Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disclosure Income Taxes Schedule Of Accumulated Deferred Income Tax Assets And Liabilities [Abstract]
Deferred State and Local Income Tax Expense (Benefit)	$ 7.9	$ 0.1	$ 7.8
Deferred income tax liabilities:
Property, plant and equipment	289.7	252.1
Acquisition adjustment	5.2	5.6
Deferred Income Tax Liability - Deferred Gas Costs	2.5	3.6
Deferred Income Tax Liability, Natural Gas Conversion Costs	4.6	4.8
Deferred Income Tax Liability, Storm Reserve	5.8	5.8
Deferred Income Tax Liability, Customer Based Intangibles	6.4
Other	8.4	9.7
Total deferred income tax liabilities	322.6	281.6
Deferred Tax Assets, Regulatory Assets and Liabilities	6.0	4.1
Deferred Income Tax Asset, Intangibles		2.8
Deferred income tax assets:
Pension and other employee benefits	5.1	5.0
Net operating loss carryforwards	1.1	1.8
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals	4.0	3.3
Other	10.3	5.5
Total deferred income tax assets	26.5	22.5
Deferred Income Tax Liabilities, Net	$ 296.1	$ 259.1