Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended
Dec. 31, 2012	Mar. 31, 2012 Interest rate contract [Member]	Mar. 31, 2012 Con Edison [Member] Other assets [Member]	Mar. 31, 2012 CECONY [Member] Other assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value measurement assets, transfers from Level 3 to Level 2			$ 105	$ 95
Fair value measurement liabilities, transfers from Level 1 to Level 2	2
Fair value measurement liabilities, transfers from Level 2 to Level 1	9
Fair value measurement liabilities, transfers from Level 2 to Level 3	2
Fair value measurement liabilities, transfers from Level 3 to Level 2	$ 11	$ 8

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets Transfers From Level Three To Level Two

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Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Transfers From Level One To Level Two

No definition available.

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Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Transfers From Level Three To Level Two

No definition available.

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Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Transfers From Level Two To Level One

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Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Liability Transfers From Level Two To Level Three

No definition available.

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