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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
Sep. 10, 2020
Aug. 03, 2020
Apr. 08, 2020
Jan. 10, 2020
Sep. 30, 2020
Jul. 06, 2020
Dec. 31, 2019
Debt Instrument [Line Items]              
Short-Term Debt - Principal         $ 404.8   $ 213.3
Short-Term Debt - Unamortized Debt Issuance Costs         (0.3)   (0.4)
Short-Term Debt - Total         404.5   212.9
Long-Term Debt - Principal         1,617.7   1,409.3
Long-Term Debt - Unamortized Debt Issuance Costs         (9.7)   (8.4)
Long-Term Debt - Total         1,608.0   1,400.9
Total Debt - Principal         2,022.5   1,622.6
Total Debt - Unamortized Debt Issuance Costs         (10.0)   (8.8)
Total Debt - Total         2,012.5   1,613.8
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%          
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        
Debt Instrument, Unused Borrowing Capacity, Amount           $ 20.0  
Variable Rate Basis     LIBOR        
Basis Spread on Variable Rate     1.70%        
Debt Instrument, Interest Rate Floor     0.75%        
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%          
Line of Credit [Member]              
Debt Instrument [Line Items]              
Letters of Credit Outstanding, Amount         22.3   62.0
Line of Credit, Current         $ 0.0   $ 0.0
Unsecured Senior Notes Due September 2025 [Member]              
Debt Instrument [Line Items]              
Interest Rate 2.65%            
Unsecured Senior Notes, Amount $ 150.0