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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
Mar. 01, 2019
Jun. 30, 2019
Jan. 10, 2019
Dec. 31, 2018
Debt Instrument [Line Items]        
Short-Term Debt - Principal   $ 28.1   $ 57.9
Short-Term Debt - Unamortized Debt Issuance Costs   (0.4)   (0.4)
Short-Term Debt - Total   27.7   57.5
Long-Term Debt - Principal   1,514.9   1,437.3
Long-Term Debt - Unamortized Debt Issuance Costs   (9.0)   (8.8)
Long-Term Debt - Total   1,505.9   1,428.5
Total Debt - Principal   1,543.0   1,495.2
Total Debt - Unamortized Debt Issuance Costs   (9.4)   (9.2)
Total Debt - Total   1,533.6   1,486.0
Letters of Credit Outstanding, Amount   56.3   18.4
Line of Credit, Current   $ 0.0   $ 0.0
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