Fair Value Fair Value of Derivative Contracts (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 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	$ 63	$ 86
Liability fair value, derivative	(57)	(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	522	603
Liability fair value, derivative	(3)	(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	2	3
Liability fair value, derivative	(17)	(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	57	76
Liability fair value, derivative	(39)	(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	522	603
Liability fair value, derivative	(3)	(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	4	7
Liability fair value, derivative	(1)	(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