Regulatory Assets and Liabilities - Additional Information (Detail) $ in Millions		1 Months Ended	3 Months Ended		12 Months Ended
	Oct. 16, 2017	Dec. 31, 2016	Mar. 31, 2018 USD ($) Station	Mar. 31, 2017 USD ($)	Apr. 30, 2019	Dec. 31, 2018	Apr. 30, 2018	Apr. 30, 2017	Dec. 31, 2017 USD ($)
Regulatory Assets And Liabilities [Line Items]
Unrecorded Regulatory Assets			$ 1,867.0
Equity ratio		50.00%
Public utilities regulatory authority distribution rate		9.10%
Unfunded future Income tax expense collection period			50 years
Federal statutory income tax rate			35.00%	35.00%
UIL Holdings [Member]
Regulatory Assets And Liabilities [Line Items]
Business combination merger related rate credits			$ 1.0	$ 2.0
NEW YORK
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
NEW YORK | Prior Tax Rate [Member]
Regulatory Assets And Liabilities [Line Items]
Statutory income tax rate, state			7.10%
NEW YORK | Revised Tax Rate [Member]
Regulatory Assets And Liabilities [Line Items]
Statutory income tax rate, state			6.50%
NEW YORK | Oswego [Member]
Regulatory Assets And Liabilities [Line Items]
Number of nuclear generating stations | Station			2
Asset Sale Gain Account [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
Carrying Costs On Deferred Income Tax Bonus Depreciation [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
Economic Development [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
Merger Capital Expense [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
Positive Benefit Adjustment [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
Post Term Amortization [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
Theoretical Reserve Flow Thru Impact [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
Maximum [Member]
Regulatory Assets And Liabilities [Line Items]
Approved return on equity			11.74%
Maximum [Member] | NEW YORK
Regulatory Assets And Liabilities [Line Items]
Deferred income tax recovery period			39 years
Minimum [Member] | NEW YORK
Regulatory Assets And Liabilities [Line Items]
Deferred income tax recovery period			27 years
Scenario, Plan [Member]
Regulatory Assets And Liabilities [Line Items]
Federal statutory income tax rate						21.00%
Electric and Gas Service Rate Plan Year One [Member]
Regulatory Assets And Liabilities [Line Items]
Customer receiving percentage			50.00%
Return on equity								9.50%
Electric and Gas Service Rate Plan Year One [Member] | Scenario, Forecast [Member]
Regulatory Assets And Liabilities [Line Items]
Return on equity					9.75%		9.65%
Electric and Gas Service Rate Plan Year Two [Member]
Regulatory Assets And Liabilities [Line Items]
Customer receiving percentage			75.00%
Return on equity								10.00%
Electric and Gas Service Rate Plan Year Two [Member] | Scenario, Forecast [Member]
Regulatory Assets And Liabilities [Line Items]
Return on equity					10.25%		10.15%
Electric and Gas Service Rate Plan Year Three [Member]
Regulatory Assets And Liabilities [Line Items]
Customer receiving percentage			90.00%
Return on equity								10.50%
Electric and Gas Service Rate Plan Year Three [Member] | Scenario, Forecast [Member]
Regulatory Assets And Liabilities [Line Items]
Return on equity					10.75%		10.65%
New York State Electric And Gas Corporation
Regulatory Assets And Liabilities [Line Items]
Annual amortization of regulatory items			$ 16.5
Regulatory items amortization period			3 years
Approved return on equity			9.00%
Equity ratio			48.00%
New York State Electric And Gas Corporation | Maximum [Member]
Regulatory Assets And Liabilities [Line Items]
Equity ratio for earnings sharing			50.00%
New York State Electric And Gas Corporation | Storm Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			10 years
New York State Electric And Gas Corporation | Regulatory Items Other Than Storm Costs [ Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
New York State Electric And Gas Corporation | Deferred Income Tax Charge [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			50 years
RG&E [Member]
Regulatory Assets And Liabilities [Line Items]
Approved return on equity			9.00%
Equity ratio			48.00%
RG&E [Member] | Deferred Property Tax [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
RG&E [Member] | Maximum [Member]
Regulatory Assets And Liabilities [Line Items]
Equity ratio for earnings sharing			50.00%
RG&E [Member] | Deferred Income Tax Charge [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			50 years
United Illuminating Company (UI)
Regulatory Assets And Liabilities [Line Items]
Public utilities nature of allowance for earnings on equity, description			UI and its customers share on a 50/50 basis all distribution earnings above the allowed ROE in a calendar year
SCG [Member]
Regulatory Assets And Liabilities [Line Items]
Equity ratio	52.00%
Public utilities regulatory authority distribution rate	9.25%
Proposed ROE for the year 2018			9.95%
Proposed ROE for the year 2019			9.95%
Proposed ROE for the year 2020			9.95%
Amount of proposed ROE for the year 2018			$ 1.5
Amount of proposed ROE for the year 2019			4.7
Amount of proposed ROE for the year 2020			5.0
Central Maine Power Company [Member]
Regulatory Assets And Liabilities [Line Items]
Recovery of deferred storm costs			$ 123.0
Central Maine Power Company [Member] | Storm Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			10 years
Central Maine Power Company [Member] | Regulatory Items Other Than Storm Costs [ Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years
UI [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets			$ 103.0						$ 93.0
UI [Member] | Storm Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Deferred costs			1.0
Regulatory assets			$ 0.0
NYSEG [Member] | Deferred Property Tax [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period			5 years