Goodwill and Other Intangible Assets - Schedule of Carrying Value of Goodwill (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		$ 27.0	$ 27.0
Finite-Lived Intangible Assets, Accumulated Amortization		12.0	10.1
Goodwill		507.7	508.2
Goodwill, Transfers	[1]	(0.5)
Regulated Energy
Goodwill [Line Items]
Goodwill		468.3	468.7
Goodwill, Transfers		(0.4)
Unregulated Energy
Goodwill [Line Items]
Goodwill		39.4	39.5
Goodwill, Transfers		(0.1)
Customer Lists [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		17.0	17.0
Finite-Lived Intangible Assets, Accumulated Amortization		8.2	7.1
Noncompete Agreements [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		3.1	3.1
Finite-Lived Intangible Assets, Accumulated Amortization		2.3	1.9
Patents [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross	[2]	6.6	6.6
Finite-Lived Intangible Assets, Accumulated Amortization	[2]	1.2	0.9
Other Intangible Assets [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross		0.3	0.3
Finite-Lived Intangible Assets, Accumulated Amortization		$ 0.3	$ 0.2

[1] (1) Adjustments to goodwill in our Regulated Energy segment attributable to purchase accounting adjustments related to our acquisition of FCG in November 2023. Adjustments in the Unregulated Energy segment relate to the sale of certain assets in our propane distribution business during 2024. See Note 4 for additional details.

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