Fair Value Fair Value (Details 2) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 2,794,000,000	$ 2,805,000,000
Derivative Liability, Fair Value, Gross Liability	(2,902,000,000)	(2,811,000,000)
Commodity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	30,000,000	33,000,000
Derivative Liability, Fair Value, Gross Liability	(108,000,000)	(104,000,000)
Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	318,000,000	257,000,000
Derivative Liability, Fair Value, Gross Liability	(229,000,000)	(140,000,000)
Fair Value, Inputs, Level 3 [Member] | Commodity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	31,000,000	20,000,000
Derivative Liability, Fair Value, Gross Liability	(25,000,000)	(14,000,000)
Minimum Weighted Average Range for Unobservable Inputs	(0.16)
Maximum Weighted Average Range for Unobservable Inputs	0.28
Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	363,000,000	224,000,000
Derivative Liability, Fair Value, Gross Liability	(338,000,000)	(192,000,000)
Minimum Weighted Average Range for Unobservable Inputs	9
Maximum Weighted Average Range for Unobservable Inputs	27
Fair Value, Inputs, Level 3 [Member] | Commodity Contract 1 [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Minimum Weighted Average Range for Unobservable Inputs	(1)
Maximum Weighted Average Range for Unobservable Inputs	$ 10