Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($)		12 Months Ended
		Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 787,200,000		$ 670,600,000		$ 680,700,000
Other Cost of Sales	[1]	(24,600,000)		(26,000,000.0)		(29,400,000)
Unregulated Energy Cost of Sales		100,200,000		102,500,000		162,700,000
Cost of Sales		219,800,000		216,500,000		260,500,000
Intersegment Revenues (1)
Total intersegment revenues		787,200,000		670,600,000		680,700,000
Operating Income
Operating Income		228,200,000		150,800,000		142,900,000
Other income		2,000,000.0		1,400,000		5,100,000
Interest charges		68,400,000		36,900,000		24,400,000
Income taxes		43,200,000		28,100,000		33,800,000
Net Income		118,600,000		87,200,000		89,800,000
Depreciation and Amortization
Total depreciation and amortization		65,700,000		65,500,000		69,000,000.0
Segment Reporting, Other Segment Item, Amount	[2]	40,900,000		39,000,000.0		25,600,000
Payments to Acquire Productive Assets		355,800,000	[3]	1,138,000,000	[3]	140,800,000
Identifiable Assets
Total identifiable assets		3,577,000,000		3,304,700,000
RSAM Adjustment		15,500,000		5,100,000
Natural gas and electricity costs		144,200,000		140,000,000.0		127,200,000
Utilities Operating Expense, Maintenance and Operations		232,600,000		198,800,000		182,700,000
Total before income taxes		161,800,000		115,300,000		123,600,000
Florida City Gas
Identifiable Assets
Business Combination, Consideration Transferred		922,800,000
Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		583,400,000		473,600,000		429,400,000
Intersegment Revenues (1)
Total intersegment revenues		583,400,000		473,600,000		429,400,000
Operating Income
Operating Income		196,200,000		126,200,000		115,300,000
Depreciation and Amortization
Total depreciation and amortization		48,800,000		48,200,000		52,700,000
Segment Reporting, Other Segment Item, Amount	[2]	35,700,000		33,900,000		21,200,000
Payments to Acquire Productive Assets		320,200,000	[3]	1,095,900,000	[3]	97,600,000
Identifiable Assets
Utilities Operating Expense, Maintenance and Operations		158,500,000		125,300,000		113,000,000.0
Regulated Energy | Florida City Gas
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		140,300,000		12,100,000
Intersegment Revenues (1)
Total intersegment revenues		140,300,000		12,100,000
Operating Income
Operating Income		13,500,000		(3,300,000)
Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		228,400,000		223,100,000		280,800,000
Intersegment Revenues (1)
Total intersegment revenues		228,400,000		223,100,000		280,800,000
Operating Income
Operating Income		31,700,000		24,400,000		27,500,000
Depreciation and Amortization
Total depreciation and amortization		16,900,000		17,300,000		16,300,000
Segment Reporting, Other Segment Item, Amount	[2]	5,200,000		5,100,000		4,300,000
Payments to Acquire Productive Assets		33,900,000	[3]	40,300,000	[3]	40,800,000
Identifiable Assets
Utilities Operating Expense, Maintenance and Operations		74,400,000		73,800,000		70,000,000.0
Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]	(24,600,000)		(26,100,000)		(29,500,000)
Intersegment Revenues (1)
Total intersegment revenues	[1]	(24,600,000)		(26,100,000)		(29,500,000)
Depreciation and Amortization
Segment Reporting, Other Segment Item, Amount	[1],[2]					100,000
Payments to Acquire Productive Assets		1,700,000	[3]	1,800,000	[3]	2,400,000
Identifiable Assets
Utilities Operating Expense, Maintenance and Operations	[1]	(300,000)		(300,000)		(300,000)
Other and eliminations
Operating Income
Operating Income	[1]	300,000		200,000		100,000
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		787,400,000		670,800,000		681,000,000.0
Intersegment Revenues (1)
Total intersegment revenues		787,400,000		670,800,000		681,000,000.0
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		578,300,000		471,600,000		422,900,000
Intersegment Revenues (1)
Total intersegment revenues		578,300,000		471,600,000		422,900,000
Identifiable Assets
Total identifiable assets		3,042,900,000		2,781,600,000
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		208,900,000		199,000,000.0		257,800,000
Intersegment Revenues (1)
Total intersegment revenues		208,900,000		199,000,000.0		257,800,000
Identifiable Assets
Total identifiable assets		486,400,000		477,400,000
Operating Segments | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]	200,000		200,000		300,000
Intersegment Revenues (1)
Total intersegment revenues	[1]	200,000		200,000		300,000
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[4]	(200,000)		(200,000)		(300,000)
Intersegment Revenues (1)
Total intersegment revenues	[4]	(200,000)		(200,000)		(300,000)
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[4]	5,100,000		2,000,000.0		6,500,000
Intersegment Revenues (1)
Total intersegment revenues	[4]	5,100,000		2,000,000.0		6,500,000
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[4]	19,500,000		24,100,000		23,000,000.0
Intersegment Revenues (1)
Total intersegment revenues	[4]	19,500,000		24,100,000		23,000,000.0
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1],[4]	(24,800,000)		(26,300,000)		(29,800,000)
Intersegment Revenues (1)
Total intersegment revenues	[1],[4]	(24,800,000)		(26,300,000)		$ (29,800,000)
Other And Intersegment Eliminations
Identifiable Assets
Total identifiable assets		$ 47,700,000		$ 45,700,000

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

[2] Other segment items for each reportable segment include: Regulated - Other taxes and transaction and transition costs related to the acquisition and integration of FCG; Unregulated - Other taxes

[3] Regulated capital expenditures in 2023 include our acquisition of FCG for $922.8 million. See Note 4 for additional details.

[4] All significant intersegment revenues are billed at market rates and have been eliminated from consolidated revenues.