v3.25.4
Business Segment Information: Information Relating to Segments Statement of Income (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Segment Reporting Information      
Revenue $ 2,310.0 $ 2,127.7 $ 2,331.3
Fuel, purchased power and cost of natural gas sold 831.5 730.3 982.9
Operations and maintenance - Direct 288.7 266.6 261.5
Operations and maintenance - Allocated 301.1 290.4 290.5
Depreciation, depletion and amortization 283.8 270.1 256.8
Taxes other than income taxes 67.4 67.2 66.9
Operating income (loss) 537.5 503.1 472.7
Interest expense, net (200.1) (181.7) (167.9)
Other income (expense), net 6.1 (1.4) (3.2)
Income tax (expense) (43.7) (36.3) (25.6)
Net income 299.8 283.7 276.0
Net income attributable to non-controlling interest (8.2) (10.6) (13.8)
Net income available for common stock 291.6 273.1 262.2
Reportable Segment, Aggregation before Other Operating Segment [Member]      
Segment Reporting Information      
Revenue 2,325.6 2,145.5 2,349.2
Fuel, purchased power and cost of natural gas sold 831.9 730.7 983.3
Operations and maintenance - Direct 298.1 282.9 274.4
Operations and maintenance - Allocated 301.1 290.4 290.5
Depreciation, depletion and amortization 283.8 270.0 256.5
Taxes other than income taxes 67.4 67.2 66.9
Operating income (loss) 543.3 504.3 477.6
Corporate, Non-Segment      
Segment Reporting Information      
Revenue (15.6) (17.8) (17.9)
Fuel, purchased power and cost of natural gas sold (0.4) (0.4) (0.4)
Operations and maintenance - Direct (9.4) (16.3) (12.9)
Depreciation, depletion and amortization   0.1 0.3
Operating income (loss) (5.8) (1.2) (4.9)
Operating Segments | Electric Utilities      
Segment Reporting Information      
Revenue 942.8 876.1 865.0
Fuel, purchased power and cost of natural gas sold 259.6 206.4 200.1
Operations and maintenance - Direct 138.6 127.5 112.2
Operations and maintenance - Allocated 132.6 125.1 124.0
Depreciation, depletion and amortization 152.4 145.3 142.6
Taxes other than income taxes 37.1 38.8 37.3
Operating income (loss) 222.5 233.0 248.8
Operating Segments | Gas Utilities      
Segment Reporting Information      
Revenue 1,382.8 1,269.4 1,484.2
Fuel, purchased power and cost of natural gas sold 572.3 524.3 783.2
Operations and maintenance - Direct 159.5 155.4 162.2
Operations and maintenance - Allocated 168.5 165.3 166.5
Depreciation, depletion and amortization 131.4 124.7 113.9
Taxes other than income taxes 30.3 28.4 29.6
Operating income (loss) 320.8 271.3 228.8
External Customers      
Segment Reporting Information      
Revenue 2,310.0 2,127.7 2,331.3
External Customers | Reportable Segment, Aggregation before Other Operating Segment [Member]      
Segment Reporting Information      
Revenue 2,310.0 2,127.7 2,331.3
External Customers | Operating Segments | Electric Utilities      
Segment Reporting Information      
Revenue 933.2 864.4 853.6
External Customers | Operating Segments | Gas Utilities      
Segment Reporting Information      
Revenue 1,376.8 1,263.3 1,477.7
Inter-segment | Reportable Segment, Aggregation before Other Operating Segment [Member]      
Segment Reporting Information      
Revenue 15.6 17.8 17.9
Inter-segment | Corporate, Non-Segment      
Segment Reporting Information      
Revenue (15.6) (17.8) (17.9)
Inter-segment | Operating Segments | Electric Utilities      
Segment Reporting Information      
Revenue 9.6 11.7 11.4
Inter-segment | Operating Segments | Gas Utilities      
Segment Reporting Information      
Revenue $ 6.0 $ 6.1 $ 6.5