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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations, Beginning Balance	$ 377	$ 253
Liabilities Incurred	24	2
Liabilities Settled	(98)	(19)
Revision of Estimate	30	9
Accretion Expense	21	15
Other	(34)	0
Asset Retirement Obligations, Ending Balance	$ 320	$ 260

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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