Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

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

- Definition

Amount of gain (loss) related to nonoperating activities, classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7,9)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_OtherNonoperatingGainsLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow associated with the purchase of noncontrolling interest during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	dei_LegalEntityAxis=ale_ALLETECleanEnergyMember
Namespace Prefix:
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Balance Type:
Period Type:

Acquisitions - Acquisition of Non-Controlling Interest (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Apr. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Acquisition of Non-Controlling Interest [Line Items]
Payment to Acquire Non-Controlling Interest		$ 0.0	$ 8.0
ALLETE Clean Energy [Member]
Acquisition of Non-Controlling Interest [Line Items]
Payment to Acquire Non-Controlling Interest	$ 8.0
Gain or Loss in Net Income or Other Comprehensive Income	$ 0.0