Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Gas) (Details) - O&R - Gas - USD ($)	12 Months Ended						36 Months Ended	72 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2018	Oct. 31, 2015
Public Utilities, General Disclosures [Line Items]
Amortization to income of net regulatory assets								$ (2,000,000)
Regulatory assets, amortization period								3 years
Deferred revenues	$ 1,700,000	$ 6,200,000	$ (800,000)	$ (100,000)	$ 700,000	$ 4,700,000
Negative revenue adjustments	0	0	0	0	0	0
Cost reconciliations	(3,500,000)	6,600,000	(4,500,000)	8,300,000	8,300,000	700,000
Net utility plant reconciliations	0	0	0	0	0	700,000
Weighted average cost of capital (after-tax) (percent)								8.49%
Authorized return on common equity (percent)								10.40%
Earnings sharing, threshold limit	$ 200,000	$ 4,000,000	$ 0	$ 0	$ 0	$ 0
Cost of long-term debt (percent)								6.81%
Common equity ratio (percent)								48.00%
Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Authorized return on common equity (percent)							9.00%
Earnings sharing (percentage)							9.60%
Common equity ratio (percent)							48.00%
Scenario, Forecast | Property Tax and Interest Rate Reconciliations
Public Utilities, General Disclosures [Line Items]
Regulatory assets not recoverable							$ 14,000,000
Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes								$ 9,000,000
Average rate base								$ 280,000,000
Actual return on common equity (percent)								10.20%
Earnings sharing (percentage)				11.40%
Year 1 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)								$ 1,400,000
Year 1 | Scenario, Forecast
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
Year 1 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							3,700,000
Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes								9,000,000
Average rate base								$ 296,000,000
Actual return on common equity (percent)								9.60%
Year 2 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)								$ 1,400,000
Year 2 | Scenario, Forecast
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
Year 2 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							4,700,000
Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes								4,600,000
Base rate change through surcharge								4,300,000
Average rate base								$ 309,000,000
Actual return on common equity (percent)								12.60%
Year 3 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)								$ 1,400,000
Year 3 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate changes							5,800,000
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%
Cost of long-term debt (percent)							5.35%
Rate exclusion amount with balance below regulatory threshold							$ 7,200,000
Year 3 | Scenario, Forecast | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							4,900,000
Year 4
Public Utilities, General Disclosures [Line Items]
Base rate changes								$ 0
Actual return on common equity (percent)								10.20%
Year 4 | Scenario, Forecast
Public Utilities, General Disclosures [Line Items]
Base rate change through surcharge							$ 10,600,000
Year 5
Public Utilities, General Disclosures [Line Items]
Base rate changes								$ 0
Actual return on common equity (percent)								6.10%