v3.26.3
Debt (Narrative) (Details)
$ in Millions
9 Months Ended
Sep. 30, 2026
USD ($)
Sep. 30, 2025
USD ($)
Dec. 31, 2025
USD ($)
Debt Instrument [Line Items]      
Proceeds from Issuance of Long-term Debt $ 0.0 $ 375.0  
Less: current portion (139.9)   $ (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 0.0   0.0
Deferred Finance Costs, Net, Including Acquisition Adjustments (8.3)   (9.1)
Debt Instrument, Unamortized Discount (Premium), Net (2.3)   (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 $ 16.3   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%    
4.13% Senior Notes due 2025      
Debt Instrument [Line Items]      
Long-term Debt, Gross $ 415.0    
Debt instrument interest percentage 4.125%