v3.26.1
Segment Information - Schedule of Segment Reporting Information by Segment (Detail)
$ in Thousands
 3 Months Ended
Mar. 31, 2026
USD ($)
Mar. 31, 2025
USD ($)
Dec. 31, 2025
USD ($)
Segment Reporting [Line Items]      
Number of Reportable Segments 2    
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers $ 353,100 $ 298,700  
Operating Income      
Total operating income 99,400 86,800  
Other expense, net 0 600  
Interest 18,700 18,100  
Income Before Income Taxes 80,700 69,300  
Income taxes 21,400 18,400  
Net Income 59,300 50,900  
Identifiable Assets      
Total identifiable assets 4,095,900   $ 3,994,800
Cost of Sales 146,900 116,300  
Utilities Operating Expense, Maintenance and Operations 75,300 63,400  
Depreciation and amortization 21,500 22,500  
Segment Reporting, Other Segment Item, Amount [1] 10,000 9,700  
Regulated natural gas and electric costs 101,600 71,500  
Unregulated Energy Cost of Sales 55,100 52,200  
Other Cost of Sales [2] (9,800) (7,400)  
Payments to Acquire Productive Assets 121,900 112,900  
Intersegment Eliminations [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers [3] 0 0  
Operating Revenues, Unaffiliated Customers [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers 353,100 298,700  
Regulated Energy [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers 249,300 199,600  
Operating Income      
Total operating income 71,100 60,500  
Identifiable Assets      
Total identifiable assets 3,509,900   3,425,300
Utilities Operating Expense, Maintenance and Operations 52,000 41,700  
Depreciation and amortization 16,100 17,600  
Segment Reporting, Other Segment Item, Amount [1] 8,500 8,300  
Payments to Acquire Productive Assets 94,100 96,100  
Regulated Energy [Member] | Intersegment Eliminations [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers [3] 600 400  
Regulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers 248,700 199,200  
Unregulated Energy [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers 113,700 106,700  
Operating Income      
Total operating income 28,300 26,300  
Identifiable Assets      
Total identifiable assets 497,400   495,000
Utilities Operating Expense, Maintenance and Operations 23,500 21,800  
Depreciation and amortization 5,400 4,900  
Segment Reporting, Other Segment Item, Amount [1] 1,400 1,500  
Payments to Acquire Productive Assets 13,500 14,800  
Unregulated Energy [Member] | Intersegment Eliminations [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers [3] 9,300 7,200  
Unregulated Energy [Member] | Operating Revenues, Unaffiliated Customers [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers 104,400 99,500  
Other [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers [2] (9,900) (7,600)  
Identifiable Assets      
Total identifiable assets 88,600   $ 74,500
Utilities Operating Expense, Maintenance and Operations [2] (200) (100)  
Segment Reporting, Other Segment Item, Amount [1],[2] 100 (100)  
Other [Member] | Intersegment Eliminations [Member]      
Operating Revenues, Unaffiliated Customers      
Total operating revenues, unaffiliated customers [2],[3] (9,900) (7,600)  
Other and eliminations [Member]      
Operating Income      
Total operating income [2] 0 0  
Other [Member]      
Identifiable Assets      
Payments to Acquire Productive Assets 14,300 2,000  
Retained Earnings [Member]      
Operating Income      
Net Income $ 59,300 $ 50,900  
[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]
(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.