Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2025 USD ($)		Sep. 30, 2024 USD ($)		Sep. 30, 2025 USD ($)		Sep. 30, 2024 USD ($)		Dec. 31, 2024 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments					2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 179,600		$ 160,200		$ 671,100		$ 572,200
Operating Income
Total operating income	45,000		40,900		182,100		161,300
Other expense, net	200		500		1,200		1,700
Interest	18,200		17,100		54,100		50,900
Income Before Income Taxes	27,000		24,300		129,200		112,100
Income taxes	7,600		6,800		35,000		30,200
Net Income	19,400		17,500		94,200		81,900
Identifiable Assets
Total identifiable assets	3,861,400				3,861,400				$ 3,577,000
Cost of Sales	42,400		38,200		208,700		159,100
Utilities Operating Expense, Maintenance and Operations	59,700		54,600		184,000		170,000
Depreciation and amortization	23,300		16,800		67,700		51,700
Segment Reporting, Other Segment Item, Amount	9,200	[1]	9,700	[1]	28,600	[2]	30,100	[2]
Regulated natural gas and electric costs	31,700		28,400		137,300		105,700
Unregulated Energy Cost of Sales	18,300		15,800		93,400		70,900
Other Cost of Sales	(7,600)	[3]	(6,000)	[3]	(22,000)	[4]	(17,500)	[4]
Payments to Acquire Productive Assets	122,800		97,300		335,600		256,800
Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	0	[5]	0	[5]	0	[6]	0	[6]
Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	179,600		160,200		671,100		572,200
Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	146,400		130,600		497,800		429,700
Operating Income
Total operating income	48,900		44,000		161,200		142,600
Identifiable Assets
Total identifiable assets	3,325,000				3,325,000				3,042,900
Utilities Operating Expense, Maintenance and Operations	39,700		37,600		122,100		115,900
Depreciation and amortization	18,200		12,300		52,600		39,500
Segment Reporting, Other Segment Item, Amount	7,900	[1]	8,300	[1]	24,600	[2]	26,000	[2]
Payments to Acquire Productive Assets	108,100		89,000		295,700		230,900
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	600	[5]	400	[5]	1,400	[6]	4,600	[6]
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	145,800		130,200		496,400		425,100
Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	40,700		35,600		195,300		160,100
Operating Income
Total operating income	(3,900)		(3,100)		20,900		18,700
Identifiable Assets
Total identifiable assets	470,200				470,200				486,400
Utilities Operating Expense, Maintenance and Operations	19,900		17,000		61,900		54,200
Depreciation and amortization	5,100		4,500		15,100
Segment Reporting, Other Segment Item, Amount	1,300	[1]	1,400	[1]	4,000	[2]	4,100	[2]
Payments to Acquire Productive Assets	9,600		8,300		32,100		24,600
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	6,900	[5]	5,600	[5]	20,600	[6]	13,000	[6]
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	33,800		30,000		174,700		147,100
Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	(7,500)	[3]	(6,000)	[3]	(22,000)	[4]	(17,600)	[4]
Identifiable Assets
Total identifiable assets	66,200				66,200				$ 47,700
Utilities Operating Expense, Maintenance and Operations	100	[3]	0	[3]	0	[4]	(100)	[4]
Segment Reporting, Other Segment Item, Amount	0	[1],[3]	0	[1],[3]	0	[2],[4]	0	[2],[4]
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	(7,500)	[3],[5]	(6,000)	[3],[5]	(22,000)	[4],[6]	(17,600)	[4],[6]
Other and eliminations [Member]
Operating Income
Total operating income	0	[3]	0	[3]	0	[4]	0	[4]
Other [Member]
Identifiable Assets
Payments to Acquire Productive Assets	5,100		$ 0		7,800		1,300
Retained Earnings [Member]
Operating Income
Net Income	$ 19,400				$ 94,200		$ 81,900

[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] 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.

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

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

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

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