Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ (162)	$ (163)
Gross Amounts Offset	49	140
Net Amounts of Assets/ (Liabilities) (a)	(113)	(23)
Margin deposits	22	27
Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	70	111
Gross Amounts Offset	(50)	(67)
Net Amounts of Assets/ (Liabilities) (a)	20	44
Fair Value of Derivative Assets, Assets Held for Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	55	0
Gross Amounts Offset	(53)	0
Net Amounts of Assets/ (Liabilities) (a)	2	0
Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	24	34
Gross Amounts Offset	(21)	(23)
Net Amounts of Assets/ (Liabilities) (a)	3	11
Fair Value of Derivative Assets
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	149	145
Gross Amounts Offset	(124)	(90)
Net Amounts of Assets/ (Liabilities) (a)	25	55
Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(110)	(242)
Gross Amounts Offset	78	139
Net Amounts of Assets/ (Liabilities) (a)	(32)	(103)
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(100)	0
Gross Amounts Offset	43	0
Net Amounts of Assets/ (Liabilities) (a)	(57)	0
Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(66)	(66)
Gross Amounts Offset	42	91
Net Amounts of Assets/ (Liabilities) (a)	(24)	25
Fair Value of Derivative Liabilities, Liabilities Held For Sale, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(35)	0
Gross Amounts Offset	10	0
Net Amounts of Assets/ (Liabilities) (a)	(25)	0
Fair Value of Derivative Liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(311)	(308)
Gross Amounts Offset	173	230
Net Amounts of Assets/ (Liabilities) (a)	(138)	(78)
CECONY
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(78)	(90)
Gross Amounts Offset	48	45
Net Amounts of Assets/ (Liabilities) (a)	(30)	(45)
Margin deposits	21	25
CECONY | Fair Value of Derivative Assets, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	46	26
Gross Amounts Offset	(36)	(15)
Net Amounts of Assets/ (Liabilities) (a)	10	11
CECONY | Fair Value of Derivative Assets, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	19	22
Gross Amounts Offset	(17)	(20)
Net Amounts of Assets/ (Liabilities) (a)	2	2
CECONY | Fair Value of Derivative Assets
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	65	48
Gross Amounts Offset	(53)	(35)
Net Amounts of Assets/ (Liabilities) (a)	12	13
CECONY | Fair Value of Derivative Liabilities, Current
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(86)	(96)
Gross Amounts Offset	63	48
Net Amounts of Assets/ (Liabilities) (a)	(23)	(48)
CECONY | Fair Value of Derivative Liabilities, Noncurrent
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(57)	(42)
Gross Amounts Offset	38	32
Net Amounts of Assets/ (Liabilities) (a)	(19)	(10)
CECONY | Fair Value of Derivative Liabilities
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(143)	(138)
Gross Amounts Offset	101	80
Net Amounts of Assets/ (Liabilities) (a)	$ (42)	$ (58)