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Short-Term and Long-Term Debt - Financial Covenants (Details) | 12 Months Ended
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Dec. 31, 2015
Financial Covenants [Abstract] |
Required Indebtedness to Total Capitalization Ratio | 0.65
Actual Indebtedness to Total Capitalization Ratio | 0.47
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
Name: | ale_DebtInstrumentActualIndebtednessToTotalCapitalRatio
Namespace Prefix: | ale_
Data Type: | xbrli:pureItemType
Balance Type: | na
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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- References No definition available. + Details
Name: | ale_FinancialCovenantsAbstract
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 + Details
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