v3.26.1
Business Segment Information: Information Relating to Segments Statement of Income (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Mar. 31, 2026
Jun. 30, 2025
Mar. 31, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting Information            
Revenue $ 452.8   $ 439.0   $ 1,233.5 $ 1,244.2
Fuel, purchased power and cost of natural gas sold 113.5   124.0   451.3 483.8
Operations and maintenance - Direct 78.0   73.3   149.7 150.0
Operations and maintenance - Allocated 72.5   74.3   148.9 151.3
Depreciation and amortization 75.3   69.8   150.1 139.0
Taxes other than income taxes 16.5   15.1   34.5 32.7
Total operating expenses 355.8   356.5   934.5 956.8
Operating income (loss) 97.0   82.5   299.0 287.4
Interest expense, net (51.6)   (48.9)   (103.5) (100.3)
Other income, net 0.2   (0.4)   0.8 0.6
Income tax (expense) (5.2)   (4.4)   (22.8) (22.5)
Net income 40.4 $ 133.1 28.8 $ 136.4 173.5 165.2
Net income attributable to non-controlling interest (2.2)   (1.3)   (4.3) (3.5)
Net income available for common stock 38.2   27.5   169.2 161.7
Corporate, Non-Segment            
Segment Reporting Information            
Revenue (4.0)   (3.9)   (8.0) (7.8)
Fuel, purchased power and cost of natural gas sold (0.1)   (0.2)   (0.3) (0.1)
Operations and maintenance - Direct 3.3   (1.7)   1.5 (4.8)
Total operating expenses 3.2   (1.9)   1.2 (4.9)
Operating income (loss) (7.2)   (2.0)   (9.2) (2.9)
Operating Segments            
Segment Reporting Information            
Revenue 456.8   442.9   1,241.5 1,252.0
Fuel, purchased power and cost of natural gas sold 113.6   124.2   451.6 483.9
Operations and maintenance - Direct 74.7   75.0   148.2 154.8
Operations and maintenance - Allocated 72.5   74.3   148.9 151.3
Depreciation and amortization 75.3   69.8   150.1 139.0
Taxes other than income taxes 16.5   15.1   34.5 32.7
Total operating expenses 352.6   358.4   933.3 961.7
Operating income (loss) 104.2   84.5   308.2 290.3
Operating Segments | Electric Utilities            
Segment Reporting Information            
Revenue 226.3   219.9   467.9 456.6
Fuel, purchased power and cost of natural gas sold 49.7   55.3   116.5 122.5
Operations and maintenance - Direct 33.4   36.1   66.5 71.8
Operations and maintenance - Allocated 32.1   33.1   65.0 66.2
Depreciation and amortization 40.9   37.5   81.4 74.6
Taxes other than income taxes 9.1   8.9   18.3 18.2
Total operating expenses 165.2   170.9   347.7 353.3
Operating income (loss) 61.1   49.0   120.2 103.3
Operating Segments | Gas Utilities            
Segment Reporting Information            
Revenue 230.5   223.0   773.6 795.4
Fuel, purchased power and cost of natural gas sold 63.9   68.9   335.1 361.4
Operations and maintenance - Direct 41.3   38.9   81.7 83.0
Operations and maintenance - Allocated 40.4   41.2   83.9 85.1
Depreciation and amortization 34.4   32.3   68.7 64.4
Taxes other than income taxes 7.4   6.2   16.2 14.5
Total operating expenses 187.4   187.5   585.6 608.4
Operating income (loss) 43.1   35.5   188.0 187.0
External Customers            
Segment Reporting Information            
Revenue 452.8   439.0   1,233.5 1,244.2
External Customers | Operating Segments            
Segment Reporting Information            
Revenue 452.8   439.0   1,233.5 1,244.2
External Customers | Operating Segments | Electric Utilities            
Segment Reporting Information            
Revenue 223.8   217.5   462.8 451.8
External Customers | Operating Segments | Gas Utilities            
Segment Reporting Information            
Revenue 229.0   221.5   770.7 792.4
Inter-segment | Corporate, Non-Segment            
Segment Reporting Information            
Revenue (4.0)   (3.9)   (8.0) (7.8)
Inter-segment | Operating Segments            
Segment Reporting Information            
Revenue 4.0   3.9   8.0 7.8
Inter-segment | Operating Segments | Electric Utilities            
Segment Reporting Information            
Revenue 2.5   2.4   5.1 4.8
Inter-segment | Operating Segments | Gas Utilities            
Segment Reporting Information            
Revenue $ 1.5   $ 1.5   $ 2.9 $ 3.0