Regulatory Matters - Summary of Utilities Rate Plans (CECONY-Gas) (Details) - CECONY - Gas - USD ($)	1 Months Ended	12 Months Ended				36 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Amortization to income of net regulatory (assets) and liabilities								$ 4,000,000
Regulatory liabilities, amortization period								3 years
Amount of revenues retained	$ 65,000,000		$ 65,000,000	$ 66,000,000	$ 70,000,000			$ 65,000,000
Percentage of revenue reserve								15.00%
Deferred revenues	71,000,000	$ 3,000,000	71,000,000	54,000,000	28,000,000			$ 71,000,000
Negative revenue adjustments		5,000,000	0	0	0
Cost reconciliations, deferred net regulatory liabilities	$ 32,000,000	2,000,000	32,000,000	11,000,000	38,000,000			$ 32,000,000
Net utility plant reconciliations		$ 2,200,000	$ 0	1,000,000	0
Earnings sharing (percentage)	9.90%		9.90%					9.90%
Earnings sharing, threshold limit			$ 0	0	0
Common equity ratio (percent)								48.00%
Base rate change deferral regulatory liability impact	$ 32,000,000		32,000,000					$ 32,000,000
Increase in gas base rate due to expiration of temporary credit under the prior rate plan	41,000,000
Difference in property taxes (percent)								90.00%
Maximum
Public Utilities, General Disclosures [Line Items]
Amount of revenues retained	$ 65,000,000		65,000,000					$ 65,000,000
Potential penalties (annually)			$ 56,000,000	$ 44,000,000	$ 33,000,000
Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Percentage of revenue reserve							15.00%
Other earnings incentives						$ 7,000,000
Earnings sharing (percentage)							9.50%
Common equity ratio (percent)							48.00%
Deferral, annual maximum (not more than) (percent)							0.10%
Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Amount of revenues retained							$ 65,000,000
Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes								(54,600,000)
Average rate base								$ 3,521,000,000
Weighted average cost of capital (after-tax) (percent)								7.10%
Authorized return on common equity (percent)								9.30%
Actual return on common equity (percent)								8.02%
Cost of long-term debt (percent)	5.17%		5.17%					5.17%
Year 1 | Gas delivery
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations								$ 3,899,000,000
Year 1 | Storm hardening
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations								3,000,000
Year 1 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes							(5,000,000)
Amortization to income of net regulatory (assets) and liabilities							39,000,000
Potential incentives if performance targets are met							7,000,000
Potential penalties (annually)							68,000,000
Average rate base							$ 4,841,000,000
Weighted average cost of capital (after-tax) (percent)							6.82%
Authorized return on common equity (percent)							9.00%
Actual return on common equity (percent)							9.22%
Cost of long-term debt (percent)							4.93%
Year 1 | Scenario, Forecast | Gas average excluding AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations							$ 5,844,000,000
Year 1 | Scenario, Forecast | Advanced metering infrastructure (AMI)
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations							27,000,000
Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes								38,600,000
Average rate base								$ 3,863,000,000
Weighted average cost of capital (after-tax) (percent)								7.13%
Actual return on common equity (percent)								8.13%
Cost of long-term debt (percent)	5.23%		5.23%					5.23%
Year 2 | Gas delivery
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations								$ 4,258,000,000
Year 2 | Storm hardening
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations								8,000,000
Year 2 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes							92,000,000
Amortization to income of net regulatory (assets) and liabilities							37,000,000
Potential incentives if performance targets are met							8,000,000
Potential penalties (annually)							63,000,000
Average rate base							$ 5,395,000,000
Weighted average cost of capital (after-tax) (percent)							6.80%
Cost of long-term debt (percent)							4.88%
Year 2 | Scenario, Forecast | Gas average excluding AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations							$ 6,512,000,000
Year 2 | Scenario, Forecast | Advanced metering infrastructure (AMI)
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations							57,000,000
Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes								56,800,000
Average rate base								$ 4,236,000,000
Weighted average cost of capital (after-tax) (percent)								7.21%
Actual return on common equity (percent)								7.83%
Cost of long-term debt (percent)	5.39%		5.39%					5.39%
Year 3 | Gas delivery
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations								$ 4,698,000,000
Year 3 | Storm hardening
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations								$ 30,000,000
Year 3 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes							90,000,000
Amortization to income of net regulatory (assets) and liabilities							36,000,000
Potential incentives if performance targets are met							8,000,000
Potential penalties (annually)							70,000,000
Average rate base							$ 6,005,000,000
Weighted average cost of capital (after-tax) (percent)							6.73%
Cost of long-term debt (percent)							4.74%
Year 3 | Scenario, Forecast | Gas average excluding AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations							$ 7,177,000,000
Year 3 | Scenario, Forecast | Advanced metering infrastructure (AMI)
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations							$ 100,000,000