Goodwill and Other Intangible Assets - Schedule of Carrying Value of Goodwill (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		$ 27.0	$ 27.0
Finite-Lived Intangible Assets, Accumulated Amortization		13.8	12.0
Goodwill		507.5	507.7
Goodwill, Transfers	[1]	(0.2)
Regulated Energy
Goodwill [Line Items]
Goodwill		468.3	468.3
Goodwill, Transfers		0.0
Unregulated Energy
Goodwill [Line Items]
Goodwill		39.2	39.4
Goodwill, Transfers		(0.2)
Customer Lists [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		17.0	17.0
Finite-Lived Intangible Assets, Accumulated Amortization		9.2	8.2
Noncompete Agreements [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		3.1	3.1
Finite-Lived Intangible Assets, Accumulated Amortization		2.7	2.3
Patents [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross	[2]	6.6	6.6
Finite-Lived Intangible Assets, Accumulated Amortization	[2]	1.6	1.2
Other Intangible Assets [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		0.3	0.3
Finite-Lived Intangible Assets, Accumulated Amortization		$ 0.3	$ 0.3

[1] (1) Adjustments in the Unregulated Energy segment relate to the sale of certain assets in our propane distribution business during 2025.

[2] (1) Includes amounts related to patented technology developed by Marlin Gas Services and the acquisition of Planet Found