Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	$ (143)	$ (99)
Gross Amounts Offset	3	6
Net Amounts of Assets/ (Liabilities)	(140)	(93)
Margin deposits	5	7
Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)		(1)
CECONY
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(131)	(95)
Gross Amounts Offset	3	9
Net Amounts of Assets/ (Liabilities)	(128)	(86)
Margin deposits	5	7
Fair Value of Derivative Liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(141)	(138)
Gross Amounts Offset	71	61
Net Amounts of Assets/ (Liabilities)	(70)	(77)
Fair Value of Derivative Liabilities, Current \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(116)	(111)
Gross Amounts Offset	57	45
Net Amounts of Assets/ (Liabilities)	(59)	(66)
Fair Value of Derivative Liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(143)	(91)
Gross Amounts Offset	60	52
Net Amounts of Assets/ (Liabilities)	(83)	(39)
Fair Value of Derivative Liabilities, Noncurrent \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(127)	(77)
Gross Amounts Offset	54	44
Net Amounts of Assets/ (Liabilities)	(73)	(33)
Fair Value of Derivative Liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(284)	(229)
Gross Amounts Offset	131	113
Net Amounts of Assets/ (Liabilities)	(153)	(116)
Fair Value of Derivative Liabilities \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(243)	(188)
Gross Amounts Offset	111	89
Net Amounts of Assets/ (Liabilities)	(132)	(99)
Fair Value of Derivative Assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	77	81
Gross Amounts Offset	(67)	(64)
Net Amounts of Assets/ (Liabilities)	10	17
Fair Value of Derivative Assets, Current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	55	52
Gross Amounts Offset	(53)	(45)
Net Amounts of Assets/ (Liabilities)	2	7
Fair Value of Derivative Assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	64	49
Gross Amounts Offset	(61)	(43)
Net Amounts of Assets/ (Liabilities)	3	6
Fair Value of Derivative Assets, Noncurrent \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	57	41
Gross Amounts Offset	(55)	(35)
Net Amounts of Assets/ (Liabilities)	2	6
Fair Value of Derivative Assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	141	130
Gross Amounts Offset	(128)	(107)
Net Amounts of Assets/ (Liabilities)	13	23
Fair Value of Derivative Assets \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	112	93
Gross Amounts Offset	(108)	(80)
Net Amounts of Assets/ (Liabilities)	$ 4	$ 13

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

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Gross Amounts of Recognized Assets Liabilities

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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 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 elected not to be offset. Excludes assets not subject to a master netting arrangement.

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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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