Summary of Significant Accounting Policies - Capitalized Cost of Utility Plant (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Public Utility, Property, Plant and Equipment [Line Items]
General	$ 2,302	$ 1,944
Held for future use	72	74
Construction work in progress	1,027	1,241
Net Utility Plant	26,301	24,965
Electric [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Generation	434	400
Electric Transmission [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Transmission	2,698	2,654
Electric Distribution [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Distribution	14,658	13,805
Gas Transmission [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas	4,170	3,858
Steam Plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Steam	1,674	1,651
General [Member]
Public Utility, Property, Plant and Equipment [Line Items]
General	1,567	1,282
CECONY [Member]
Public Utility, Property, Plant and Equipment [Line Items]
General	2,126	1,785
Held for future use	62	62
Construction work in progress	947	1,165
Net Utility Plant	24,739	23,496
CECONY [Member] | Electric [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Generation	434	400
CECONY [Member] | Electric Transmission [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Transmission	2,518	2,476
CECONY [Member] | Electric Distribution [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Distribution	13,930	13,125
CECONY [Member] | Gas Transmission [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas	3,735	3,455
CECONY [Member] | Steam Plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Steam	1,674	1,651
CECONY [Member] | General [Member]
Public Utility, Property, Plant and Equipment [Line Items]
General	$ 1,439	$ 1,162