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Equity Investments (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Jan. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
MW
Sep. 30, 2021
USD ($)
Jun. 30, 2021
USD ($)
Mar. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Sep. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2021
USD ($)
MW
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
ALLETE's Investment in ATC [Roll Forward]                                
Equity Investment Beginning Balance         $ 301.2                 $ 301.2    
Cash Investments                           17.6 $ 99.1 $ 37.9
Equity Earnings                           20.0 22.1 21.7
Equity Investment Ending Balance   $ 318.0       $ 301.2               318.0 301.2  
Assets [Abstract]                                
Total Current Assets   291.3       254.9               291.3 254.9  
Total Assets   6,435.0       6,084.6               6,435.0 6,084.6  
Liabilities and Equity [Abstract]                                
Liabilities, Current   543.4       459.6               543.4 459.6  
Members' Equity   2,946.3       2,800.2       $ 2,335.6       2,946.3 2,800.2 2,335.6
Total Liabilities and Equity   6,435.0       6,084.6               6,435.0 6,084.6  
Income Statement Data [Abstract]                                
Revenue   399.0 $ 345.4 $ 335.6 339.2 320.4 $ 293.9 $ 243.2 $ 311.6 304.6 $ 288.3 $ 290.4 $ 357.2 1,419.2 1,169.1 1,240.5
Net Income                           137.8 161.6 185.5
ALLETE's Equity in Net Income                           20.0 22.1 21.7
ATC [Member]                                
Income Statement Data [Abstract]                                
Approximate Refund for Reversal of FERC ROE Order   65.0                       65.0    
Approximate Refund for Reversale of FERC ROE Order, ALLETE's Proportionate Share                           5.0    
Approximate Reduction in Pretax Income for FERC Proposal to Limit Incentive adder, ALLETE's Proportionate Share   $ 1.0                       $ 1.0    
FERC Proposed Reduction to Incentive Adder   0.50%                       0.50%    
ATC [Member]                                
Equity Investments [Line Items]                                
Ownership Percentage   8.00%                       8.00%    
Expected Additional Investment in 2022   $ 6.0                       $ 6.0    
ALLETE's Investment in ATC [Roll Forward]                                
Equity Investment Beginning Balance         149.0       $ 141.6         149.0 141.6  
Cash Investments                           0.0 2.7  
Equity Earnings                           21.3 22.3  
Distributed Earnings                           (17.2) (18.9)  
Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments                           1.4 1.3  
Equity Investment Ending Balance   $ 154.5       149.0       $ 141.6       154.5 149.0 141.6
Income Statement Data [Abstract]                                
ALLETE's Equity in Net Income                           $ 21.3 22.3  
Authorized Return on Equity   10.02%                       10.02%    
Authorized Return on Equity, Including Incentive Adder   10.52%                       10.52%    
ATC [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees                                
ALLETE's Investment in ATC [Roll Forward]                                
Equity Earnings                           $ 21.3 22.3 21.7
Assets [Abstract]                                
Total Current Assets   $ 89.8       92.8               89.8 92.8  
Assets, Noncurrent   5,628.1       5,400.5               5,628.1 5,400.5  
Total Assets   5,717.9       5,493.3               5,717.9 5,493.3  
Liabilities and Equity [Abstract]                                
Liabilities, Current   436.9       310.8               436.9 310.8  
Long-Term Debt   2,513.0       2,512.2               2,513.0 2,512.2  
Other Non-Current Liabilities   422.0       378.2               422.0 378.2  
Members' Equity   2,346.0       2,292.1               2,346.0 2,292.1  
Total Liabilities and Equity   $ 5,717.9       5,493.3               5,717.9 5,493.3  
Income Statement Data [Abstract]                                
Revenue                           754.8 758.1 744.4
Operating Expense                           376.2 372.4 373.5
Other Expense                           113.9 110.9 110.5
Net Income                           264.7 274.8 260.4
ALLETE's Equity in Net Income                           $ 21.3 22.3 $ 21.7
ATC [Member] | Subsequent Event [Member]                                
ALLETE's Investment in ATC [Roll Forward]                                
Cash Investments $ 2.7                              
Nobles 2 [Member]                                
Equity Investments [Line Items]                                
Ownership Percentage   49.00%                       49.00%    
ALLETE's Investment in ATC [Roll Forward]                                
Equity Investment Beginning Balance         $ 152.2                 $ 152.2    
Cash Investments                           17.6    
Equity Earnings [1]                           (1.3)    
Distributed Earnings                           (5.0)    
Equity Investment Ending Balance   $ 163.5       $ 152.2               163.5 $ 152.2  
Income Statement Data [Abstract]                                
ALLETE's Equity in Net Income [1]                           $ (1.3)    
Tenaska [Member] | Nobles 2 PPA [Member] | Minnesota Power [Member] | Wind Turbine Generators [Member]                                
Equity Investments [Line Items]                                
Generating Capacity Counterparty Owned (MW) | MW   250                       250    
Contract Term (Years)                           20 years    
[1] The Company also recorded net loss attributable to non-controlling interest of $6.6 million related to its investment in Nobles 2