Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	$ 465	$ 446
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(97)	(66)
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(44)	(23)
Gross Amounts Offset	(3)	(1)
Net Amounts of Assets/ (Liabilities)	(47)	(24)
Margin deposits	4	7
Interest Rate Swap
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	1	2
Fair value of derivative liabilities
Net Amounts of Assets/ (Liabilities)	(14)	(6)
CECONY
Fair value of derivative assets
Net Amounts of Assets/ (Liabilities)	433	407
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	(24)	(7)
Gross Amounts Offset	(1)	1
Net Amounts of Assets/ (Liabilities)	(25)	(6)
Margin deposits	4	6
Fair Value of Derivative Liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(78)	(61)
Gross Amounts Offset	32	11
Net Amounts of Assets/ (Liabilities)	(46)	(50)
Fair Value of Derivative Liabilities, Current \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(53)	(31)
Gross Amounts Offset	29	6
Net Amounts of Assets/ (Liabilities)	(24)	(25)
Fair Value of Derivative Liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(44)	(19)
Gross Amounts Offset	7	9
Net Amounts of Assets/ (Liabilities)	(37)	(10)
Fair Value of Derivative Liabilities, Noncurrent \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(37)	(12)
Gross Amounts Offset	5	6
Net Amounts of Assets/ (Liabilities)	(32)	(6)
Fair Value of Derivative Liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(122)	(80)
Gross Amounts Offset	39	20
Net Amounts of Assets/ (Liabilities)	(83)	(60)
Fair Value of Derivative Liabilities \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(90)	(43)
Gross Amounts Offset	34	12
Net Amounts of Assets/ (Liabilities)	(56)	(31)
Fair Value of Derivative Assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	72	43
Gross Amounts Offset	(37)	(14)
Net Amounts of Assets/ (Liabilities)	35	29
Fair Value of Derivative Assets, Current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	62	25
Gross Amounts Offset	(31)	(6)
Net Amounts of Assets/ (Liabilities)	31	19
Fair Value of Derivative Assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	6	14
Gross Amounts Offset	(5)	(7)
Net Amounts of Assets/ (Liabilities)	1	7
Fair Value of Derivative Assets, Noncurrent \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	4	11
Gross Amounts Offset	(4)	(5)
Net Amounts of Assets/ (Liabilities)	0	6
Fair Value of Derivative Assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	78	57
Gross Amounts Offset	(42)	(21)
Net Amounts of Assets/ (Liabilities)	36	36
Fair Value of Derivative Assets \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	66	36
Gross Amounts Offset	(35)	(11)
Net Amounts of Assets/ (Liabilities)	$ 31	$ 25

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Offsetting Derivative Assets and Liabilities [Abstract]

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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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