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Short-Term and Long-Term Debt - Narrative (Details)
$ in Millions
 12 Months Ended
Apr. 27, 2023
USD ($)
Dec. 31, 2023
USD ($)
Oct. 17, 2023
USD ($)
Oct. 16, 2023
USD ($)
Dec. 31, 2022
USD ($)
Debt Instrument [Line Items]          
Short-term debt - outstanding   $ 111.4     $ 272.6
Short-term debt - unamortized debt issuance costs   0.1      
Long-Term Debt   1,679.9     1,648.2
Long-term debt - unamortized debt issuance costs   8.0      
Long-term debt maturing in 2024   111.4      
Long-term debt maturing in 2025   244.7      
Long-term debt maturing in 2026   80.2      
Long-term debt maturing in 2027   162.5      
Long-term debt maturing in 2028   55.8      
Long-term debt maturing thereafter   $ 1,144.8      
Actual indebtedness to total capitalization ratio   0.36      
Maximum          
Debt Instrument [Line Items]          
Allowed indebtedness to total capitalization ratio   0.65      
Bank Lines of Credit          
Debt Instrument [Line Items]          
Maximum borrowing capacity   $ 423.1     475.7
Standby letters of credit   19.4     32.8
Draws outstanding   34.1     $ 31.3
Credit Agreement | Line of Credit          
Debt Instrument [Line Items]          
Maximum borrowing capacity     $ 355.0 $ 400.0  
Extension period     1 year    
Issuable letters of credit     $ 100.0    
Camp Ripley Financing          
Debt Instrument [Line Items]          
Annual financing payment   $ 1.4      
Financing renewal term   2 years      
Purchase option   $ 4.0      
ALLETE Bonds 4.98% Due April 2033          
Debt Instrument [Line Items]          
Proceeds from issuance of First Mortgage Bond $ 125.0        
Interest rate 4.98%