Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	$ 835	$ 675
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(304)	(478)
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(80)	(366)
Gross Amounts Offset	0	3
Net Amounts of Assets/ (Liabilities)	(80)	(363)
Margin deposits		3
Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	3	0
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(74)	(106)
CECONY \| Subsidiaries
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	738	550
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(25)	(252)
Gross Amounts Offset	(5)	0
Net Amounts of Assets/ (Liabilities)	(30)	(252)
Clean Energy Businesses \| Interest Rate Swap
Net fair value derivative assets/(liabilities)
Notional amount	1,057	863
Buyer \| Interest Rate Swap
Net fair value derivative assets/(liabilities)
Notional amount	33
Fair value of derivative liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(232)	(225)
Gross Amounts Offset	53	(13)
Net Amounts of Assets/ (Liabilities)	(179)	(238)
Fair value of derivative liabilities, Current \| Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(27)	(24)
Fair value of derivative liabilities, Current \| CECONY \| Subsidiaries
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(159)	(174)
Gross Amounts Offset	30	11
Net Amounts of Assets/ (Liabilities)	(129)	(163)
Fair value of derivative liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(127)	(207)
Gross Amounts Offset	2	(33)
Net Amounts of Assets/ (Liabilities)	(125)	(240)
Fair value of derivative liabilities, Noncurrent \| Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(47)	(82)
Fair value of derivative liabilities, Noncurrent \| CECONY \| Subsidiaries
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(70)	(114)
Gross Amounts Offset	16	9
Net Amounts of Assets/ (Liabilities)	(54)	(105)
Total fair value of derivative liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(359)	(432)
Gross Amounts Offset	55	(46)
Net Amounts of Assets/ (Liabilities)	(304)	(478)
Total fair value of derivative liabilities \| CECONY \| Subsidiaries
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(229)	(288)
Gross Amounts Offset	46	20
Net Amounts of Assets/ (Liabilities)	(183)	(268)
Total fair value of derivative assets, Noncurrent \| Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	3
Fair value of derivative assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	214	44
Gross Amounts Offset	(55)	14
Net Amounts of Assets/ (Liabilities)	159	58
Fair value of derivative assets, Current \| CECONY \| Subsidiaries
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	157	20
Gross Amounts Offset	(35)	(12)
Net Amounts of Assets/ (Liabilities)	122	8
Total fair value of derivative assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	65	22
Gross Amounts Offset	0	35
Net Amounts of Assets/ (Liabilities)	65	57
Total fair value of derivative assets, Noncurrent \| CECONY \| Subsidiaries
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	47	16
Gross Amounts Offset	(16)	(8)
Net Amounts of Assets/ (Liabilities)	31	8
Total fair value of derivative assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	279	66
Gross Amounts Offset	(55)	49
Net Amounts of Assets/ (Liabilities)	224	115
Total fair value of derivative assets \| CECONY \| Subsidiaries
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	204	36
Gross Amounts Offset	(51)	(20)
Net Amounts of Assets/ (Liabilities)	$ 153	$ 16

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Offsetting Derivative Assets and Liabilities [Abstract]

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

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Nominal or face amount used to calculate payment on derivative.

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