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BUSINESS SEGMENTS - Business Segments (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Business Segments [Line Items]          
Total Operating Revenue $ 407.2 $ 378.8 $ 1,165.0 $ 1,477.1  
Net Income (Loss) Attributable to ALLETE 45.0 85.9 128.7 195.6  
Assets 6,737.2   6,737.2   $ 6,656.4
Corporate and Other          
Business Segments [Line Items]          
Total Operating Revenue 407.2 378.8 1,165.0 1,477.1  
Regulated Operations          
Business Segments [Line Items]          
Net Income (Loss) Attributable to ALLETE 34.0 34.0 111.9 112.4  
Assets 4,410.3   4,410.3   4,335.0
Regulated Operations | Operating Segments          
Business Segments [Line Items]          
Total Operating Revenue 310.5 314.3 928.6 919.1  
Regulated Operations | Residential | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 41.9 37.9 130.5 123.3  
Regulated Operations | Commercial | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 49.5 49.2 143.0 140.9  
Regulated Operations | Municipal | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 9.2 8.7 25.6 25.2  
Regulated Operations | Industrial | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 149.8 154.3 456.1 439.4  
Regulated Operations | Other Power Suppliers | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 32.0 31.0 102.4 103.2  
Regulated Operations | Other | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 28.1 33.2 71.0 87.1  
ALLETE Clean Energy          
Business Segments [Line Items]          
Net Income (Loss) Attributable to ALLETE 3.9 54.8 10.1 66.4  
Assets 1,581.6   1,581.6   1,594.1
Gain from legal settlements - after tax         44.3
ALLETE Clean Energy | Operating Segments          
Business Segments [Line Items]          
Total Operating Revenue 34.1 14.2 68.9 396.8  
ALLETE Clean Energy | Long-term PSA | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 9.9 12.9 42.2 44.5  
ALLETE Clean Energy | Sale of Wind Energy Facilities | Operating Segments          
Business Segments [Line Items]          
Operating Revenue 22.9 0.0 22.9 348.4  
ALLETE Clean Energy | Other | Operating Segments          
Business Segments [Line Items]          
Operating revenue, other than customer revenue 1.3 1.3 3.8 3.9  
Corporate and Other          
Business Segments [Line Items]          
Net Income (Loss) Attributable to ALLETE 7.1 (2.9) 6.7 16.8  
Assets 745.3   745.3   727.3
Purchase price accounting - after tax         $ 3.8
Corporate and Other | Corporate and Other          
Business Segments [Line Items]          
Total Operating Revenue 62.6 50.3 167.5 161.2  
Corporate and Other | Long-term Contract | Corporate and Other          
Business Segments [Line Items]          
Operating Revenue 26.7 24.6 79.4 74.0  
Corporate and Other | Sale of Renewable Development Projects | Corporate and Other          
Business Segments [Line Items]          
Operating Revenue 29.2 20.8 68.7 73.2  
Corporate and Other | Other | Corporate and Other          
Business Segments [Line Items]          
Operating Revenue $ 6.7 $ 4.9 $ 19.4 $ 14.0