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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Company Before Tax Obligation
Balance at January 1	$ 12,488	$ 10,175	$ 9,395
Liabilities incurred	62	129	144
Liabilities settled	(1,316)	(755)	(757)
Accretion expense	628	513	463
Revisions in estimated cash flows	905	2,426	930
Balance at December 31	12,767	12,488	10,175
Asset Retirement Obligations (Textual) [Abstract]
Long-term portion of the company's before-tax asset retirement obligations	11,999
Company's before-tax asset retirement obligations	$ 12,767	$ 12,488	$ 10,175

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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