Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ (132)	$ (163)
Gross Amounts Offset	158	140
Net Amounts of Assets/(Liabilities)	26	(23)
Fair Value of Derivative Assets, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	108	111
Gross Amounts Offset	(23)	(67)
Net Amounts of Assets/(Liabilities)	85	44
Fair Value of Derivative Assets, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	30	34
Gross Amounts Offset	(28)	(23)
Net Amounts of Assets/(Liabilities)	2	11
Fair Value of Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	138	145
Gross Amounts Offset	(51)	(90)
Net Amounts of Assets/(Liabilities)	87	55
Fair Value of Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(174)	(242)
Gross Amounts Offset	137	139
Net Amounts of Assets/(Liabilities)	(37)	(103)
Fair Value of Derivative Liabilities, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(96)	(66)
Gross Amounts Offset	72	91
Net Amounts of Assets/(Liabilities)	(24)	25
Fair Value of Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(270)	(308)
Gross Amounts Offset	209	230
Net Amounts of Assets/(Liabilities)	(61)	(78)
CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(65)	(90)
Gross Amounts Offset	39	45
Net Amounts of Assets/(Liabilities)	(26)	(45)
CECONY [Member] | Fair Value of Derivative Assets, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	40	26
Gross Amounts Offset	(29)	(15)
Net Amounts of Assets/(Liabilities)	11	11
CECONY [Member] | Fair Value of Derivative Assets, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	17	22
Gross Amounts Offset	(15)	(20)
Net Amounts of Assets/(Liabilities)	2	2
CECONY [Member] | Fair Value of Derivative Assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	57	48
Gross Amounts Offset	(44)	(35)
Net Amounts of Assets/(Liabilities)	13	13
CECONY [Member] | Fair Value of Derivative Liabilities, Current [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(69)	(96)
Gross Amounts Offset	46	48
Net Amounts of Assets/(Liabilities)	(23)	(48)
CECONY [Member] | Fair Value of Derivative Liabilities, Noncurrent [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(53)	(42)
Gross Amounts Offset	37	32
Net Amounts of Assets/(Liabilities)	(16)	(10)
CECONY [Member] | Fair Value of Derivative Liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(122)	(138)
Gross Amounts Offset	83	80
Net Amounts of Assets/(Liabilities)	$ (39)	$ (58)