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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Income Taxes [Line Items]
Regulatory Liability			$ 61.1		$ 58.5		$ 60.1
Regulatory liability, expected flow back to customers			47.1			$ 48.9
Tax provision					$ 2.4
NOL carryforward assets expiration date					2029
Alternative minimum tax credit carryforwards					$ 3.1
Scenario Forecast
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers		$ 0.4		$ 1.6
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						5.8
Tax Cuts And Jobs Act Of 2017 Change In Tax Rate Regulatory Liability recognized			$ 1.4
Minimum
Income Taxes [Line Items]
Average Rate Assumption Method estimated flow back period			15 years
Maximum
Income Taxes [Line Items]
Average Rate Assumption Method estimated flow back period			20 years
Fitchburg Gas and Electric Light Company
Income Taxes [Line Items]
Average Rate Assumption Method estimated flow back period			15 years
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability	$ 48.9		$ 47.6		$ 47.0	$ 48.9	$ 47.9
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
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax	21.00%		21.00%
Internal Revenue Service [Member] | Tax Year 2018
Income Taxes [Line Items]
Net Operating Loss Carryforwards Utilized For Income Taxes			$ 5.7
Federal, Maine, Massachusetts, and New Hampshire Tax Authorities
Income Taxes [Line Items]
Tax examination description			The Company remains subject to examination by Maine, Massachusetts, and New Hampshire tax authorities for the tax periods ended December 31, 2016; December 31, 2017; and December 31, 2018.
Domestic Country And State Jurisdiction [Member]
Income Taxes [Line Items]
Net Operating Loss Carryforwards			$ 11.9

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