v3.26.1
Business Segments (Segment Financial Information) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Segment Reporting [Line Items]      
Revenues $ 341.1 $ 254.3  
Depreciation and amortization 31.7 29.2  
Interest expense 20.0 23.9  
Total assets 3,773.5   $ 3,781.0
Purchased power 119.3 95.1  
Operation and maintenance 80.2 65.5  
Taxes other than income taxes 38.3 36.3  
Other income / (expense), net 0.3 (0.6)  
Income tax expense 12.6 1.0  
Net Income (Loss) Attributable to Parent 26.6 2.7  
Segment Reporting, Other Segment Item, Amount (0.3) 0.6  
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest 51.9 3.7  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 114.3 85.3  
Net income 39.3 2.7  
Revenue from external customers      
Segment Reporting [Line Items]      
Revenues 341.1 254.3  
Intersegment revenue      
Segment Reporting [Line Items]      
Revenues 0.0 0.0  
Utility [Member]      
Segment Reporting [Line Items]      
Revenues 338.7 252.1  
Operating Segments [Member] | Utility [Member]      
Segment Reporting [Line Items]      
Revenues 338.7 252.1  
Depreciation and amortization 31.3 28.8  
Interest expense 15.4 14.7  
Purchased power 119.0 94.8  
Operation and maintenance 80.2 63.9  
Taxes other than income taxes 38.2 36.2  
Income tax expense 12.2 1.7  
Net Income (Loss) Attributable to Parent   10.7  
Segment Reporting, Other Segment Item, Amount 0.2 1.3  
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest 54.4 12.4  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 114.1 85.3  
Net income 42.2    
Operating Segments [Member] | Utility [Member] | Revenue from external customers      
Segment Reporting [Line Items]      
Revenues 338.7 251.8  
Operating Segments [Member] | Utility [Member] | Intersegment revenue      
Segment Reporting [Line Items]      
Revenues 0.0 0.3  
Corporate, Non-Segment [Member]      
Segment Reporting [Line Items]      
Revenues 3.8 3.7  
Depreciation and amortization 0.4 0.4  
Interest expense 4.6 9.2  
Total assets 56.2   50.2
Purchased power 0.6 0.6  
Operation and maintenance 1.1 2.8  
Taxes other than income taxes 0.1 0.1  
Income tax expense 0.4 (0.7)  
Net Income (Loss) Attributable to Parent   (8.0)  
Segment Reporting, Other Segment Item, Amount (0.5) (0.7)  
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest (2.5) (8.7)  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 0.2 0.0  
Net income (2.9)    
Corporate, Non-Segment [Member] | Revenue from external customers      
Segment Reporting [Line Items]      
Revenues 2.7 2.5  
Corporate, Non-Segment [Member] | Intersegment revenue      
Segment Reporting [Line Items]      
Revenues 1.1 1.2  
Intersegment Eliminations      
Segment Reporting [Line Items]      
Revenues (1.4) (1.5)  
Depreciation and amortization 0.0 0.0  
Interest expense 0.0 0.0  
Purchased power (0.3) (0.3)  
Operation and maintenance (1.1) (1.2)  
Taxes other than income taxes 0.0 0.0  
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest 0.0 0.0  
Income tax expense 0.0 0.0  
Net Income (Loss) Attributable to Parent   0.0  
Segment Reporting, Other Segment Item, Amount 0.0 0.0  
Segment Reporting, Long-Lived Asset, Expenditure for Addition 0.0 0.0  
Net income 0.0    
Intersegment Eliminations | Revenue from external customers      
Segment Reporting [Line Items]      
Revenues (0.3) 0.0  
Intersegment Eliminations | Intersegment revenue      
Segment Reporting [Line Items]      
Revenues (1.1) $ (1.5)  
Subsidiaries [Member]      
Segment Reporting [Line Items]      
Total assets $ 3,717.3   $ 3,730.8