Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Income Taxes [Line Items]
Corporate federal income tax		21.00%
Regulatory Liability	$ 58.1	$ 60.0	$ 58.1		$ 13.5
Regulatory liability, expected flow back to customers		$ 47.1
Regulatory liability, expected pass back to ratepayers			1.8
Tax benefit for New Hampshire business enterprise tax credits				$ 0.7
Deferred tax assets, operating loss carryforwards, federal	1.1		1.1
Net Operating Loss Carryforwards	12.7		$ 12.7
NOL carryforward assets expiration date			2029
Alternative minimum tax credit carryforwards	3.5		$ 3.5
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	$ 49.1	48.9
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax	21.00%
Deferred tax assets, operating loss carryforwards, federal	$ 10.1		$ 10.1
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, 2014; December 31, 2015; and December 31, 2016