Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Corporate federal income tax	34.00%	34.00%	34.00%
Regulatory Liability	$ 58,100	$ 13,000
Deferred tax assets, operating loss carryforwards, federal	1,100
Net Operating Loss Carryforwards	$ 12,686	23,921
NOL carryforward assets expiration date	2029
Tax benefit for New Hampshire business enterprise tax credits		$ 700
Alternative minimum tax credit carryforwards	$ 3,500
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability	$ 48,900
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax	21.00%
Deferred tax assets, operating loss carryforwards, federal	$ 10,100
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.