Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2025 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2024 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		$ 298.7	$ 245.7
Operating Income
Total operating income		86.8	79.6
Other expense, net		0.6	0.2
Interest		18.1	17.0
Income Before Income Taxes		69.3	62.8
Income taxes		18.4	16.6
Net Income		50.9	46.2
Identifiable Assets
Total identifiable assets		3,664.6		$ 3,577.0
Cost of Sales		116.3	81.2
Utilities Operating Expense, Maintenance and Operations		63.4	57.5
Depreciation and amortization		22.5	17.0
Segment Reporting, Other Segment Item, Amount	[1]	9.7	10.4
Regulated natural gas and electric costs		71.5	49.9
Unregulated Energy Cost of Sales		52.2	37.1
Other Cost of Sales	[2]	(7.4)	(5.8)
Payments to Acquire Productive Assets		112.9	70.6
Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[3]	0.0	0.0
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		298.7	245.7
Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		199.6	168.4
Operating Income
Total operating income		60.5	58.1
Identifiable Assets
Total identifiable assets		3,128.9		3,042.9
Utilities Operating Expense, Maintenance and Operations		41.7	39.0
Depreciation and amortization		17.6	12.5
Segment Reporting, Other Segment Item, Amount	[1]	8.3	8.9
Payments to Acquire Productive Assets		96.1	59.9
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[3]	0.4	0.5
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		199.2	167.9
Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		106.7	83.1
Operating Income
Total operating income		26.3	21.4
Identifiable Assets
Total identifiable assets		488.2		486.4
Utilities Operating Expense, Maintenance and Operations		21.8	18.6
Depreciation and amortization		4.9	4.5
Segment Reporting, Other Segment Item, Amount	[1]	1.5	1.5
Payments to Acquire Productive Assets		14.8	9.8
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[3]	7.2	5.3
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers		99.5	77.8
Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[2]	(7.6)	(5.8)
Identifiable Assets
Total identifiable assets		47.5		$ 47.7
Utilities Operating Expense, Maintenance and Operations	[2]	(0.1)	(0.1)
Segment Reporting, Other Segment Item, Amount	[1],[2]	(0.1)	0.0
Payments to Acquire Productive Assets		2.0	0.9
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	[2],[3]	(7.6)	(5.8)
Other and eliminations [Member]
Operating Income
Total operating income	[2]	0.0	0.1
Retained Earnings [Member]
Operating Income
Net Income		$ 50.9	$ 46.2

[1] (3) Other segment items for each reportable segment include: Regulated - Other taxes and transaction and transition costs related to the integration of FCG; Unregulated - Other taxes.

[2] (1) Other revenues and other operating income (loss) amounts are attributable to eliminations and unregulated subsidiaries that own real estate leased to the Company.

[3] (2) All significant intersegment revenues are billed at market rates and have been eliminated from consolidated revenues.