Summary of Significant Accounting Policies and Other Matters - Plant and Depreciation (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Accumulated amortization		$ 29	$ 15
Annual aggregate depreciation allowance		1,323
Software Licenses
Public Utility, Property, Plant and Equipment [Line Items]
Gross asset		100
Amortization period	15 years
Estimated aggregate annual amortization expense	$ 7
Accumulated amortization		$ 3
Non-Utility Plant | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		3 years
Non-Utility Plant | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		30 years
CECONY
Public Utility, Property, Plant and Equipment [Line Items]
AFUDC rates (percent)		5.40%	5.50%	4.70%
Average depreciation rates (percent)		3.10%	3.10%	3.10%
Annual aggregate depreciation allowance		$ 1,253
CECONY | Software Licenses
Public Utility, Property, Plant and Equipment [Line Items]
Gross asset		$ 95
CECONY | Electric | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		5 years
CECONY | Electric | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		95 years
CECONY | Gas | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		5 years
CECONY | Gas | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		100 years
CECONY | Steam | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		5 years
CECONY | Steam | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		80 years
CECONY | General Plant | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		5 years
CECONY | General Plant | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		55 years
O&R
Public Utility, Property, Plant and Equipment [Line Items]
AFUDC rates (percent)		2.20%	2.50%	3.50%
Average depreciation rates (percent)		2.90%	2.90%	2.90%
O&R | Electric | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		5 years
O&R | Electric | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		75 years
O&R | Gas | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		5 years
O&R | Gas | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		75 years
O&R | General Plant | Minimum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		5 years
O&R | General Plant | Maximum
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful lives (years)		50 years