Document and Entity Information Document (USD $)
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
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   $ 2,175,249,161
Entity Common Stock, Shares Outstanding 48,850,462  
Document Fiscal Year Focus 2015  
Document Fiscal Period Focus Q2  
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2015