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Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018
Income Taxes [Line Items]
Corporate federal income tax	21.00%	34.00%	34.00%
Regulatory Liability	$ 58,500,000	$ 58,100,000
Regulatory liability, expected flow back to customers	47,100,000	48,900,000
Tax provision	2,400,000
Net Operating Loss Carryforwards	10,773,000	12,686,000		$ 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
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
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability	$ 47,000,000	48,900,000
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
Fitchburg Gas and Electric Light Company
Income Taxes [Line Items]
Average Rate Assumption Method estimated flow back period	15 years
Fitchburg Gas and Electric Light Company | Electric Ratepayers
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers	$ 10,100,000
Fitchburg Gas and Electric Light Company | Gas Ratepayers
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%
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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