Fair Value Fair Value (Details 3) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 1,955,000,000	$ 2,805,000,000
Derivative Liability, Fair Value, Gross Liability	(1,932,000,000)	(2,811,000,000)
Commodity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	36,000,000	33,000,000
Derivative Liability, Fair Value, Gross Liability	(53,000,000)	(104,000,000)
Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	250,000,000	257,000,000
Derivative Liability, Fair Value, Gross Liability	(227,000,000)	(140,000,000)
Fair Value, Inputs, Level 3 [Member] | Commodity [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	35,000,000	20,000,000
Derivative Liability, Fair Value, Gross Liability	(33,000,000)	(14,000,000)
Minimum Weighted Average Range for Unobservable Inputs	(0.32)
Maximum Weighted Average Range for Unobservable Inputs	1.68
Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Unobservable Inputs Valuation Techniques [Line Items]
Derivative Asset, Fair Value, Gross Asset	270,000,000	224,000,000
Derivative Liability, Fair Value, Gross Liability	(220,000,000)	(192,000,000)
Minimum Weighted Average Range for Unobservable Inputs	13
Maximum Weighted Average Range for Unobservable Inputs	23
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	$ 16