Summary of Significant Accounting Policies - Capitalized Cost of Utility Plant (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
General	$ 3,008	$ 2,719
Held for future use	76	74
Construction work in progress	1,605	1,175
NET UTILITY PLANT	35,273	33,045
Electric Generation
Public Utility, Property, Plant and Equipment [Line Items]
Generation	544	479
Electric Transmission
Public Utility, Property, Plant and Equipment [Line Items]
Transmission	3,210	3,184
Electric Distribution
Public Utility, Property, Plant and Equipment [Line Items]
Distribution	18,959	18,150
Gas
Public Utility, Property, Plant and Equipment [Line Items]
Gas	6,976	6,285
Steam
Public Utility, Property, Plant and Equipment [Line Items]
Steam	1,798	1,882
General
Public Utility, Property, Plant and Equipment [Line Items]
General	2,105	1,816
CECONY
Public Utility, Property, Plant and Equipment [Line Items]
General	2,753	2,490
Held for future use	67	65
Construction work in progress	1,502	1,104
NET UTILITY PLANT	33,205	31,115
CECONY | Electric Generation
Public Utility, Property, Plant and Equipment [Line Items]
Generation	544	479
CECONY | Electric Transmission
Public Utility, Property, Plant and Equipment [Line Items]
Transmission	2,990	2,963
CECONY | Electric Distribution
Public Utility, Property, Plant and Equipment [Line Items]
Distribution	17,996	17,234
CECONY | Gas
Public Utility, Property, Plant and Equipment [Line Items]
Gas	6,403	5,749
CECONY | Steam
Public Utility, Property, Plant and Equipment [Line Items]
Steam	1,798	1,882
CECONY | General
Public Utility, Property, Plant and Equipment [Line Items]
General	$ 1,905	$ 1,639