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Asset Retirement Obligation (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
Asset Retirement Obligation (Details) (USD $) In Millions	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Asset Retirement Obligation
Asset retirement obligation at beginning of year			$ 2,872
Liabilities incurred			186
Liabilities settled			(238)
Accretion expense	38	25	75	49
Asset retirement obligation at end of the period	2,895		2,895
Less current portion	(360)		(360)		(407)
Asset retirement obligation - long-term	$ 2,535		$ 2,535		$ 2,465

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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