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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
Aug. 03, 2020
Apr. 08, 2020
Jan. 10, 2020
Jul. 06, 2020
Jun. 30, 2020
Dec. 31, 2019
Debt Instrument [Line Items]            
Short-Term Debt - Principal         $ 510.0 $ 213.3
Short-Term Debt - Unamortized Debt Issuance Costs         (0.3) (0.4)
Short-Term Debt - Total         509.7 212.9
Long-Term Debt - Principal         1,389.0 1,409.3
Long-Term Debt - Unamortized Debt Issuance Costs         (8.0) (8.4)
Long-Term Debt - Total         1,381.0 1,400.9
Total Debt - Principal         1,899.0 1,622.6
Total Debt - Unamortized Debt Issuance Costs         (8.3) (8.8)
Total Debt - Total         1,890.7 1,613.8
Unsecured Term Loan Due in 2021 [Member]            
Debt Instrument [Line Items]            
Unsecured Term Loan, Amount     $ 200.0      
Variable Rate Basis     LIBOR      
Basis Spread on Variable Rate     0.55%      
Unsecured Term Loan Due April 2021 [Member]            
Debt Instrument [Line Items]            
Unsecured Term Loan, Amount   $ 115.0        
Proceeds from Issuance of Unsecured Debt   $ 95.0        
Variable Rate Basis   LIBOR        
Basis Spread on Variable Rate   1.70%        
Debt Instrument, Interest Rate Floor   0.75%        
Line of Credit [Member]            
Debt Instrument [Line Items]            
Letters of Credit Outstanding, Amount         23.5 62.0
Line of Credit, Current         $ 0.0 $ 0.0
Subsequent Event [Member] | ALLETE Bonds 2.50% Due August 2030 [Member]            
Debt Instrument [Line Items]            
Proceeds from Issuance of First Mortgage Bond $ 46.0          
Interest Rate 2.50%          
Subsequent Event [Member] | Unsecured Term Loan Due April 2021 [Member]            
Debt Instrument [Line Items]            
Debt Instrument, Unused Borrowing Capacity, Amount       $ 20.0    
Subsequent Event [Member] | ALLETE Bonds 3.30% Due August 2050 [Member]            
Debt Instrument [Line Items]            
Proceeds from Issuance of First Mortgage Bond $ 94.0          
Interest Rate 3.30%