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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2011	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligations [Line Items]
Asset retirement obligations at January 1, 2012			$ 1,593
Accretion			100
Liabilities incurred			27
Liabilities settled			(11)
Revision in estimated cash flows	19	20	10	22
Asset retirement obligations at December 31, 2012			$ 1,719	$ 1,593

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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