Document and Entity Information Document (USD $)
9 Months Ended
Sep. 30, 2013
Jun. 30, 2012
Document and Entity Information [Abstract]    
Entity Registrant Name ALLETE INC  
Entity Central Index Key 0000066756  
Current Fiscal Year End Date --12-31  
Entity Well-known Seasoned Issuer Yes  
Entity Voluntary Filers No  
Entity Current Reporting Status Yes  
Entity Filer Category Large Accelerated Filer  
Entity Public Float   $ 1,591,836,880
Entity Common Stock, Shares Outstanding 40,736,213  
Document Fiscal Year Focus 2013  
Document Fiscal Period Focus Q3  
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2013