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Asset Retirement Obligations - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Related regulatory liabilities	$ 503	$ 448
Asset retirement obligations	158	145
Reduction of related regulatory liabilities	13	7
Change in estimated cash flows	24
Change in accretion expense	6
Offset in part by liabilities settled	17
CECONY [Member]
Related regulatory liabilities	$ 420	$ 372

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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