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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2017	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2020	Sep. 30, 2019
Income Taxes [Line Items]
Regulatory Liability			$ 55.3	$ 54.0				$ 61.1
Regulatory liability, expected flow back to customers			47.1		$ 48.9
Regulatory liability, expected pass back to ratepayers			0.2	0.3
Net Operating Loss Carryforwards Utilized For Income Taxes					8.2
Deferred tax assets, operating loss carryforwards, federal			2.7
Alternative minimum tax credit carryforwards			0.9				$ 0.1
Additional Alternative minimum tax credit carryforwards			$ 1.9
Percentage of employment retention credit			50.00%
Employment retention duties capacity			100.00%
Employment tax expense			$ 0.6
Reconciling Mechanisms
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers						$ 1.8
Net Operating Loss Carryforward Assets
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers					$ 2.0
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability		$ 48.9	45.8	$ 47.6		$ 48.9		$ 47.6
Electric Ratepayers | Fitchburg Gas and Electric Light Company
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			10.1
Gas Ratepayers | Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			1.6
Gas Ratepayers | Fitchburg Gas and Electric Light Company
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			$ 10.4
Gas Ratepayers | Northern Utilities Inc
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers	$ 12.3
Average Rate Assumption Method estimated flow back period	20 years
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax		21.00%	21.00%

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible domestic operating loss carryforwards. Excludes state and local operating loss carryforwards.

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Employment retention duties capacity.

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