Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	$ 1,017	$ 675
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(329)	(478)
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	181	(366)
Gross Amounts Offset	(108)	3
Net Amounts of Assets/ (Liabilities)	73	(363)
Margin deposits		3
Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	5	0
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(65)	(106)
CECONY \| Subsidiaries
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	898	550
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	238	(252)
Gross Amounts Offset	(139)	0
Net Amounts of Assets/ (Liabilities)	99	(252)
Margin deposits		3
Clean Energy Businesses \| Interest Rate Swap
Net fair value derivative assets/(liabilities)
Notional amount	1,046	863
Fair value of derivative liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(428)	(225)
Gross Amounts Offset	265	(13)
Net Amounts of Assets/ (Liabilities)	(163)	(238)
Fair value of derivative liabilities, Current \| Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(26)	(24)
Fair value of derivative liabilities, Current \| CECONY \| Subsidiaries
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(158)	(174)
Gross Amounts Offset	44	11
Net Amounts of Assets/ (Liabilities)	(114)	(163)
Fair value of derivative liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(183)	(207)
Gross Amounts Offset	17	(33)
Net Amounts of Assets/ (Liabilities)	(166)	(240)
Fair value of derivative liabilities, Noncurrent \| Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(39)	(82)
Fair value of derivative liabilities, Noncurrent \| CECONY \| Subsidiaries
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(125)	(114)
Gross Amounts Offset	28	9
Net Amounts of Assets/ (Liabilities)	(97)	(105)
Total fair value of derivative liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(611)	(432)
Gross Amounts Offset	282	(46)
Net Amounts of Assets/ (Liabilities)	(329)	(478)
Total fair value of derivative liabilities \| CECONY \| Subsidiaries
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(283)	(288)
Gross Amounts Offset	72	20
Net Amounts of Assets/ (Liabilities)	(211)	(268)
Total fair value of derivative assets, Noncurrent \| Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	5
Fair value of derivative assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	652	44
Gross Amounts Offset	(375)	14
Net Amounts of Assets/ (Liabilities)	277	58
Fair value of derivative assets, Current \| CECONY \| Subsidiaries
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	407	20
Gross Amounts Offset	(183)	(12)
Net Amounts of Assets/ (Liabilities)	224	8
Total fair value of derivative assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	140	22
Gross Amounts Offset	(15)	35
Net Amounts of Assets/ (Liabilities)	125	57
Total fair value of derivative assets, Noncurrent \| CECONY \| Subsidiaries
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	114	16
Gross Amounts Offset	(28)	(8)
Net Amounts of Assets/ (Liabilities)	86	8
Total fair value of derivative assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	792	66
Gross Amounts Offset	(390)	49
Net Amounts of Assets/ (Liabilities)	402	115
Total fair value of derivative assets \| CECONY \| Subsidiaries
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	521	36
Gross Amounts Offset	(211)	(20)
Net Amounts of Assets/ (Liabilities)	$ 310	$ 16

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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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