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Asset Retirement Obligation - Asset Retirement Obligation (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligation at December 31, 2012	$ 4,578
Liabilities incurred	132
Liabilities acquired	53
Liabilities settled	(163)
Accretion expense	65	55
Asset retirement obligation at March 31, 2013	4,665
Less current portion	(468)		(478)
Asset retirement obligation, long-term	$ 4,197		$ 4,100

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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