Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Net Amounts of Assets/ (Liabilities)	$ 326	$ 361
Net Amounts of Assets/ (Liabilities)	(112)	(78)
Gross Amounts of Recognized Assets/(Liabilities)	(135)	(163)
Gross Amounts Offset	39	140
Net Amounts of Assets/ (Liabilities)	(96)	(23)
Margin deposits	17	27
Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	47	111
Gross Amounts Offset	(33)	(67)
Net Amounts of Assets/ (Liabilities)	14	44
Fair Value of Derivative Assets, Assets Held for Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	39	0
Gross Amounts Offset	(39)	0
Net Amounts of Assets/ (Liabilities)	0	0
Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	19	34
Gross Amounts Offset	(17)	(23)
Net Amounts of Assets/ (Liabilities)	2	11
Fair Value of Derivative Assets
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	105	145
Gross Amounts Offset	(89)	(90)
Net Amounts of Assets/ (Liabilities)	16	55
Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(84)	(242)
Gross Amounts Offset	58	139
Net Amounts of Assets/ (Liabilities)	(26)	(103)
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(77)	0
Gross Amounts Offset	30	0
Net Amounts of Assets/ (Liabilities)	(47)	0
Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(48)	(66)
Gross Amounts Offset	31	91
Net Amounts of Assets/ (Liabilities)	(17)	25
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(31)	0
Gross Amounts Offset	9	0
Net Amounts of Assets/ (Liabilities)	(22)	0
Fair Value of Derivative Liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(240)	(308)
Gross Amounts Offset	128	230
Net Amounts of Assets/ (Liabilities)	(112)	(78)
CECONY
Derivatives, Fair Value [Line Items]
Net Amounts of Assets/ (Liabilities)	297	299
Gross Amounts of Recognized Assets/(Liabilities)	(62)	(90)
Gross Amounts Offset	38	45
Net Amounts of Assets/ (Liabilities)	(24)	(45)
Margin deposits	17	25
CECONY \| Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	36	26
Gross Amounts Offset	(28)	(15)
Net Amounts of Assets/ (Liabilities)	8	11
CECONY \| Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	16	22
Gross Amounts Offset	(15)	(20)
Net Amounts of Assets/ (Liabilities)	1	2
CECONY \| Fair Value of Derivative Assets
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	52	48
Gross Amounts Offset	(43)	(35)
Net Amounts of Assets/ (Liabilities)	9	13
CECONY \| Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(72)	(96)
Gross Amounts Offset	53	48
Net Amounts of Assets/ (Liabilities)	(19)	(48)
CECONY \| Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(42)	(42)
Gross Amounts Offset	28	32
Net Amounts of Assets/ (Liabilities)	(14)	(10)
CECONY \| Fair Value of Derivative Liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(114)	(138)
Gross Amounts Offset	81	80
Net Amounts of Assets/ (Liabilities)	$ (33)	$ (58)

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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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

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