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Short-Term and Long-Term Debt - Financial Covenants (Details) | Dec. 31, 2018
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Financial Covenants [Abstract] |
Allowed Indebtedness to Total Capitalization Ratio | 0.65
Actual Indebtedness to Total Capitalization Ratio | 0.41

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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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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
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration