Regulatory Matters - Summary of Utilities Rate Plans (O&R New York-Electric) (Details) - O&R - Electric - USD ($)	12 Months Ended						24 Months Ended	40 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2017	Oct. 31, 2015
Public Utilities, General Disclosures [Line Items]
Deferred revenues	$ 11,200,000	$ 6,300,000	$ 0	$ (3,400,000)	$ 3,200,000	$ 2,600,000
Negative revenue adjustments	0	0	1,250,000	0	0	0
Deferral of net increase (decrease) to regulatory assets	3,200,000	7,400,000	300,000	(200,000)	4,100,000	7,800,000
Net utility plant reconciliations	(1,900,000)	(1,900,000)	$ 2,200,000	(2,300,000)	$ (1,100,000)	$ 4,200,000
Authorized return on common equity (percent)							9.00%
Earnings sharing (percentage)							9.60%
Earnings sharing, threshold limit	$ 300,000	$ 6,100,000		$ 1,000,000
Common equity ratio (percent)							48.00%	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
Property Tax and Interest Rate Reconciliations
Public Utilities, General Disclosures [Line Items]
Regulatory assets not recoverable							4,000,000
Year 1
Public Utilities, General Disclosures [Line Items]
Base rate changes							9,300,000	$ 19,400,000
Amortization to income of net regulatory assets							(8,500,000)	(32,200,000)
Net utility plant reconciliations							928,000,000	678,000,000
Average rate base							$ 763,000,000	$ 671,000,000
Weighted average cost of capital (after-tax) (percent)							7.10%	7.61%
Authorized return on common equity (percent)								9.40%
Actual return on common equity (percent)							10.80%	12.90%
Cost of long-term debt (percent)							5.42%	6.07%
Deferred storm and property reserve deficiency, noncurrent							$ 11,850,000
Rate exclusion amount with balance below regulatory threshold							1,000,000
Year 1 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							4,000,000	$ 3,000,000
Year 2
Public Utilities, General Disclosures [Line Items]
Base rate changes							8,800,000	$ 8,800,000
Amortization to income of net regulatory assets							(9,400,000)
Regulatory assets, amortization period								3 years
Net utility plant reconciliations							970,000,000	$ 704,000,000
Average rate base							$ 805,000,000	$ 708,000,000
Weighted average cost of capital (after-tax) (percent)							7.06%	7.65%
Authorized return on common equity (percent)								9.50%
Actual return on common equity (percent)							9.70%	8.70%
Cost of long-term debt (percent)							5.35%	6.07%
Deferred storm and property reserve deficiency, noncurrent							$ 11,850,000
Rate exclusion amount with balance below regulatory threshold							9,000,000
Year 2 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							4,000,000	$ 3,000,000
Year 3
Public Utilities, General Disclosures [Line Items]
Base rate changes								15,200,000
Net utility plant reconciliations								753,000,000
Average rate base								$ 759,000,000
Weighted average cost of capital (after-tax) (percent)								7.48%
Authorized return on common equity (percent)								9.60%
Actual return on common equity (percent)								9.40%
Cost of long-term debt (percent)								5.64%
Year 3 | Maximum
Public Utilities, General Disclosures [Line Items]
Potential penalties (annually)							$ 4,000,000	$ 3,000,000