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Short-Term and Long-Term Debt - Narrative (Details)
 12 Months Ended
Jul. 31, 2024
USD ($)
Apr. 23, 2024
USD ($)
Dec. 31, 2024
USD ($)
Sep. 05, 2024
USD ($)
Dec. 31, 2023
USD ($)
Debt Instrument [Line Items]          
Short-term debt - outstanding     $ 94,700,000   $ 111,400,000
Short-term debt - unamortized debt issuance costs     0    
Long-term debt outstanding     1,704,700,000   1,679,900,000
Long-term debt - unamortized debt issuance costs     8,600,000    
Long-term debt maturing in 2025     94,700,000    
Long-term debt maturing in 2026     80,200,000    
Long-term debt maturing in 2027     182,500,000    
Long-term debt maturing in 2028     55,800,000    
Long-term debt maturing in 2029     220,300,000    
Long-term debt maturing thereafter     $ 1,174,500,000    
Actual indebtedness to total capitalization ratio     0.36    
Dividend payable percentage limit     5.00%    
Dividend payable period     12 months    
Maximum          
Debt Instrument [Line Items]          
Allowed indebtedness to total capitalization ratio     0.65    
Bank Lines of Credit          
Debt Instrument [Line Items]          
Maximum borrowing capacity     $ 362,000,000   423,100,000
Standby letters of credit     16,200,000   19,400,000
Draws outstanding     20,000,000.0   $ 34,100,000
Senior Notes          
Debt Instrument [Line Items]          
Proceeds from issuance of long-term debt       $ 150,000,000  
Camp Ripley Financing          
Debt Instrument [Line Items]          
Annual financing payment     $ 1,400,000    
Financing renewal term     2 years    
Purchase option     $ 4,000,000    
ALLETE Bonds 5.72% Due April 2039          
Debt Instrument [Line Items]          
Proceeds from issuance of first mortgage bond   $ 100,000,000      
Interest rate   5.72%      
2.65% Senior Notes Due September 10, 2025 | Senior Notes          
Debt Instrument [Line Items]          
Interest rate 2.65%        
Repayments of senior debt $ 150,000,000        
Percentage of principal amount prepaid 100.00%        
5.94% Senior Unsecured Notes Due September 2029 | Senior Notes          
Debt Instrument [Line Items]          
Interest rate       5.94%  
Proceeds from issuance of long-term debt       $ 100,000,000  
6.18% Senior Unsecured Notes Due September 2034 | Senior Notes          
Debt Instrument [Line Items]          
Interest rate       6.18%  
Proceeds from issuance of long-term debt       $ 50,000,000