Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Electric) (Details) - USD ($)	12 Months Ended				24 Months Ended	36 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2017	Dec. 31, 2021
Public Utilities, General Disclosures [Line Items]
Deferred revenues	$ 20,000,000	$ 87,000,000
Deferred revenues	$ 514,000,000	598,000,000
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 | Electric
Public Utilities, General Disclosures [Line Items]
Deferred revenues			$ 6,300,000	$ 0
Deferred revenues	$ 500,000	11,200,000
Negative revenue adjustments	0	0	0	1,250,000
Deferral of net increase (decrease) to regulatory assets	5,000,000	3,200,000	7,400,000	300,000
Net utility plant reconciliations	$ 1,400,000	(1,900,000)	(1,900,000)	$ 2,200,000
Authorized return on common equity (percent)					9.00%
Earnings sharing (percent)					9.60%
Earnings sharing, threshold limit		$ 300,000	$ 6,100,000
Common equity ratio (percent)					48.00%
Deferred storm and property reserve deficiency, noncurrent					$ 59,300,000
Deferred storm and property reserve deficiency, recovery period					5 years
Deferred storm and property reserve deficiency not recovered					$ 1,000,000
O&R | Electric | Property Tax and Interest Rate Reconciliations
Public Utilities, General Disclosures [Line Items]
Regulatory assets not recoverable					4,000,000
O&R | Electric | Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes					9,300,000
Amortization to income of net regulatory assets					(8,500,000)
Net utility plant reconciliations					928,000,000
Average rate base					$ 763,000,000
Weighted average cost of capital (after-tax) (percent)					7.10%
Actual return on common equity (percent)					10.80%
Cost of long-term debt (percent)					5.42%
Deferred storm and property reserve deficiency, noncurrent					$ 11,850,000
Rate exclusion amount with balance below regulatory threshold					1,000,000
O&R | Electric | Year 1 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)					4,000,000
O&R | Electric | Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes					8,800,000
Amortization to income of net regulatory assets					(9,400,000)
Net utility plant reconciliations					970,000,000
Average rate base					$ 805,000,000
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%
Deferred storm and property reserve deficiency, noncurrent					$ 11,850,000
Rate exclusion amount with balance below regulatory threshold					9,000,000
O&R | Electric | Year 2 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)					4,000,000
O&R | Electric | Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes					0
Amortization to income of net regulatory assets					0
Average rate base					$ 805,000,000
Weighted average cost of capital (after-tax) (percent)					7.06%
Actual return on common equity (percent)					7.20%
Cost of long-term debt (percent)					5.35%
O&R | Electric | Year 3 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)					$ 4,000,000
O&R | Electric | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Authorized return on common equity (percent)						9.00%
Earnings sharing (percent)						9.60%
Common equity ratio (percent)						48.00%
Deferrals for property taxes limitation from rates (percent)						90.00%
O&R | Electric | Scenario, Forecast | Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes						$ 13,400,000
Amortization to income of net regulatory assets						(1,500,000)
Potential earnings adjustment mechanism incentives						3,600,000
Potential incentive if target is met, related to service terminations						500,000
Average rate base						$ 878,000,000
Weighted average cost of capital (after-tax) (percent)						6.97%
Cost of long-term debt (percent)						5.17%
Requested rate increase (decrease), amount						$ 8,600,000
Deferral, annual maximum (not more than) (percent)						0.10%
O&R | Electric | Scenario, Forecast | Year 1 | Electric average excluding AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						$ 1,008,000,000
O&R | Electric | Scenario, Forecast | Year 1 | AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						48,000,000
O&R | Electric | Scenario, Forecast | Year 1 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						4,000,000
O&R | Electric | Scenario, Forecast | Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes						8,000,000
Amortization to income of net regulatory assets						(1,500,000)
Potential earnings adjustment mechanism incentives						4,000,000
Potential incentive if target is met, related to service terminations						500,000
Average rate base						$ 906,000,000
Weighted average cost of capital (after-tax) (percent)						6.96%
Cost of long-term debt (percent)						5.14%
Requested rate increase (decrease), amount						$ 12,100,000
Deferral, annual maximum (not more than) (percent)						0.075%
O&R | Electric | Scenario, Forecast | Year 2 | Electric average excluding AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						$ 1,032,000,000
O&R | Electric | Scenario, Forecast | Year 2 | AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						58,000,000
O&R | Electric | Scenario, Forecast | Year 2 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						4,000,000
O&R | Electric | Scenario, Forecast | Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes						5,800,000
Amortization to income of net regulatory assets						(1,500,000)
Potential earnings adjustment mechanism incentives						4,200,000
Potential incentive if target is met, related to service terminations						500,000
Average rate base						$ 948,000,000
Weighted average cost of capital (after-tax) (percent)						6.96%
Cost of long-term debt (percent)						5.14%
Requested rate increase (decrease), amount						$ 12,200,000
Deferral, annual maximum (not more than) (percent)						0.05%
O&R | Electric | Scenario, Forecast | Year 3 | Electric average excluding AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						$ 1,083,000,000
O&R | Electric | Scenario, Forecast | Year 3 | AMI
Public Utilities, General Disclosures [Line Items]
Net utility plant reconciliations						61,000,000
O&R | Electric | Scenario, Forecast | Year 3 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)						$ 5,000,000