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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Company Before Tax Obligation
Balance at January 1	$ 12,767	$ 12,488	$ 10,175
Liabilities incurred	133	62	129
Liabilities settled	(966)	(1,316)	(755)
Accretion expense	629	628	513
Revisions in estimated cash flows	708	905	2,426
Balance at December 31	13,271	12,767	12,488
Long-term portion of the company's before-tax asset retirement obligations	$ 12,375

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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