Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Current Income Tax Expense
Federal		$ 3.7	$ 14.7	$ 8.3
State		2.7	5.5	1.9
Other		(0.4)	0.0	0.0
Total current income tax expense		6.0	20.2	10.2
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	37.2	7.9	23.6
Total Income Tax Expense		43.2	28.1	33.8
Deferred State and Local Income Tax Expense (Benefit)		7.9	0.1	7.8
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		33.2	17.8	15.0
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		(1.0)	(7.7)	8.9
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		(0.3)	(1.0)	1.1
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		(2.5)	(0.4)	0.0
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		0.0	$ (0.8)	$ (1.4)
Florida City Gas Goodwill Amortization
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ 7.8

[1] (1) Includes $7.9 million, less than $0.1 million, and $7.8 million of deferred state income taxes for the years 2024, 2023, and 2022, respectively.