Fair Value Fair Value of Derivative Contracts (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Estimate of Fair Value [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	$ 138	$ 86
Liability fair value, derivative	(39)	(33)
Estimate of Fair Value [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	261	603
Liability fair value, derivative	(49)	(1)
Quoted prices in active markets for identical assets (Level 1) [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	14	3
Liability fair value, derivative	(4)	(3)
Quoted prices in active markets for identical assets (Level 1) [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	0	0
Liability fair value, derivative	0	0
Significant other observable inputs (Level 2) [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	89	76
Liability fair value, derivative	(18)	(26)
Significant other observable inputs (Level 2) [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	261	603
Liability fair value, derivative	(49)	(1)
Significantunobservable inputs (Level 3) [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	35	7
Liability fair value, derivative	(17)	(4)
Significantunobservable inputs (Level 3) [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset fair value, derivative	0	0
Liability fair value, derivative	0	0
Energy Related Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, net	(13)	(11)
Derivative Liability, Fair Value, Gross Liability	39	33
Balance sheet, derivative liability reduction, gross	0	0
Derivative liabilities presented on balance sheet	39	33
Energy Related Derivative [Member] | Not offset on balance sheet [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Collateral, Right to Reclaim Cash	0	5
Financial instrument, liability	26	17
Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, net	(39)	(1)
Derivative Liability, Fair Value, Gross Liability	49	1
Balance sheet, derivative liability reduction, gross	0	0
Derivative liabilities presented on balance sheet	49	1
Interest Rate Swap [Member] | Not offset on balance sheet [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, Collateral, Right to Reclaim Cash	0	0
Financial instrument, liability	$ 10	$ 0