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Commitments, Guarantees and Contingencies - Other Matters (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2005
Dec. 31, 2022
Guarantor Obligations [Line Items]        
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      
ALLETE, Inc. | Letters of Credit        
Guarantor Obligations [Line Items]        
Standby letters of credit   $ 149.8    
Regulated Operations | Letters of Credit        
Guarantor Obligations [Line Items]        
Standby letters of credit   24.2    
ALLETE Clean Energy | Letters of Credit        
Guarantor Obligations [Line Items]        
Standby letters of credit   91.6    
BNI Energy Reclamation Liability        
Guarantor Obligations [Line Items]        
Estimated obligation   82.1    
BNI Energy Reclamation Liability | Surety Bonds        
Guarantor Obligations [Line Items]        
Collateral   82.4    
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   23.9    
ALLETE Properties Development and Maintenance Obligations        
Guarantor Obligations [Line Items]        
Estimated obligation   1.0    
ALLETE Properties Development and Maintenance Obligations | Surety Bonds and Letters of Credit        
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   33.00%   42.00%
Annual assessment   $ 0.7