Segment Information - Schedule of Segment Reporting Information by Segment (Detail) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2025 USD ($)		Jun. 30, 2024 USD ($)		Jun. 30, 2025 USD ($)		Jun. 30, 2024 USD ($)		Dec. 31, 2024 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments					2
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	$ 192,800		$ 166,300		$ 491,500		$ 412,000
Operating Income
Total operating income	50,300		40,800		137,100		120,400
Other expense, net	400		1,000		1,000		1,200
Interest	17,800		16,800		35,900		33,800
Income Before Income Taxes	32,900		25,000		102,200		87,800
Income taxes	9,000		6,800		27,400		23,400
Net Income	23,900		18,200		74,800		64,400
Identifiable Assets
Total identifiable assets	3,737,800				3,737,800				$ 3,577,000
Cost of Sales	50,000		39,700		166,300		120,900
Utilities Operating Expense, Maintenance and Operations	60,900		57,900		124,300		115,400
Depreciation and amortization	21,900		17,900		44,400		34,900
Segment Reporting, Other Segment Item, Amount	9,700	[1]	10,000	[1]	19,400	[2]	20,400	[2]
Regulated natural gas and electric costs	34,100		27,400		105,600		77,300
Unregulated Energy Cost of Sales	22,900		18,000		75,100		55,100
Other Cost of Sales	(7,000)	[3]	(5,700)	[3]	(14,400)	[4]	(11,500)	[4]
Payments to Acquire Productive Assets	99,900		88,900		212,800		159,500
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	192,800		166,300		491,500		412,000
Regulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	151,800		130,700		351,400		299,100
Operating Income
Total operating income	51,800		40,500		112,300		98,600
Identifiable Assets
Total identifiable assets	3,212,800				3,212,800				3,042,900
Utilities Operating Expense, Maintenance and Operations	40,700		39,300		82,400		78,300
Depreciation and amortization	16,800		14,700		34,400		27,200
Segment Reporting, Other Segment Item, Amount	8,400	[1]	8,800	[1]	16,700	[2]	17,700	[2]
Payments to Acquire Productive Assets	91,500		82,000		187,600		141,900
Regulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	400	[5]	3,700	[5]	800	[6]	4,200	[6]
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	151,400		127,000		350,600		294,900
Unregulated Energy [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	47,900		41,400		154,600		124,500
Operating Income
Total operating income	(1,500)		400		24,800		21,800
Identifiable Assets
Total identifiable assets	466,700				466,700				486,400
Utilities Operating Expense, Maintenance and Operations	20,200		18,600		42,000		37,200
Depreciation and amortization	5,100		3,200		10,000		7,700
Segment Reporting, Other Segment Item, Amount	1,200	[1]	1,200	[1]	2,700	[2]	2,700	[2]
Payments to Acquire Productive Assets	7,700		6,500		22,500		16,300
Unregulated Energy [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	6,500	[5]	2,100	[5]	13,700	[6]	7,400	[6]
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	41,400		39,300		140,900		117,100
Other [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	(6,900)	[3]	(5,800)	[3]	(14,500)	[4]	(11,600)	[4]
Identifiable Assets
Total identifiable assets	58,300				58,300				$ 47,700
Utilities Operating Expense, Maintenance and Operations	0	[3]	0	[3]	(100)	[4]	(100)	[4]
Segment Reporting, Other Segment Item, Amount	100	[1],[3]	0	[1],[3]	0	[2],[4]	0	[2],[4]
Payments to Acquire Productive Assets	700	[3]	400	[3]	2,700	[4]	1,300	[4]
Other [Member] | Intersegment Eliminations [Member]
Operating Revenues, Unaffiliated Customers
Total operating revenues, unaffiliated customers	(6,900)	[3],[5]	(5,800)	[3],[5]	(14,500)	[4],[6]	(11,600)	[4],[6]
Other and eliminations [Member]
Operating Income
Total operating income	0	[3]	$ (100)	[3]	0	[4]	0	[4]
Retained Earnings [Member]
Operating Income
Net Income	$ 23,900				$ 74,800		$ 64,400

[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.