Document and Entity Information Document
3 Months Ended
Mar. 31, 2018
Document and Entity Information [Line Items]  
Entity Registrant Name ALLETE INC
Entity Central Index Key 0000066756
Entity Tax Identification Number 410418150
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 Common Stock, Shares Outstanding 51,271,007
Document Fiscal Year Focus 2018
Document Fiscal Period Focus Q1
Document Type 10-Q
Amendment Flag false
Document Period End Date Mar. 31, 2018