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Operations and Significant Accounting Policies - New Accounting Standards (Details) | Jun. 30, 2015
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New Accounting Standards [Abstract] |
Percentage of Long-Term Debt, Issuance Costs | 1.00%

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- Definition New Accounting Standards [Abstract] + References No definition available. + Details
Name: | ale_NewAccountingStandardsAbstract
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the percentage of debt issuance costs in long-term debt + References No definition available. + Details
Name: | ale_PercentageofLongTermDebtIssuanceCosts
Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant