Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	$ (1)	$ (17)
Gross Amounts Offset	0	0
Net Amounts of Assets/ (Liabilities)	(1)	(17)
Margin deposits	7	12
CECONY
Net fair value derivative assets/(liabilities)
Gross Amounts of Recognized Assets/(Liabilities)	9	(14)
Gross Amounts Offset	(1)	(1)
Net Amounts of Assets/ (Liabilities)	8	(15)
Margin deposits	6	11
Fair Value of Derivative Liabilities, Current
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(41)	(67)
Gross Amounts Offset	23	50
Net Amounts of Assets/ (Liabilities)	(18)	(17)
Fair Value of Derivative Liabilities, Current \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(19)	(26)
Gross Amounts Offset	15	14
Net Amounts of Assets/ (Liabilities)	(4)	(12)
Fair Value of Derivative Liabilities, Noncurrent
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(26)	(43)
Gross Amounts Offset	13	5
Net Amounts of Assets/ (Liabilities)	(13)	(38)
Fair Value of Derivative Liabilities, Noncurrent \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(20)	(36)
Gross Amounts Offset	11	4
Net Amounts of Assets/ (Liabilities)	(9)	(32)
Fair Value of Derivative Liabilities
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(67)	(110)
Gross Amounts Offset	36	55
Net Amounts of Assets/ (Liabilities)	(31)	(55)
Fair Value of Derivative Liabilities \| CECONY
Fair value of derivative liabilities
Gross Amounts of Recognized Assets/(Liabilities)	(39)	(62)
Gross Amounts Offset	26	18
Net Amounts of Assets/ (Liabilities)	(13)	(44)
Fair Value of Derivative Assets, Current
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	45	83
Gross Amounts Offset	(23)	(51)
Net Amounts of Assets/ (Liabilities)	22	32
Fair Value of Derivative Assets, Current \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	32	39
Gross Amounts Offset	(16)	(15)
Net Amounts of Assets/ (Liabilities)	16	24
Fair Value of Derivative Assets, Noncurrent
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	21	10
Gross Amounts Offset	(13)	(4)
Net Amounts of Assets/ (Liabilities)	8	6
Fair Value of Derivative Assets, Noncurrent \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	16	9
Gross Amounts Offset	(11)	(4)
Net Amounts of Assets/ (Liabilities)	5	5
Fair Value of Derivative Assets
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	66	93
Gross Amounts Offset	(36)	(55)
Net Amounts of Assets/ (Liabilities)	30	38
Fair Value of Derivative Assets \| CECONY
Fair value of derivative assets
Gross Amounts of Recognized Assets/(Liabilities)	48	48
Gross Amounts Offset	(27)	(19)
Net Amounts of Assets/ (Liabilities)	$ 21	$ 29

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