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Asset Retirement Obligations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Changes to the asset retirement obligations
Beginning Balance	$ 1,599	$ 1,497
Liabilities settled	(8)	(2)
Accretion	27	104
Revisions in estimated cash flows	149
Ending Balance	1,767	1,599
FES
Changes to the asset retirement obligations
Beginning Balance	965	904
Liabilities settled	(9)	(1)
Accretion	16	62
Revisions in estimated cash flows	149
Ending Balance	$ 1,121	$ 965

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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