Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Gas) (Details) - USD ($)	12 Months Ended				36 Months Ended
	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	$ 514,000,000	$ 598,000,000
Deferred revenues	$ 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	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
Net utility plant reconciliations	$ 400,000	0	0	$ 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
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
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