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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2017	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2020	Mar. 31, 2019
Income Taxes [Line Items]
Regulatory Liability	$ 55.6		$ 55.6	$ 54.0				$ 62.4
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					5.7
Deferred tax assets, operating loss carryforwards, federal				1.6
Alternative minimum tax credit carryforwards	0.9		0.9	0.9			$ 0.1
Additional Alternative minimum tax credit carryforwards				1.9
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	46.1	$ 48.9	46.1	47.6		$ 48.9		$ 48.2
Electric Ratepayers | Fitchburg Gas and Electric Light Company
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			10.1
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%
Tax Year 2019
Income Taxes [Line Items]
Net Operating Loss Carryforwards Utilized For Income Taxes				$ 3.5

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