Segment Information - Schedule of Segment Reporting Information by Segment (Detail) - USD ($)		12 Months Ended
		Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		$ 930,000,000.0	$ 787,200,000	$ 670,600,000
Other Cost of Sales	[1]	(29,600,000)	(24,600,000)	(26,000,000.0)
Unregulated Energy Cost of Sales		127,300,000	100,200,000	102,500,000
Cost of Sales		291,500,000	219,800,000	216,500,000
Intersegment Revenues (1)
Total intersegment revenues		930,000,000.0	787,200,000	670,600,000
Operating Income
Operating Income		255,900,000	228,200,000	150,800,000
Other income		9,600,000	2,000,000.0	1,400,000
Interest charges		72,500,000	68,400,000	36,900,000
Income taxes		52,700,000	43,200,000	28,100,000
Net Income		140,300,000	118,600,000	87,200,000
Depreciation and Amortization
Total depreciation and amortization		91,700,000	65,700,000	65,500,000
Segment Reporting, Other Segment Item, Amount	[2]	37,700,000	40,900,000	39,000,000.0
Payments to Acquire Productive Assets		470,400,000	355,800,000	1,138,000,000	[3]
Identifiable Assets
Total identifiable assets		3,994,800,000	3,577,000,000
RSAM Adjustment		15,500,000	5,100,000
Natural gas and electricity costs		193,800,000	144,200,000	140,000,000.0
Utilities Operating Expense, Maintenance and Operations		253,200,000	232,600,000	198,800,000
Total before income taxes		193,000,000.0	161,800,000	115,300,000
Florida City Gas
Identifiable Assets
Business Combination, Consideration Transferred		922,800,000
Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		687,800,000	583,400,000	473,600,000
Intersegment Revenues (1)
Total intersegment revenues		687,800,000	583,400,000	473,600,000
Operating Income
Operating Income		222,000,000.0	196,200,000	126,200,000
Depreciation and Amortization
Total depreciation and amortization		70,900,000	48,800,000	48,200,000
Segment Reporting, Other Segment Item, Amount	[2]	32,100,000	35,700,000	33,900,000
Payments to Acquire Productive Assets		409,800,000	320,200,000	1,095,900,000	[3]
Identifiable Assets
Utilities Operating Expense, Maintenance and Operations		169,000,000.0	158,500,000	125,300,000
Regulated Energy | Florida City Gas
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		170,400,000	140,300,000
Intersegment Revenues (1)
Total intersegment revenues		170,400,000	140,300,000
Operating Income
Operating Income		5,200,000	13,500,000
Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		271,900,000	228,400,000	223,100,000
Intersegment Revenues (1)
Total intersegment revenues		271,900,000	228,400,000	223,100,000
Operating Income
Operating Income		33,600,000	31,700,000	24,400,000
Depreciation and Amortization
Total depreciation and amortization		20,800,000	16,900,000	17,300,000
Segment Reporting, Other Segment Item, Amount	[2]	5,600,000	5,200,000	5,100,000
Payments to Acquire Productive Assets		39,700,000	33,900,000	40,300,000	[3]
Identifiable Assets
Utilities Operating Expense, Maintenance and Operations		84,600,000	74,400,000	73,800,000
Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]	(29,700,000)	(24,600,000)	(26,100,000)
Intersegment Revenues (1)
Total intersegment revenues	[1]	(29,700,000)	(24,600,000)	(26,100,000)
Depreciation and Amortization
Segment Reporting, Other Segment Item, Amount	[1],[2]			0
Payments to Acquire Productive Assets		20,900,000	1,700,000	1,800,000	[3]
Identifiable Assets
Utilities Operating Expense, Maintenance and Operations	[1]	(400,000)	(300,000)	(300,000)
Other and eliminations
Operating Income
Operating Income	[1]	300,000	300,000	200,000
Operating Segments
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		930,200,000	787,400,000	670,800,000
Intersegment Revenues (1)
Total intersegment revenues		930,200,000	787,400,000	670,800,000
Operating Segments | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		685,700,000	578,300,000	471,600,000
Intersegment Revenues (1)
Total intersegment revenues		685,700,000	578,300,000	471,600,000
Identifiable Assets
Total identifiable assets		3,425,300,000	3,042,900,000
Operating Segments | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues		244,300,000	208,900,000	199,000,000.0
Intersegment Revenues (1)
Total intersegment revenues		244,300,000	208,900,000	199,000,000.0
Identifiable Assets
Total identifiable assets		495,000,000.0	486,400,000
Operating Segments | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1]	200,000	200,000	200,000
Intersegment Revenues (1)
Total intersegment revenues	[1]	200,000	200,000	200,000
Intersegment Eliminations
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[4]	(200,000)	(200,000)	(200,000)
Intersegment Revenues (1)
Total intersegment revenues	[4]	(200,000)	(200,000)	(200,000)
Intersegment Eliminations | Regulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[4]	2,100,000	5,100,000	2,000,000.0
Intersegment Revenues (1)
Total intersegment revenues	[4]	2,100,000	5,100,000	2,000,000.0
Intersegment Eliminations | Unregulated Energy
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[4]	27,600,000	19,500,000	24,100,000
Intersegment Revenues (1)
Total intersegment revenues	[4]	27,600,000	19,500,000	24,100,000
Intersegment Eliminations | Other
Operating Revenues, Unaffiliated Customers
Total intersegment revenues	[1],[4]	(29,900,000)	(24,800,000)	(26,300,000)
Intersegment Revenues (1)
Total intersegment revenues	[1],[4]	(29,900,000)	(24,800,000)	$ (26,300,000)
Other And Intersegment Eliminations
Identifiable Assets
Total identifiable assets		$ 74,500,000	$ 47,700,000

[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 Energy- Other taxes and transaction and transition costs related to the acquisition and integration of FCG; Unregulated Energy - Other taxes

[3] 4) 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.