Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Recurring Basis (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Financial Assets [Abstract]
Mutual Fund Investments	$ 109		$ 103
Commodity Derivative Instruments	49		84
Financial Liabilities [Abstract]
Commodity Derivative Instruments	(53)		(10)
Portion of Deferred Compensation Liability Measured at Fair Value	(174)		(160)
Quoted Prices in Active Markets (Level 1)
Financial Assets [Abstract]
Mutual Fund Investments	109	[1]	103	[1]
Commodity Derivative Instruments	0	[1]	0	[1]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	0	[1]	0	[1]
Portion of Deferred Compensation Liability Measured at Fair Value	(174)	[1]	(160)	[1]
Significant Other Observable Inputs (Level 2)
Financial Assets [Abstract]
Mutual Fund Investments	0	[2]	0	[2]
Commodity Derivative Instruments	62	[2]	113	[2]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	(66)	[2]	(39)	[2]
Portion of Deferred Compensation Liability Measured at Fair Value	0	[2]	0	[2]
Significant Unobservable Inputs (Level 3)
Financial Assets [Abstract]
Mutual Fund Investments	0	[3]	0	[3]
Commodity Derivative Instruments	0	[3]	0	[3]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	0	[3]	0	[3]
Portion of Deferred Compensation Liability Measured at Fair Value	0	[3]	0	[3]
Fair Value Inputs Adjustment
Financial Assets [Abstract]
Mutual Fund Investments	0	[4]	0	[4]
Commodity Derivative Instruments	(13)	[4]	(29)	[4]
Financial Liabilities [Abstract]
Commodity Derivative Instruments	13	[4]	29	[4]
Portion of Deferred Compensation Liability Measured at Fair Value	$ 0	[4]	$ 0	[4]

[1] Level 1 measurements are fair value measurements which use quoted market prices (unadjusted) in active markets for identical assets or liabilities. We use Level 1 inputs when available as Level 1 inputs generally provide the most reliable evidence of fair value.

[2] Level 2 measurements are fair value measurements which use inputs, other than quoted prices included within Level 1, which are observable for the asset or liability, either directly or indirectly.

[3] Level 3 measurements are fair value measurements which use unobservable inputs.

[4] Amount represents the impact of netting clauses within our master agreements that allow us to net cash settle asset and liability positions with the same counterparty.