Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Gas) (Details) - USD ($)	12 Months Ended					36 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2021	Oct. 31, 2018
Public Utilities, General Disclosures [Line Items]
Deferred revenues	$ 599,000,000	$ 514,000,000
Deferred revenues	$ 17,000,000	20,000,000	$ 87,000,000
NYSPSC
Public Utilities, General Disclosures [Line Items]
Percentage of total consolidated revenues	15.00%
Percentage of debt to total consolidated debt	20.00%
Maximum | NYSPSC
Public Utilities, General Disclosures [Line Items]
Percentage of total consolidated revenues	15.00%
O&R | Gas
Public Utilities, General Disclosures [Line Items]
Deferred revenues					$ 800,000
Deferred revenues		6,300,000	1,700,000	$ 6,200,000
Negative revenue adjustments	$ 200,000	100,000	0	0	0
Cost reconciliation, deferred net regulatory liabilities		7,400,000	3,500,000		4,500,000
Cost reconciliation, deferred net regulatory assets				6,600,000
Deferred regulatory asset (liability)		$ (400,000)			$ 0
Authorized return on common equity (percent)							9.00%
Earnings sharing (percent)							9.60%
Earnings sharing, threshold limit			$ 200,000	$ 4,000,000
Common equity ratio (percent)							48.00%
O&R | Gas | Property Tax and Interest Rate Reconciliations
Public Utilities, General Disclosures [Line Items]
Regulatory assets not recoverable							$ 14,000,000
O&R | Gas | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Amount of revenues retained						$ 4,000,000
Share in variances in annual revenue retained (percent)						20.00%
Potential earnings adjustment mechanism incentives						$ 300,000
Authorized return on common equity (percent)						9.00%
Earnings sharing (percent)						9.60%
Common equity ratio (percent)						48.00%
O&R | Gas | Scenario, Forecast | Customers
Public Utilities, General Disclosures [Line Items]
Share in variances in annual revenue retained (percent)						80.00%
O&R | Gas | Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes							16,400,000
Amortization to income of net regulatory assets							1,700,000
Net utility plant reconciliations							492,000,000
Average rate base							$ 366,000,000
Weighted average cost of capital (after-tax) (percent)							7.10%
Actual return on common equity (percent)							11.20%
Cost of long-term debt (percent)							5.42%
Rate exclusion amount with balance below regulatory threshold							$ 500,000
O&R | Gas | Year 1 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							3,700,000
O&R | Gas | Year 1 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ (7,500,000)
Amortization to income of net regulatory assets						1,800,000
Potential earnings adjustment mechanism incentives						1,200,000
Net utility plant reconciliations						593,000,000
Average rate base						$ 454,000,000
Weighted average cost of capital (after-tax) (percent)						6.97%
Cost of long-term debt (percent)						5.17%
Requested rate increase (decrease), amount						$ (5,900,000)
O&R | Gas | Year 1 | Scenario, Forecast | AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						20,000,000
O&R | Gas | Year 1 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						5,500,000
O&R | Gas | Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes							16,400,000
Amortization to income of net regulatory assets							2,100,000
Net utility plant reconciliations							518,000,000
Average rate base							$ 391,000,000
Weighted average cost of capital (after-tax) (percent)							7.06%
Actual return on common equity (percent)							9.70%
Cost of long-term debt (percent)							5.35%
Rate exclusion amount with balance below regulatory threshold							$ 4,200,000
O&R | Gas | Year 2 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							4,700,000
O&R | Gas | Year 2 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes						3,600,000
Amortization to income of net regulatory assets						1,800,000
Potential earnings adjustment mechanism incentives						1,300,000
Net utility plant reconciliations						611,000,000
Average rate base						$ 476,000,000
Weighted average cost of capital (after-tax) (percent)						6.96%
Cost of long-term debt (percent)						5.14%
Requested rate increase (decrease), amount						$ 1,000,000.0
O&R | Gas | Year 2 | Scenario, Forecast | AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						24,000,000
O&R | Gas | Year 2 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						5,700,000
O&R | Gas | Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes							5,800,000
Base rate change through surcharge							10,600,000
Amortization to income of net regulatory assets							2,500,000
Net utility plant reconciliations							546,000,000
Average rate base							$ 417,000,000
Weighted average cost of capital (after-tax) (percent)							7.06%
Actual return on common equity (percent)							8.00%
Cost of long-term debt (percent)							5.35%
Rate exclusion amount with balance below regulatory threshold							$ 7,200,000
O&R | Gas | Year 3 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							$ 4,900,000
O&R | Gas | Year 3 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes						700,000
Amortization to income of net regulatory assets						1,800,000
Potential earnings adjustment mechanism incentives						1,300,000
Net utility plant reconciliations						632,000,000
Average rate base						$ 498,000,000
Weighted average cost of capital (after-tax) (percent)						6.96%
Cost of long-term debt (percent)						5.14%
Requested rate increase (decrease), amount						$ 1,000,000.0
O&R | Gas | Year 3 | Scenario, Forecast | AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						25,000,000
O&R | Gas | Year 3 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						$ 6,000,000
Energy Efficiency | O&R | Gas
Public Utilities, General Disclosures [Line Items]
Actual earnings adjustment mechanism incentives	300,000
Service Termination | O&R | Gas
Public Utilities, General Disclosures [Line Items]
Actual earnings adjustment mechanism incentives	$ 700,000