Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013 Contract
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	$ 157
Makeup of net credit exposure, with investment-grade counterparties	55
Makeup of net credit exposure with commodity exchange brokers	44
Makeup of net credit exposure independent system operators	55
Makeup of net credit exposure non-investment grade/non-rated counterparties	3
Number of contracts considered derivatives	1,096
Interest Rate Swap [Member]
Investment Holdings [Line Items]
Unrealized gain (loss) on derivatives	(5)
Derivative, fixed interest rate	6.09%
Increase in the fair value of derivative	1
CECONY [Member]
Investment Holdings [Line Items]
Energy supply and hedging activities credit exposure total	21
Makeup of net credit exposure, with investment-grade counterparties	6
Makeup of net credit exposure with commodity exchange brokers	$ 15
Number of contracts considered derivatives	553

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Credit Exposure Independent System Operators

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Credit exposure nonrated counterparties

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Description Of Derivatives Activity Volume

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Maximum Potential Future Exposure On Credit Risk Derivatives With Investment Grade Counterparties

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Fixed interest rate related to the interest rate derivative.

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Maximum amount the entity would be obligated to pay on the credit risk derivatives upon occurrence of a credit event.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

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