Regulatory Assets and Liabilities - Additional Information (Detail) - USD ($) $ in Millions										1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 17, 2019	Feb. 22, 2019	Jan. 18, 2019	Dec. 19, 2018	Oct. 15, 2018	Aug. 30, 2018	May 17, 2018	Aug. 25, 2014	Jul. 01, 2014	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2017	Dec. 31, 2018
Regulatory Assets And Liabilities [Line Items]
Unrecorded regulatory assets												$ 1,762.0		$ 1,762.0
Equity ratio											50.00%
Public utilities regulatory authority distribution rate											9.10%
Equity ratio, year three												55.00%		55.00%
Rate increase agreement, term						3 years
Regulatory assets												$ 2,801.0		$ 2,801.0					$ 2,945.0
Unfunded future Income tax expense collection period														50 years
NEW YORK
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
Asset sale gain account
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
Carrying costs on deferred income tax bonus depreciation
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
Economic development
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
Merger capital expense target customer credit
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
Positive benefit adjustment
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
Theoretical reserve flow thru impact
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
UIL Holdings
Regulatory Assets And Liabilities [Line Items]
Business combination merger related rate credits												0.0	$ 1.0	$ 1.0	$ 2.0
Maximum | NEW YORK
Regulatory Assets And Liabilities [Line Items]
Deferred income tax recovery period														39 years
Minimum | NEW YORK
Regulatory Assets And Liabilities [Line Items]
Deferred income tax recovery period														27 years
Electric and Gas Service Rate Plan Year One
Regulatory Assets And Liabilities [Line Items]
Customer receiving percentage														50.00%
Return on equity																9.75%	9.65%	9.50%
Electric and Gas Service Rate Plan Year Two
Regulatory Assets And Liabilities [Line Items]
Customer receiving percentage														75.00%
Return on equity																10.25%	10.15%	10.00%
Electric and Gas Service Rate Plan Year Three
Regulatory Assets And Liabilities [Line Items]
Customer receiving percentage														90.00%
Return on equity																10.75%	10.65%	10.50%
Storm costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets												$ 342.0		$ 342.0					$ 346.0
Central Maine Power
Regulatory Assets And Liabilities [Line Items]
Annual distribution tariff increase								$ 24.3
Distribution tariff rate increased based on ROE									9.45%
Distribution tariff rate increased based on equity capital									50.00%
Recovery of deferred storm costs														84.0
Equity ratio					55.00%
Return on equity	8.75%	9.35%
Current Cap On Shared Service Costs	$ 31.4
Public utilities regulatory authority distribution rate					10.00%
Central Maine Power | Maximum
Regulatory Assets And Liabilities [Line Items]
Proposed tariff rate decrease		$ 3.6
Proposed tariff rate decrease based on ROE	1.00%	1.00%
Central Maine Power | Minimum
Regulatory Assets And Liabilities [Line Items]
Proposed tariff rate decrease		$ 2.0
Proposed tariff rate decrease based on ROE	0.75%	0.75%
NYSEG
Regulatory Assets And Liabilities [Line Items]
Recovery of deferred storm costs														$ 51.0
Regulatory items amortization period														3 years
Storm costs not included in joint proposal														$ 143.0
Annual amortization of regulatory items														$ 16.5
Approved return on equity														9.00%
Equity ratio												48.00%		48.00%
NYSEG | Maximum
Regulatory Assets And Liabilities [Line Items]
Equity ratio for earnings sharing												50.00%		50.00%
NYSEG | Storm costs
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														10 years
NYSEG | Regulatory Items Other Than Storm Costs
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
NYSEG | Deferred Income Tax Charge
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														50 years
RG&E
Regulatory Assets And Liabilities [Line Items]
Storm costs not included in joint proposal														$ 52.0
Approved return on equity														9.00%
Equity ratio												48.00%		48.00%
RG&E | Deferred property tax
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years
RG&E | Maximum
Regulatory Assets And Liabilities [Line Items]
Equity ratio for earnings sharing												50.00%		50.00%
RG&E | Deferred Income Tax Charge
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														50 years
Southern Connecticut Gas Company S C G
Regulatory Assets And Liabilities [Line Items]
Equity ratio										52.00%
Public utilities regulatory authority distribution rate										9.25%
Amount of approved ROE for the year 2018														$ 1.5
Amount of approved ROE for the year 2019														4.7
Amount of approved ROE for the year 2020														$ 5.0
Berkshire Gas Company
Regulatory Assets And Liabilities [Line Items]
Proposed alternative ratemaking mechanism term							5 years
Connecticut Natural Gas Corporation
Regulatory Assets And Liabilities [Line Items]
Amount of proposed ROE for the year 2019				$ 9.9
Amount of proposed ROE for the year 2020				4.6
Amount of proposed ROE for the year 2021				5.2
Amount of proposed ROE, total				$ 19.7
PURA
Regulatory Assets And Liabilities [Line Items]
Equity ratio												54.00%		54.00%
Percentage of proposed return on equity, year one														9.30%
Equity ratio, year two												54.50%		54.50%
BGC
Regulatory Assets And Liabilities [Line Items]
Equity ratio			55.00%
Public utilities regulatory authority distribution rate			9.70%
Amount of approved ROE for the year 2018			$ 1.6
Amount of approved ROE for the year 2019			$ 0.7
NYSEG | Deferred property tax
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period														5 years