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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Asset Retirement Obligations [Line Items]
Asset retirement obligations at December 31, 2011		$ 1,593
Accretion	73
Revision in estimate cash flows	11
Liabilities incurred	14
Liabilities settled	(8)
Asset retirement obligations at September 30, 2012	$ 1,683	$ 1,591

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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