Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ (163)	$ 53
Gross Amounts Offset	140	(3)
Net Amounts of Assets/(Liabilities)	(23)	50
Fair Value of Derivative Assets, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	111	134
Gross Amounts Offset	(67)	(77)
Net Amounts of Assets/(Liabilities)	44	57
Fair Value of Derivative Assets, Non-current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	34	32
Gross Amounts Offset	(23)	(24)
Net Amounts of Assets/(Liabilities)	11	8
Fair Value of Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	145	166
Gross Amounts Offset	(90)	(101)
Net Amounts of Assets/(Liabilities)	55	65
Fair Value of Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(242)	(82)
Gross Amounts Offset	139	72
Net Amounts of Assets/(Liabilities)	(103)	(10)
Fair Value of Derivative Liabilities, Non-current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(66)	(31)
Gross Amounts Offset	91	26
Net Amounts of Assets/(Liabilities)	25	(5)
Fair Value of Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(308)	(113)
Gross Amounts Offset	230	98
Net Amounts of Assets/(Liabilities)	(78)	(15)
CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(90)	(10)
Gross Amounts Offset	45	5
Net Amounts of Assets/(Liabilities)	(45)	(5)
CECONY [Member] | Fair Value of Derivative Assets, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	26	27
Gross Amounts Offset	(15)	(19)
Net Amounts of Assets/(Liabilities)	11	8
CECONY [Member] | Fair Value of Derivative Assets, Non-current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	22	14
Gross Amounts Offset	(20)	(13)
Net Amounts of Assets/(Liabilities)	2	1
CECONY [Member] | Fair Value of Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	48	41
Gross Amounts Offset	(35)	(32)
Net Amounts of Assets/(Liabilities)	13	9
CECONY [Member] | Fair Value of Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(96)	(32)
Gross Amounts Offset	48	21
Net Amounts of Assets/(Liabilities)	(48)	(11)
CECONY [Member] | Fair Value of Derivative Liabilities, Non-current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(42)	(19)
Gross Amounts Offset	32	16
Net Amounts of Assets/(Liabilities)	(10)	(3)
CECONY [Member] | Fair Value of Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(138)	(51)
Gross Amounts Offset	80	37
Net Amounts of Assets/(Liabilities)	$ (58)	$ (14)