Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ 133	$ 182
Gross Amounts Offset	(117)	(160)
Net Amounts of Assets/ (Liabilities)	16	22
Gross Amounts of Recognized Assets/(Liabilities)	(254)	(389)
Gross Amounts Offset	132	206
Net Amounts of Assets/ (Liabilities)	(122)	(183)
Gross Amounts of Recognized Assets/(Liabilities)	(121)	(207)
Gross Amounts Offset	15	46
Net Amounts of Assets/ (Liabilities)	(106)	(161)
Margin deposits	12	26
Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	75	59
Gross Amounts Offset	(60)	(41)
Net Amounts of Assets/ (Liabilities)	15	18
Fair Value of Derivative Assets, Assets Held for Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	0	51
Gross Amounts Offset	0	(50)
Net Amounts of Assets/ (Liabilities)	0	1
Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	58	57
Gross Amounts Offset	(57)	(54)
Net Amounts of Assets/ (Liabilities)	1	3
Fair Value of Derivative Assets, Assets Held for Sale, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	0	15
Gross Amounts Offset	0	(15)
Net Amounts of Assets/ (Liabilities)	0	0
Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(143)	(144)
Gross Amounts Offset	73	78
Net Amounts of Assets/ (Liabilities)	(70)	(66)
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	0	(115)
Gross Amounts Offset	0	50
Net Amounts of Assets/ (Liabilities)	0	(65)
Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(111)	(102)
Gross Amounts Offset	59	63
Net Amounts of Assets/ (Liabilities)	(52)	(39)
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	0	(28)
Gross Amounts Offset	0	15
Net Amounts of Assets/ (Liabilities)	0	(13)
CECONY
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	97	88
Gross Amounts Offset	(91)	(79)
Net Amounts of Assets/ (Liabilities)	6	9
Gross Amounts of Recognized Assets/(Liabilities)	(214)	(213)
Gross Amounts Offset	108	127
Net Amounts of Assets/ (Liabilities)	(106)	(86)
Gross Amounts of Recognized Assets/(Liabilities)	(117)	(125)
Gross Amounts Offset	17	48
Net Amounts of Assets/ (Liabilities)	(100)	(77)
Margin deposits	12	26
CECONY \| Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	52	40
Gross Amounts Offset	(46)	(32)
Net Amounts of Assets/ (Liabilities)	6	8
CECONY \| Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	45	48
Gross Amounts Offset	(45)	(47)
Net Amounts of Assets/ (Liabilities)	0	1
CECONY \| Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(122)	(121)
Gross Amounts Offset	61	71
Net Amounts of Assets/ (Liabilities)	(61)	(50)
CECONY \| Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(92)	(92)
Gross Amounts Offset	47	56
Net Amounts of Assets/ (Liabilities)	$ (45)	$ (36)

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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, after the effects of master netting arrangements, of a 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. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, before effects of master netting arrangements, of a 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. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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