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Short-Term and Long-Term Debt - Financial Covenants (Details)	12 Months Ended
	Dec. 31, 2017
Financial Covenants [Abstract]
Allowed Indebtedness to Total Capitalization Ratio	0.65
Actual Indebtedness to Total Capitalization Ratio	0.42
Compliance with Financial Covenants	ALLETE was in compliance with its financial covenants.

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- Definition

Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.

+ References

No definition available.

+ Details

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Period Type:	instant

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- Definition

Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant.

+ References

No definition available.

+ Details

Name:	ale_DebtInstrumentRequiredIndebtednessToTotalCapitalRatio
Namespace Prefix:	ale_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

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- Definition

States whether the entity was in compliance with the debt covenants throughout the reporting period, and describes facts and circumstances of any compliance failure.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

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Period Type:	duration

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- References

No definition available.

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