Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Deferred income tax liabilities:
Property, plant and equipment	$ 318.8	$ 289.7
Acquisition adjustment	4.9	5.2
Deferred Income Tax Liability - Deferred Gas Costs	4.7	2.5
Deferred Income Tax Liability, Natural Gas Conversion Costs	4.4	4.6
Deferred Income Tax Liability, Storm Reserve	5.8	5.8
Deferred Income Tax Liability, Customer Based Intangibles	16.0	6.4
Other	6.5	8.4
Total deferred income tax liabilities	361.1	322.6
Deferred Tax Assets, Regulatory Assets and Liabilities	7.6	6.0
Deferred income tax assets:
Pension and other employee benefits	6.5	5.1
Net operating loss carryforwards	9.0	1.1
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	4.6	4.0
Other	11.1	10.3
Total deferred income tax assets	47.8	26.5
Deferred Income Tax Liabilities, Net	313.3	$ 296.1
Operating Loss Carryforwards [Line Items]
Investment Tax Credit	$ 9.0