Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	$ (148)	$ (17)
Gross Amounts Offset	3	0
Net Amounts of Assets/ (Liabilities)	(145)	(17)
Margin deposits	6	12
CECONY
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(139)	(14)
Gross Amounts Offset	3	(1)
Net Amounts of Assets/ (Liabilities)	(136)	(15)
Margin deposits	6	11
Fair Value of Derivative Liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(109)	(67)
Gross Amounts Offset	35	50
Net Amounts of Assets/ (Liabilities)	(74)	(17)
Fair Value of Derivative Liabilities, Current \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(89)	(26)
Gross Amounts Offset	23	14
Net Amounts of Assets/ (Liabilities)	(66)	(12)
Fair Value of Derivative Liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(94)	(43)
Gross Amounts Offset	9	5
Net Amounts of Assets/ (Liabilities)	(85)	(38)
Fair Value of Derivative Liabilities, Noncurrent \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(83)	(36)
Gross Amounts Offset	7	4
Net Amounts of Assets/ (Liabilities)	(76)	(32)
Fair Value of Derivative Liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(203)	(110)
Gross Amounts Offset	44	55
Net Amounts of Assets/ (Liabilities)	(159)	(55)
Fair Value of Derivative Liabilities \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(172)	(62)
Gross Amounts Offset	30	18
Net Amounts of Assets/ (Liabilities)	(142)	(44)
Fair Value of Derivative Assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	46	83
Gross Amounts Offset	(36)	(51)
Net Amounts of Assets/ (Liabilities)	10	32
Fair Value of Derivative Assets, Current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	28	39
Gross Amounts Offset	(24)	(15)
Net Amounts of Assets/ (Liabilities)	4	24
Fair Value of Derivative Assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	9	10
Gross Amounts Offset	(5)	(4)
Net Amounts of Assets/ (Liabilities)	4	6
Fair Value of Derivative Assets, Noncurrent \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	5	9
Gross Amounts Offset	(3)	(4)
Net Amounts of Assets/ (Liabilities)	2	5
Fair Value of Derivative Assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	55	93
Gross Amounts Offset	(41)	(55)
Net Amounts of Assets/ (Liabilities)	14	38
Fair Value of Derivative Assets \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	33	48
Gross Amounts Offset	(27)	(19)
Net Amounts of Assets/ (Liabilities)	$ 6	$ 29

Derivative Asset (Liability), Gross Amount Offset in Balance Sheet

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Offsetting Derivative Assets and Liabilities [Abstract]

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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The amount of cash or securities placed with a broker or counterparty as security for a trading or derivatives securities position which was partially obtained with funds provided by the broker dealer.

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