Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2022	Dec. 31, 2023
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross	$ 27.0		$ 27.0
Acquired finite-lived intangible assets, weighted average useful life	14 years
Goodwill	$ 507.7		508.2
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	1.9		1.8
Amortization of intangible assets		$ 1.5
Finite-Lived Intangible Assets, Amortization Expense, Year Four	1.5
Amortization of intangible assets, 2020	1.6
Amortization of intangible assets, 2022	1.3
Amortization of intangible assets, 2019	1.8
Regulated Energy
Goodwill [Line Items]
Goodwill	468.3		468.7
Unregulated Energy
Goodwill [Line Items]
Goodwill	39.4		39.5
Customer list
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross	$ 17.0		17.0
Customer list | Minimum [Member]
Goodwill [Line Items]
Amortized period of acquired intangible assets	5 years
Noncompete Agreements [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross	$ 3.1		$ 3.1