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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
Oct. 17, 2018
Oct. 03, 2018
Apr. 16, 2018
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]          
Short-Term Debt - Principal       $ 57.0 $ 64.6
Short-Term Debt - Unamortized Debt Issuance Costs       (0.4) (0.5)
Short-Term Debt - Total       56.6 64.1
Long-Term Debt - Principal       1,470.9 1,448.7
Long-Term Debt - Unamortized Debt Issuance Costs       (9.2) (9.5)
Long-Term Debt - Total       1,461.7 1,439.2
Total Debt - Principal       1,527.9 1,513.3
Total Debt - Unamortized Debt Issuance Costs       (9.6) (10.0)
Total Debt - Total       $ 1,518.3 $ 1,503.3
ALLETE First Mortgage Bonds 4.07% Due April 2048 [Member]          
Debt Instrument [Line Items]          
First Mortgage Bonds - Proceeds from Sale     $ 60.0    
First Mortgage Bonds - Interest Rate     4.07%    
Unsecured Term Loan Due in 2020 [Member] | Subsequent Event [Member]          
Debt Instrument [Line Items]          
Unsecured Term Loan Payment $ 10.0 $ 20.0