Regulatory Matters - Summary of Rate Plan (Detail) - USD ($) $ in Millions	1 Months Ended		9 Months Ended	36 Months Ended
	Oct. 31, 2016	Apr. 30, 2016	Sep. 30, 2016	Dec. 31, 2019
CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Additional rate increase			$ 48
Deferrals for property taxes limitation from rates (percent)			90.00%
Deferrals for property taxes limitation from rates (percent)			80.00%
Maximum deferral (percentage)			30.00%
CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Additional rate increase			$ 41
New Jersey Board of Public Utilities | Rockland Electric Company | Year 1 | Rockland Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Increase (decrease) in requested rate amount		$ 10
Return on common equity (percent)		10.20%
Common equity ratio (percent)		49.81%
Scenario, Forecast | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Public utilities, term of rate plan				3 years
Authorized return on common equity (percent)				9.00%
Earnings sharing (percentage)				9.50%
Common equity ratio (percent)				48.00%
Scenario, Forecast | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Public utilities, term of rate plan				3 years
Retention of revenues from non-firm customers (up to)				$ 65
Percentage of revenue reserve				15.00%
Authorized return on common equity (percent)				9.00%
Earnings sharing (percentage)				9.50%
Common equity ratio (percent)				48.00%
Scenario, Forecast | Year 1 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Base rate changes				$ 195
Amortization to income of net regulatory (assets) and liabilities				84
Retention of annual transmission congestion revenues				75
Potential incentive revenue				28
Potential penalty expense				376
Average rate base				$ 18,902
Weighted average cost of capital (after-tax) (percent)				6.82%
Cost of long-term debt (percent)				4.93%
Additional rate increase				$ 199
Deferral, annual maximum (not more than) (percent)				0.10%
Scenario, Forecast | Year 1 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Base rate changes				$ (5)
Amortization to income of net regulatory (assets) and liabilities				39
Potential incentive revenue				7
Potential penalty expense				68
Average rate base				$ 4,841
Weighted average cost of capital (after-tax) (percent)				6.82%
Cost of long-term debt (percent)				4.93%
Scenario, Forecast | Year 2 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Base rate changes				$ 155
Amortization to income of net regulatory (assets) and liabilities				83
Retention of annual transmission congestion revenues				75
Potential incentive revenue				47
Potential penalty expense				383
Average rate base				$ 19,530
Weighted average cost of capital (after-tax) (percent)				6.80%
Cost of long-term debt (percent)				4.88%
Additional rate increase				$ 199
Deferral, annual maximum (not more than) (percent)				0.075%
Scenario, Forecast | Year 2 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Base rate changes				$ 92
Amortization to income of net regulatory (assets) and liabilities				37
Potential incentive revenue				8
Potential penalty expense				75
Average rate base				$ 5,395
Weighted average cost of capital (after-tax) (percent)				6.80%
Cost of long-term debt (percent)				4.88%
Scenario, Forecast | Year 3 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Base rate changes				$ 155
Amortization to income of net regulatory (assets) and liabilities				69
Retention of annual transmission congestion revenues				75
Potential incentive revenue				64
Potential penalty expense				395
Average rate base				$ 20,277
Weighted average cost of capital (after-tax) (percent)				6.73%
Cost of long-term debt (percent)				4.74%
Additional rate increase				$ 199
Deferral, annual maximum (not more than) (percent)				0.05%
Scenario, Forecast | Year 3 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Base rate changes				$ 90
Amortization to income of net regulatory (assets) and liabilities				36
Potential incentive revenue				8
Potential penalty expense				83
Average rate base				$ 6,005
Weighted average cost of capital (after-tax) (percent)				6.73%
Cost of long-term debt (percent)				4.74%
Net plant target, excluding AMI | Scenario, Forecast | Year 1 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				$ 21,689
Net plant target, excluding AMI | Scenario, Forecast | Year 1 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				5,844
Net plant target, excluding AMI | Scenario, Forecast | Year 2 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				22,338
Net plant target, excluding AMI | Scenario, Forecast | Year 2 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				6,512
Net plant target, excluding AMI | Scenario, Forecast | Year 3 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				23,002
Net plant target, excluding AMI | Scenario, Forecast | Year 3 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				7,177
AMI | Scenario, Forecast | Year 1 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				126
AMI | Scenario, Forecast | Year 1 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				27
AMI | Scenario, Forecast | Year 2 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				257
AMI | Scenario, Forecast | Year 2 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				57
AMI | Scenario, Forecast | Year 3 | CECONY | Electric
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				415
AMI | Scenario, Forecast | Year 3 | CECONY | Gas
Public Utilities, General Disclosures [Line Items]
Average net utility plant rates				$ 100
Subsequent Event | New Jersey Board of Public Utilities | Rockland Electric Company | Year 1 | Rockland Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Increase (decrease) in requested rate amount	$ (4)
Return on common equity (percent)	10.20%
Common equity ratio (percent)	50.15%
Subsequent Event | New Jersey Board of Public Utilities | Rockland Electric Company | Year 2 | Rockland Electric Company | Electric
Public Utilities, General Disclosures [Line Items]
Public utilities, requested rate increase (decrease), amount	$ 6