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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
6 Months Ended
Apr. 21, 2021
Jun. 30, 2021
Dec. 31, 2020
Debt Instrument [Line Items]      
Short-Term Debt - Principal   $ 362.8 $ 204.0
Short-Term Debt - Unamortized Debt Issuance Costs   (0.2) (0.3)
Short-Term Debt - Total   362.6 203.7
Long-Term Debt - Principal   1,673.3 1,602.4
Long-Term Debt - Unamortized Debt Issuance Costs   (8.7) (9.2)
Long-Term Debt - Total   1,664.6 1,593.2
Total Debt - Principal   2,036.1 1,806.4
Total Debt - Unamortized Debt Issuance Costs   (8.9) (9.5)
Total Debt - Total   2,027.2 1,796.9
Unsecured Term Loan Due in 2022      
Debt Instrument [Line Items]      
Unsecured Term Loan, Amount   $ 150.0  
Variable Rate Basis   LIBOR  
Basis Spread on Variable Rate   0.75%  
Debt Drawn Through Accordion Feature   $ 35.0  
ALLETE Bonds 2.79% Due September 2031      
Debt Instrument [Line Items]      
Proceeds from Issuance of First Mortgage Bond $ 100.0    
Interest Rate 2.79%    
Line of Credit [Member]      
Debt Instrument [Line Items]      
Letters of Credit Outstanding, Amount   18.7 22.3
Line of Credit, Current   $ 63.8 $ 0.0