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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
Aug. 03, 2020
Apr. 08, 2020
Jan. 10, 2020
Mar. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]          
Short-Term Debt - Principal       $ 323.3 $ 213.3
Short-Term Debt - Unamortized Debt Issuance Costs       (0.3) (0.4)
Short-Term Debt - Total       323.0 212.9
Long-Term Debt - Principal       1,408.0 1,409.3
Long-Term Debt - Unamortized Debt Issuance Costs       (8.1) (8.4)
Long-Term Debt - Total       1,399.9 1,400.9
Total Debt - Principal       1,731.3 1,622.6
Total Debt - Unamortized Debt Issuance Costs       (8.4) (8.8)
Total Debt - Total       1,722.9 1,613.8
Letters of Credit Outstanding, Amount       66.3 62.0
Line of Credit, Current       $ 0.2 $ 0.0
Unsecured Term Loan Due in 2021 [Member]          
Debt Instrument [Line Items]          
Unsecured Term Loan, Amount     $ 200.0    
Proceeds from Issuance of Unsecured Debt     110.0    
Debt Instrument, Unused Borrowing Capacity, Amount     $ 90.0    
Variable Rate Basis     LIBOR    
Basis Spread on Variable Rate     0.55%    
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]          
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%      
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%