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Commitments, Guarantees and Contingencies - Other Matters (Details)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Aug. 06, 2024
complaint
Sep. 30, 2023
USD ($)
Dec. 31, 2024
USD ($)
Dec. 31, 2005
USD ($)
Dec. 31, 2023
Guarantor Obligations [Line Items]          
Production tax credits sold     $ 64.3    
Proceeds from legal settlements, gross   $ 68.3      
Litigation settlement, amount awarded from other party, prejudgment interest   5.1      
Litigation settlement, amount awarded from other party, arbitration costs recovery   3.6      
Litigation settlement, amount awarded from other party, pre tax gain   58.4      
Proceeds from legal settlements   $ 60.0      
Number of complaints | complaint 3        
ALLETE, Inc. | Letters of Credit          
Guarantor Obligations [Line Items]          
Standby letters of credit     134.7    
ALLETE, Inc. | Surety Bonds          
Guarantor Obligations [Line Items]          
Standby letters of credit     122.2    
Regulated Operations          
Guarantor Obligations [Line Items]          
Standby letters of credit and surety bonds outstanding     23.3    
ALLETE Clean Energy          
Guarantor Obligations [Line Items]          
Standby letters of credit and surety bonds outstanding     94.5    
BNI Energy Reclamation Liability          
Guarantor Obligations [Line Items]          
Estimated obligation     82.1    
BNI Energy Reclamation Liability | Surety Bonds          
Guarantor Obligations [Line Items]          
Collateral     88.8    
ALLETE South Wind | Letters of Credit          
Guarantor Obligations [Line Items]          
Standby letters of credit     10.1    
South Shore Energy | Letters of Credit          
Guarantor Obligations [Line Items]          
Standby letters of credit     29.7    
New Energy | Letters of Credit          
Guarantor Obligations [Line Items]          
Collateral     10.4    
ALLETE Properties Development and Maintenance Obligations          
Guarantor Obligations [Line Items]          
Estimated obligation     1.0    
ALLETE Properties Development and Maintenance Obligations | Surety Bonds          
Guarantor Obligations [Line Items]          
Collateral     $ 2.0    
Town Center District Capital Improvement Bonds          
Guarantor Obligations [Line Items]          
Bonds       $ 26.4  
Bond interest rate       6.00%  
Bond term       31 years  
Ownership percentage of benefited property     32.00%   33.00%
Annual assessment     $ 0.7