Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	$ 670	$ 550
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(447)	(229)
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(297)	(183)
Gross Amounts Offset	(1)	17
Net Amounts of Assets/ (Liabilities)	(298)	(166)
Margin deposits		9
Margin deposit liabilities	(2)
Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	0	1
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(122)	(41)
CECONY
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	552	498
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(162)	(124)
Gross Amounts Offset	(1)	17
Net Amounts of Assets/ (Liabilities)	(163)	(107)
Margin deposits		8
Margin deposit liabilities	(2)
Clean Energy Businesses \| Interest Rate Swap
Net fair value derivative assets/(liabilities)
Notional amount	878	919
Fair Value of Derivative Liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(160)	(140)
Gross Amounts Offset	13	17
Net Amounts of Assets/ (Liabilities)	(147)	(123)
Fair Value of Derivative Liabilities, Current \| Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(24)	(7)
Fair Value of Derivative Liabilities, Current \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(113)	(100)
Gross Amounts Offset	29	19
Net Amounts of Assets/ (Liabilities)	(84)	(81)
Fair Value of Derivative Liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(258)	(122)
Gross Amounts Offset	(40)	16
Net Amounts of Assets/ (Liabilities)	(298)	(106)
Fair Value of Derivative Liabilities, Noncurrent \| Interest Rate Swap
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(98)	(34)
Fair Value of Derivative Liabilities, Noncurrent \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(147)	(80)
Gross Amounts Offset	13	16
Net Amounts of Assets/ (Liabilities)	(134)	(64)
Fair Value of Derivative Liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(418)	(262)
Gross Amounts Offset	(27)	33
Net Amounts of Assets/ (Liabilities)	(445)	(229)
Fair Value of Derivative Liabilities \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(260)	(180)
Gross Amounts Offset	42	35
Net Amounts of Assets/ (Liabilities)	(218)	(145)
Fair Value of Derivative Assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	88	60
Gross Amounts Offset	(16)	(3)
Net Amounts of Assets/ (Liabilities)	72	57
Fair Value of Derivative Assets, Current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	70	39
Gross Amounts Offset	(30)	(6)
Net Amounts of Assets/ (Liabilities)	40	33
Fair Value of Derivative Assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	33	19
Gross Amounts Offset	42	(13)
Net Amounts of Assets/ (Liabilities)	75	6
Fair Value of Derivative Assets, Noncurrent \| Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)		1
Fair Value of Derivative Assets, Noncurrent \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	28	17
Gross Amounts Offset	(13)	(12)
Net Amounts of Assets/ (Liabilities)	15	5
Fair Value of Derivative Assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	121	79
Gross Amounts Offset	26	(16)
Net Amounts of Assets/ (Liabilities)	147	63
Fair Value of Derivative Assets \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	98	56
Gross Amounts Offset	(43)	(18)
Net Amounts of Assets/ (Liabilities)	$ 55	$ 38

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