Fair Value Measurements and Disclosures of Assets and Liabilities Measured on a Nonrecurring Basis (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	$ 73	[1]	$ 131	[1]	$ 73	[1]	$ 137	[1]
Discount Rate for Impairment Model (percentage)	10.00%				10.00%
Quoted Prices in Active Markets (Level 1)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	0	[2]	0	[2]	0	[2]	0	[2]
Significant Other Observable Inputs (Level 2)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	0	[3]	0	[3]	0	[3]	0	[3]
Significant Unobservable Inputs (Level 3)
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	172	[4]	29	[4]	172	[4]	32	[4]
Net Book Value
Asset Impairment Charges [Abstract]
Impaired Oil and Gas Properties	$ 245	[5]	$ 160	[5]	$ 245	[5]	$ 169	[5]

[1] See Note 4. Asset Impairments.

[2] 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.

[3] 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.

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

[5] Amount represents net book value at the date of assessment.