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Short-Term and Long-Term Debt (Details) - USD ($)
$ in Millions
Apr. 16, 2018
Jun. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Short-Term Debt - Principal   $ 57.1 $ 64.6
Short-Term Debt - Unamortized Debt Issuance Costs   (0.4) (0.5)
Short-Term Debt - Total   56.7 64.1
Long-Term Debt - Principal   1,471.7 1,448.7
Long-Term Debt - Unamortized Debt Issuance Costs   (9.5) (9.5)
Long-Term Debt - Total   1,462.2 1,439.2
Total Debt - Principal   1,528.8 1,513.3
Total Debt - Unamortized Debt Issuance Costs   (9.9) (10.0)
Total Debt - Total   $ 1,518.9 $ 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%