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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Income Taxes [Line Items]
Net Operating Loss Carryforwards		$ 18.2
Alternative minimum tax credit carryforwards		3.4
Regulatory Liability	$ 58.1	60.1	$ 58.1	$ 15.0
Regulatory liability, expected flow back to customers		$ 47.1
Regulatory liability, expected pass back to ratepayers			1.8
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	$ 48.9	$ 47.9	$ 48.9
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.