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Operations and Significant Accounting Policies - Income Taxes (Details) - Income Tax Expense [Member] $ in Millions | 9 Months Ended
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Sep. 30, 2018 USD ($)
Measurement Period Adjustment | $ 0.9
Expected Full-year Measurement Period Adjustment | $ 1.1

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- Definition Expected Full-year Measurement Period Adjustment for new/clarified TCJA information + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Measurement period adjustment for new/clarified TCJA information. + References No definition available. + Details
Name: | ale_MeasurementPeriodAdjustment
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=ale_IncomeTaxExpenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |