v3.26.1
Debt (Narrative) (Details)
$ in Millions
3 Months Ended
Mar. 31, 2026
USD ($)
Dec. 31, 2025
USD ($)
Debt Instrument [Line Items]    
Less: current portion $ (0.2) $ (0.2)
Debt Instrument, Unamortized Discount (Premium), Net (0.4) (0.4)
Line of Credit Facility, Maximum Borrowing Capacity 350.0  
Subsidiaries [Member]    
Debt Instrument [Line Items]    
Long-term Line of Credit 30.0 0.0
Deferred Finance Costs, Net, Including Acquisition Adjustments (8.8) (9.1)
Debt Instrument, Unamortized Discount (Premium), Net (2.4) (2.5)
3.95% Senior Notes due 2049 [Member] | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 425.0 425.0
Debt Instrument, Interest Rate, Effective Percentage 3.95%  
Revolving Credit Facility [Member] | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum 0.67  
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum Upper 1.00  
Uncommitted Accordion Feature $ 150.0  
3.25% First Mortgage Bonds due 2040 [Member] | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 140.0 140.0
Debt Instrument, Interest Rate, Effective Percentage 3.20%  
U.S. Government note maturing in February 2061 - 4.20% [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 16.4 16.4
U.S. Government note maturing in February 2061 - 4.20% [Member] | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Debt instrument interest percentage 4.20%  
4.25% Tax-exempt First Mortgage Bonds due 2027 | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 100.0 100.0
Debt Instrument, Interest Rate, Effective Percentage 4.25%  
4.00% Tax-exempt First Mortgage Bonds due 2027 | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 40.0 40.0
Debt Instrument, Interest Rate, Effective Percentage 4.00%  
5.19% First Mortgage Bonds due 2033 | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 100.0 100.0
Debt Instrument, Interest Rate, Effective Percentage 5.19%  
5.49% Tax-exempt First Mortgage Bonds due 2028 | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 5.49%  
5.70% Tax-exempt First Mortgage Bonds due 2033 | Subsidiaries [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Effective Percentage 5.70%  
4.35% Senior Notes due 2029 [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Gross $ 400.0 $ 400.0
Debt instrument interest percentage 4.35%