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Regulatory Matters - Additional Information (Detail)													1 Months Ended			3 Months Ended		12 Months Ended
	Jun. 01, 2021 USD ($)	May 01, 2021 USD ($)	Apr. 02, 2021 USD ($)	Mar. 09, 2021 USD ($)	Mar. 01, 2021 USD ($)	Nov. 30, 2020 USD ($)	Nov. 02, 2020 USD ($)	Mar. 26, 2020 USD ($)	Feb. 28, 2020 USD ($)	Jan. 31, 2020 USD ($)	Oct. 29, 2019 USD ($)	Nov. 01, 2018 USD ($)	Mar. 31, 2020 USD ($)	May 01, 2019 USD ($)	May 02, 2018 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2017	Jun. 30, 2027 MW	Dec. 31, 2022 MW	Mar. 10, 2021 MW	Nov. 05, 2020	May 31, 2019 MW	Aug. 09, 2018 MW	Jul. 31, 2018 MW	Jun. 30, 2017 MW
Regulatory Assets [Line Items]
Corporate income tax rate																21.00%	21.00%
Approved annual increase in rates								$ 3,600,000	$ 4,600,000				$ 3,700,000
Increase in annual base rate								3.60%
Requested annual increase in rates		$ 1,100,000
Tax Year 2018
Regulatory Assets [Line Items]
Corporate income tax rate																21.00%		21.00%	21.00%
Other Restructuring
Regulatory Assets [Line Items]
Cost recovery period, years																2 years
Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW																				1,600
Northern Utilities Inc
Regulatory Assets [Line Items]
Approved annual increase in rates														$ 1,400,000
Northern Utilities Inc | Subsequent Event [Member]
Regulatory Assets [Line Items]
Approved annual increase in rates	$ 5,800,000		$ 12,000,000.0
Increase in annual base rate	2.70%		4.40%
Northern Utilities Inc | ME
Regulatory Assets [Line Items]
Percentage of approved return on equity								9.48%
Percentage of approved return on equity, reflecting on equity								50.00%
Percentage of approved return on equity, reflecting on debt								50.00%
Customer information system potential excess costs				$ 5,000,000.0
Customer information system costs				$ 36,800,000
Northern Utilities Inc | New Hampshire
Regulatory Assets [Line Items]
Approved annual increase in rates															$ 3,200,000
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue							$ 1,400,000
Percentage of approved return on equity										9.70%
Percentage of approved return on equity, reflecting on equity										52.45%
Percentage of approved return on equity, reflecting on debt										47.55%
Requested annual increase in rates											$ 1,100,000	$ 900,000
Revenue impact threshold										$ 100,000
Fitchburg Gas and Electric Light Company | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW																										400
Remuneration Percentage																							2.75%			2.75%	2.75%
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | First Solicitation [Member]
Regulatory Assets [Line Items]
Power generation facility | MW																									1,600
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Second Solicitation [Member]
Regulatory Assets [Line Items]
Power generation facility | MW																						1,600		1,600		800
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Second Rate Step Adjustments
Regulatory Assets [Line Items]
Power generation facility | MW																										800
Fitchburg Gas and Electric Light Company | Qualified Clean Energy
Regulatory Assets [Line Items]
Power generation capacity | MW																										9,554,940
Fitchburg Gas and Electric Light Company | Minimum [Member] | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW																											400
Fitchburg Gas and Electric Light Company | Scenario Forecast
Regulatory Assets [Line Items]
Power generation capacity | MW																					9,450,000
Fitchburg Gas Company
Regulatory Assets [Line Items]
Percentage of approved return on equity									9.70%
Percentage of approved return on equity, reflecting on equity									52.45%
Percentage of approved return on equity, reflecting on debt									47.55%
Regulatory assets approved increase in revenue due to be recovered		$ 2,200,000
Approved annual increase in rates					$ 900,000					$ 1,100,000
Approved annual increase in rates																$ 900,000
Apporved annual decrease in rates																200,000
Revenue impact threshold									$ 40,000
Granite State
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue						$ 1,300,000
Spending cap																$ 14,600,000

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