Document and Entity Information Document - USD ($)
12 Months Ended
Dec. 31, 2019
Feb. 01, 2020
Jun. 30, 2019
Document and Entity Information [Line Items]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 1-3548    
Entity Registrant Name ALLETE, Inc.    
Entity Incorporation, State MN    
Entity Tax Identification Number 41-0418150    
Entity Address, Address Line 30 West Superior Street    
Entity Address, City Duluth    
Entity Address, State MN    
Entity Address, Postal Zip Code 55802-2093    
City Area Code 218    
Local Phone Number 279-5000    
Title of 12(b) Security Common Stock, without par value    
Trading Symbol ALE    
Security Exchange Name NYSE    
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 Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 4,285,299,935
Entity Common Stock, Shares Outstanding   51,696,497  
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false