Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	$ 453	$ 363
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(55)	(117)
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/ (Liabilities)	(17)	(99)
Gross Amounts Offset	0	6
Net Amounts of Assets/ (Liabilities)	(17)	(93)
Margin deposits	12	7
Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(1)	(1)
CECONY
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	414	326
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/ (Liabilities)	(14)	(95)
Gross Amounts Offset	(1)	9
Net Amounts of Assets/ (Liabilities)	(15)	(86)
Margin deposits	11	7
Fair Value of Derivative Liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/ (Liabilities)	(67)	(138)
Gross Amounts Offset	50	61
Net Amounts of Assets/ (Liabilities)	(17)	(77)
Fair Value of Derivative Liabilities, Current \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/ (Liabilities)	(26)	(111)
Gross Amounts Offset	14	45
Net Amounts of Assets/ (Liabilities)	(12)	(66)
Fair Value of Derivative Liabilities, Non-current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/ (Liabilities)	(43)	(91)
Gross Amounts Offset	5	52
Net Amounts of Assets/ (Liabilities)	(38)	(39)
Fair Value of Derivative Liabilities, Non-current \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/ (Liabilities)	(36)	(77)
Gross Amounts Offset	4	44
Net Amounts of Assets/ (Liabilities)	(32)	(33)
Fair Value of Derivative Liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/ (Liabilities)	(110)	(229)
Gross Amounts Offset	55	113
Net Amounts of Assets/ (Liabilities)	(55)	(116)
Fair Value of Derivative Liabilities \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/ (Liabilities)	(62)	(188)
Gross Amounts Offset	18	89
Net Amounts of Assets/ (Liabilities)	(44)	(99)
Fair Value of Derivative Assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/ (Liabilities)	83	81
Gross Amounts Offset	(51)	(64)
Net Amounts of Assets/ (Liabilities)	32	17
Fair Value of Derivative Assets, Current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/ (Liabilities)	39	52
Gross Amounts Offset	(15)	(45)
Net Amounts of Assets/ (Liabilities)	24	7
Fair Value of Derivative Assets, Non-current
Fair value of derivative assets
Gross Amounts of Recognized Assets/ (Liabilities)	10	49
Gross Amounts Offset	(4)	(43)
Net Amounts of Assets/ (Liabilities)	6	6
Fair Value of Derivative Assets, Non-current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/ (Liabilities)	9	41
Gross Amounts Offset	(4)	(35)
Net Amounts of Assets/ (Liabilities)	5	6
Fair Value of Derivative Assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/ (Liabilities)	93	130
Gross Amounts Offset	(55)	(107)
Net Amounts of Assets/ (Liabilities)	38	23
Fair Value of Derivative Assets \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/ (Liabilities)	48	93
Gross Amounts Offset	(19)	(80)
Net Amounts of Assets/ (Liabilities)	$ 29	$ 13

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

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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, 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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