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v3.20.2

Short-Term and Long-Term Debt - Financial Covenants (Details) | Jun. 30, 2020
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Debt Instrument [Line Items] |
Actual Ratio of Indebtedness to Total Capitalization | 0.44
Maximum [Member] |
Debt Instrument [Line Items] |
Required Ratio of Indebtedness to Total Capitalization | 0.65

X
- 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

X
- Definition 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

X
- Definition 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. + References No definition available. + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |