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Short-Term and Long-Term Debt - Long-Term Debt (Details) - USD ($)
$ in Millions
12 Months Ended
Nov. 02, 2017
Aug. 25, 2017
Jun. 01, 2017
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]          
Long-Term Debt       $ 1,439.2 $ 1,370.4
Long-Term Debt - Unamortized Debt Issuance Costs       9.5  
Debt Instrument [Abstract]          
Long-Term Debt Maturing in 2018       64.6  
Long-Term Debt Maturing in 2019       57.6  
Long-Term Debt Maturing in 2020       143.0  
Long-Term Debt Maturing in 2021       97.8  
Long-Term Debt Maturing in 2022       88.0  
Long-Term Debt Maturing Thereafter       1,062.3  
Camp Ripley Financing [Member]          
Debt Instrument [Abstract]          
Annual Financing Payment       $ 1.4  
Financing Renewal Term (Years)       2 years  
Purchase Option       $ 4.0  
ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member]          
Debt Instrument [Abstract]          
Proceeds from Issuance of Unsecured Debt     $ 80.0    
Interest Rate     3.11% 3.11%  
ALLETE Term Loan Variable Rate Due 2020 [Member]          
Debt Instrument [Abstract]          
Proceeds from Issuance of Unsecured Debt   $ 40.0      
Variable Rate Basis   LIBOR      
Basis Spread on Variable Rate   1.025%      
ALLETE Bonds 4.07% Due April 2048 [Member]          
Debt Instrument [Abstract]          
Expected Proceeds from Future Issuance of Senior First Mortgage Bonds $ 60.0        
Expected Future Interest Rate 4.07%