v3.26.3
Business Segments (Segment Financial Information) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Jun. 30, 2026
Mar. 31, 2026
Sep. 30, 2025
Jun. 30, 2025
Mar. 31, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Segment Reporting [Line Items]                  
Revenues $ 355.6     $ 294.8     $ 965.4 $ 779.0  
Depreciation and amortization 30.5     29.8     93.7 88.6  
Interest expense 19.5     19.2     58.8 63.0  
Total assets 3,948.7           3,948.7   $ 3,781.0
Purchased power 129.0     113.7     341.4 291.6  
Operation and maintenance 85.0     70.8     242.3 201.8  
Taxes other than income taxes 40.6     36.3     117.8 100.4  
Other income / (expense), net 0.4     (0.1)     2.0 0.3  
Income tax expense 12.7     11.5     28.0 15.5  
Net Income (Loss) Attributable to Parent 26.0 $ 4.2 $ 26.6 8.3 $ 0.7 $ 2.7 56.8 11.7  
Segment Reporting, Other Segment Item, Amount (0.3)     0.1     (1.8) (0.3)  
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest 51.3     24.9     113.2 33.9  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 148.5     100.0     366.3 258.3  
NET INCOME 38.6     13.4     85.2 18.4  
Revenue from external customers                  
Segment Reporting [Line Items]                  
Revenues 355.6     294.8     965.4 779.0  
Intersegment revenue                  
Segment Reporting [Line Items]                  
Revenues 0.0     0.0     0.0 0.0  
Utility [Member]                  
Segment Reporting [Line Items]                  
Revenues 353.2     292.7     958.3 772.4  
Operating Segments [Member] | Utility [Member]                  
Segment Reporting [Line Items]                  
Revenues 353.2     292.7     958.3 772.4  
Depreciation and amortization 30.0     29.4     92.4 87.2  
Interest expense 14.9     14.6     45.1 43.0  
Purchased power 128.6     113.6     340.4 290.9  
Operation and maintenance 84.7     70.4     241.4 199.1  
Taxes other than income taxes 40.5     36.3     117.6 100.2  
Income tax expense 12.5     10.9     27.5 16.6  
Segment Reporting, Other Segment Item, Amount 0.0     0.6     (0.8) 2.3  
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest 54.5     27.8     122.2 49.7  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 148.2     98.6     365.2 256.1  
NET INCOME 42.0     16.9     94.7 33.1  
Operating Segments [Member] | Utility [Member] | Revenue from external customers                  
Segment Reporting [Line Items]                  
Revenues 353.1     292.3     958.2 771.5  
Operating Segments [Member] | Utility [Member] | Intersegment revenue                  
Segment Reporting [Line Items]                  
Revenues 0.1     0.4     0.1 0.9  
Corporate, Non-Segment [Member]                  
Segment Reporting [Line Items]                  
Revenues 3.9     3.6     11.4 11.0  
Depreciation and amortization 0.5     0.4     1.3 1.4  
Interest expense 4.6     4.6     13.7 20.0  
Total assets 65.2           65.2   50.2
Purchased power 0.7     0.5     1.9 1.6  
Operation and maintenance 1.5     1.5     4.3 6.2  
Taxes other than income taxes 0.1     0.0     0.2 0.2  
Income tax expense 0.2     0.6     0.5 (1.1)  
Segment Reporting, Other Segment Item, Amount (0.3)     (0.5)     (1.0) (2.6)  
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest (3.2)     (2.9)     (9.0) (15.8)  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 0.3     1.4     1.1 2.2  
NET INCOME (3.4)     (3.5)     (9.5) (14.7)  
Corporate, Non-Segment [Member] | Revenue from external customers                  
Segment Reporting [Line Items]                  
Revenues 2.8     2.5     8.1 7.5  
Corporate, Non-Segment [Member] | Intersegment revenue                  
Segment Reporting [Line Items]                  
Revenues 1.1     1.1     3.3 3.5  
Intersegment Eliminations                  
Segment Reporting [Line Items]                  
Revenues (1.5)     (1.5)     (4.3) (4.4)  
Depreciation and amortization 0.0     0.0     0.0 0.0  
Interest expense 0.0     0.0     0.0 0.0  
Purchased power (0.3)     (0.4)     (0.9) (0.9)  
Operation and maintenance (1.2)     (1.1)     (3.4) (3.5)  
Taxes other than income taxes 0.0     0.0     0.0 0.0  
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest 0.0     0.0     0.0 0.0  
Income tax expense 0.0     0.0     0.0 0.0  
Net Income (Loss) Attributable to Parent       0.0       0.0  
Segment Reporting, Other Segment Item, Amount 0.0     0.0     0.0 0.0  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 0.0     0.0     0.0 0.0  
NET INCOME 0.0           0.0    
Intersegment Eliminations | Revenue from external customers                  
Segment Reporting [Line Items]                  
Revenues (0.3)     0.0     (0.9) 0.0  
Intersegment Eliminations | Intersegment revenue                  
Segment Reporting [Line Items]                  
Revenues (1.2)     $ (1.5)     (3.4) $ (4.4)  
Subsidiaries [Member]                  
Segment Reporting [Line Items]                  
Total assets $ 3,883.5           $ 3,883.5   $ 3,730.8