	0.03
Minimum Weighted Average Range for Unobservable Inputs	(2.63)	(0.63)
Derivative Liability, Fair Value, Gross Liability	(62,000,000)	(62,000,000)
Derivative Asset, Fair Value, Gross Asset	$ 32,000,000	$ 24,000,000