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Short-Term and Long-Term Debt - Long-Term Debt (Details)
$ in Millions
Nov. 05, 2015
USD ($)
Sep. 24, 2015
USD ($)
Aug. 25, 2015
USD ($)
Dec. 31, 2015
USD ($)
Jul. 01, 2015
USD ($)
Dec. 31, 2014
USD ($)
Debt Instrument [Line Items]            
Long-Term Debt       $ 1,568.7   $ 1,272.8
Required Indebtedness to Total Capitalization Ratio       0.65    
Proceeds from Issuance of First Mortgage Bond   $ 100.0        
Long-term Debt Maturities [Abstract]            
Long-Term Debt, Maturing in 2016       $ 36.3    
Long-Term Debt, Maturing in 2017       193.6    
Long-Term Debt, Maturing in 2018       64.1    
Long-Term Debt, Maturing in 2019       56.5    
Long-Term Debt, Maturing in 2020       103.0    
Long-Term Debt, Maturing Thereafter       $ 1,151.5    
Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member]            
Debt Instrument [Line Items]            
Proceeds from Issuance of Senior Secured Notes $ 84.5          
Interest Rate 3.26%          
ALLETE Term Loan Variable Rate Due 2017 [Member]            
Debt Instrument [Line Items]            
Principal Amount     $ 125.0      
Description of Variable Rate Basis     LIBOR      
Basis Spread on Variable Rate     0.625%      
Debt Refinanced     $ 75.0      
Required Indebtedness to Total Capitalization Ratio     0.65      
Cross-Default Amount     $ 35.0      
ALLETE First Mortgage Bonds 2.80% Due September 2020 [Member]            
Debt Instrument [Line Items]            
Proceeds from Issuance of First Mortgage Bond   $ 40.0        
Interest Rate   2.80%        
ALLETE First Mortgage Bonds 3.86% Due September 2030 [Member]            
Debt Instrument [Line Items]            
Proceeds from Issuance of First Mortgage Bond   $ 60.0        
Interest Rate   3.86%        
Armenia Mountain [Member]            
Debt Instrument [Line Items]            
Long-Term Debt         $ 60.9  
Long-Term Debt Due Within One Year         $ 5.9