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Income Taxes - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018	Mar. 31, 2018
Income Taxes [Line Items]
Regulatory Liability		$ 62,400,000	$ 58,500,000			$ 60,000,000
Regulatory liability, expected flow back to customers		$ 47,100,000		$ 48,900,000
Tax provision			2,400,000
Net Operating Loss Carryforwards			10,800,000		$ 3,700,000
Calculated federal current tax			7,700,000
Deferred tax assets, operating loss carryforwards, federal			7,700,000
Federal current taxes payable			$ 0
NOL carryforward assets expiration date			2029
Alternative minimum tax credit carryforwards	$ 3,500,000			3,500,000
Reconciling Mechanisms
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers				1,800,000
Net Operating Loss Carryforward Assets
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers			$ 3,400,000	5,800,000
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	$ 48,900,000	$ 48,200,000	$ 47,000,000	$ 48,900,000		$ 49,100,000
Electric Ratepayers | Fitchburg Gas and Electric Light Company
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers		10,100,000
Gas Ratepayers | Fitchburg Gas and Electric Light Company
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers		$ 10,400,000
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax	21.00%	21.00%
Federal, Maine, Massachusetts, and New Hampshire Tax Authorities
Income Taxes [Line Items]
Tax examination description		The Company remains subject to examination by Federal, Maine, Massachusetts, and New Hampshire tax authorities for the tax periods ended December 31, 2015; December 31, 2016; and December 31, 2017.

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