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Asset Retirement Obligation (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Asset Retirement Obligation
Asset retirement obligation at beginning of year	$ 3,887
Liabilities incurred	109
Liabilities settled	(121)
Accretion expense	55	37
Asset retirement obligation at end of the period	3,930
Less current portion	(448)		(447)
Asset retirement obligation - long-term	$ 3,482		$ 3,440

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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