Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Net Amounts of Assets/ (Liabilities)	$ 369	$ 345
Net Amounts of Assets/ (Liabilities)	(143)	(183)
Gross Amounts of Recognized Assets/(Liabilities)	(127)	(207)
Gross Amounts Offset	19	46
Net Amounts of Assets/ (Liabilities)	(108)	(161)
Margin deposits	16	26
Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	78	59
Gross Amounts Offset	(50)	(41)
Net Amounts of Assets/ (Liabilities)	28	18
Fair Value of Derivative Assets, Assets Held for Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	45	51
Gross Amounts Offset	(42)	(50)
Net Amounts of Assets/ (Liabilities)	3	1
Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	33	57
Gross Amounts Offset	(32)	(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)	13	15
Gross Amounts Offset	(10)	(15)
Net Amounts of Assets/ (Liabilities)	3	0
Fair Value of Derivative Assets
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	169	182
Gross Amounts Offset	(134)	(160)
Net Amounts of Assets/ (Liabilities)	35	22
Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(129)	(144)
Gross Amounts Offset	67	78
Net Amounts of Assets/ (Liabilities)	(62)	(66)
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(76)	(115)
Gross Amounts Offset	42	50
Net Amounts of Assets/ (Liabilities)	(34)	(65)
Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(67)	(102)
Gross Amounts Offset	34	63
Net Amounts of Assets/ (Liabilities)	(33)	(39)
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(24)	(28)
Gross Amounts Offset	10	15
Net Amounts of Assets/ (Liabilities)	(14)	(13)
Fair Value of Derivative Liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(296)	(389)
Gross Amounts Offset	153	206
Net Amounts of Assets/ (Liabilities)	(143)	(183)
CECONY
Derivatives, Fair Value [Line Items]
Net Amounts of Assets/ (Liabilities)	323	311
Gross Amounts of Recognized Assets/(Liabilities)	(87)	(125)
Gross Amounts Offset	23	48
Net Amounts of Assets/ (Liabilities)	(64)	(77)
Margin deposits	16	26
CECONY \| Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	57	40
Gross Amounts Offset	(47)	(32)
Net Amounts of Assets/ (Liabilities)	10	8
CECONY \| Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	24	48
Gross Amounts Offset	(24)	(47)
Net Amounts of Assets/ (Liabilities)	0	1
CECONY \| Fair Value of Derivative Assets
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	81	88
Gross Amounts Offset	(71)	(79)
Net Amounts of Assets/ (Liabilities)	10	9
CECONY \| Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(109)	(121)
Gross Amounts Offset	65	71
Net Amounts of Assets/ (Liabilities)	(44)	(50)
CECONY \| Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(59)	(92)
Gross Amounts Offset	29	56
Net Amounts of Assets/ (Liabilities)	(30)	(36)
CECONY \| Fair Value of Derivative Liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(168)	(213)
Gross Amounts Offset	94	127
Net Amounts of Assets/ (Liabilities)	$ (74)	$ (86)

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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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