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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
Aug. 14, 2019
Mar. 01, 2019
Sep. 30, 2019
Jan. 10, 2019
Dec. 31, 2018
Debt Instrument [Line Items]          
Short-Term Debt - Principal     $ 213.1   $ 57.9
Short-Term Debt - Unamortized Debt Issuance Costs     (0.4)   (0.4)
Short-Term Debt - Total     212.7   57.5
Long-Term Debt - Principal     1,413.6   1,437.3
Long-Term Debt - Unamortized Debt Issuance Costs     (8.7)   (8.8)
Long-Term Debt - Total     1,404.9   1,428.5
Total Debt - Principal     1,626.7   1,495.2
Total Debt - Unamortized Debt Issuance Costs     (9.1)   (9.2)
Total Debt - Total     1,617.6   1,486.0
Letters of Credit Outstanding, Amount     60.1   18.4
Line of Credit, Current     $ 0.0   $ 0.0
Unsecured Term Loan Due in 2020 [Member]          
Debt Instrument [Line Items]          
Proceeds from Issuance of Unsecured Debt $ 110.0        
Variable Rate Basis LIBOR        
Basis Spread on Variable Rate 1.025%        
ALLETE Bonds 4.08% Due March 2029 [Member]          
Debt Instrument [Line Items]          
Proceeds from Issuance of First Mortgage Bond   $ 70.0      
Interest Rate   4.08%      
ALLETE Bonds 4.47% Due March 2049 [Member]          
Debt Instrument [Line Items]          
Proceeds from Issuance of First Mortgage Bond   $ 30.0      
Interest Rate   4.47%      
Revolving Credit Facility [Member]          
Debt Instrument [Line Items]          
Line of Credit Facility, Maximum Borrowing Capacity       $ 400.0  
Line of Credit Facility Additional Borrowing Capacity       150.0  
Letters of Credit Maximum Issuances       $ 100.0