Acquisitions (Details)	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Business Combinations [Abstract]
Reason for Business Acquisitions	The following acquisition is consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services businesses to complement its regulated businesses, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth.	The following acquisition is consistent with ALLETE?s stated strategy of investing in energy infrastructure and related services businesses to complement its regulated businesses, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth.

This element represents a description of the primary reason for the business combination which may consist of general categories such as top-line growth, synergistic benefits, market share, and diversification and the more detailed factors that might apply.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

Name:	us-gaap_BusinessCombinationReasonForBusinessCombination
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration