Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Asset Retirement Obligations [Line Items]
Asset retirement obligations	$ 1,719
Accretion	26
Asset retirement obligations	$ 1,745

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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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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