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Operations and Significant Accounting Policies - Minnesota Land Acres (Details) (Investments and Other [Member]) | Dec. 31, 2012
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Investments and Other [Member] |
Business Segments [Line Items] |
Number of acres of land in Minnesota (in acres) | 6,100

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- Definition Number of acres of land available-for-sale. + References No definition available. + Details
Name: | ale_NumberOfAcresOfLandAvailableForSaleInAcres
Namespace Prefix: | ale_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration