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Operations and Significant Accounting Policies - Subsequent Events (Details) - Subsequent Event [Member] $ in Millions | Feb. 08, 2019 USD ($)
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Subsequent Event [Line Items] |
Cash Purchase Price on the Sale of U.S. Water Services | $ 270
Estimated Gain on the Sale of U.S. Water Services | $ 10

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- Definition Estimated Gain on Sale of Affiliate + References No definition available. + Details
Name: | ale_EstimatedGainonSaleofAffiliate
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Expected Cash Proceeds from Sale of Affiliate + References No definition available. + Details
Name: | ale_ExpectedCashProceedsfromSaleofAffiliate
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |