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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
3 Months Ended
Apr. 21, 2021
Mar. 31, 2021
Dec. 31, 2020
Debt Instrument [Line Items]      
Short-Term Debt - Principal   $ 298.8 $ 204.0
Short-Term Debt - Unamortized Debt Issuance Costs   (0.3) (0.3)
Short-Term Debt - Total   298.5 203.7
Long-Term Debt - Principal   1,661.3 1,602.4
Long-Term Debt - Unamortized Debt Issuance Costs   (8.9) (9.2)
Long-Term Debt - Total   1,652.4 1,593.2
Total Debt - Principal   1,960.1 1,806.4
Total Debt - Unamortized Debt Issuance Costs   (9.2) (9.5)
Total Debt - Total   1,950.9 1,796.9
Unsecured Term Loan Due in 2022      
Debt Instrument [Line Items]      
Unsecured Term Loan, Amount   150.0  
Proceeds from Short-term Debt   100.0  
Debt Instrument, Unused Borrowing Capacity, Amount   $ 50.0  
Variable Rate Basis   LIBOR  
Basis Spread on Variable Rate   0.75%  
Line of Credit [Member]      
Debt Instrument [Line Items]      
Letters of Credit Outstanding, Amount   $ 24.3 22.3
Line of Credit, Current   $ 60.0 $ 0.0
Subsequent Event [Member] | ALLETE Bonds 2.79% Due September 2031      
Debt Instrument [Line Items]      
Proceeds from Issuance of First Mortgage Bond $ 100.0    
Interest Rate 2.79%