Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	$ 697	$ 675
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(386)	(478)
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(261)	(366)
Gross Amounts Offset	5	3
Net Amounts of Assets/ (Liabilities)	(256)	(363)
Margin deposits	1	3
Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	11	0
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(58)	(106)
CECONY
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	570	550
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(217)	(252)
Gross Amounts Offset	(1)	0
Net Amounts of Assets/ (Liabilities)	(218)	(252)
Margin deposits	1
Clean Energy Businesses \| Interest Rate Swap
Net fair value derivative assets/(liabilities)
Notional amount	1,103	863
Fair Value of Derivative Liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(236)	(225)
Gross Amounts Offset	5	(13)
Net Amounts of Assets/ (Liabilities)	(231)	(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
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(184)	(174)
Gross Amounts Offset	22	11
Net Amounts of Assets/ (Liabilities)	(162)	(163)
Fair Value of Derivative Liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(131)	(207)
Gross Amounts Offset	(24)	(33)
Net Amounts of Assets/ (Liabilities)	(155)	(240)
Fair Value of Derivative Liabilities, Noncurrent \| Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(30)	(82)
Fair Value of Derivative Liabilities, Noncurrent \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(93)	(114)
Gross Amounts Offset	9	9
Net Amounts of Assets/ (Liabilities)	(84)	(105)
Fair Value of Derivative Liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(367)	(432)
Gross Amounts Offset	(19)	(46)
Net Amounts of Assets/ (Liabilities)	(386)	(478)
Fair Value of Derivative Liabilities \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(277)	(288)
Gross Amounts Offset	31	20
Net Amounts of Assets/ (Liabilities)	(246)	(268)
Fair Value of Derivative Assets, Noncurrent \| Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	11
Fair Value of Derivative Assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	75	44
Gross Amounts Offset	(1)	14
Net Amounts of Assets/ (Liabilities)	74	58
Fair Value of Derivative Assets, Current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	47	20
Gross Amounts Offset	(23)	(12)
Net Amounts of Assets/ (Liabilities)	24	8
Fair Value of Derivative Assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	31	22
Gross Amounts Offset	25	35
Net Amounts of Assets/ (Liabilities)	56	57
Fair Value of Derivative Assets, Noncurrent \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	13	16
Gross Amounts Offset	(9)	(8)
Net Amounts of Assets/ (Liabilities)	4	8
Fair Value of Derivative Assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	106	66
Gross Amounts Offset	24	49
Net Amounts of Assets/ (Liabilities)	130	115
Fair Value of Derivative Assets \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	60	36
Gross Amounts Offset	(32)	(20)
Net Amounts of Assets/ (Liabilities)	$ 28	$ 16

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

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