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Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) - Accounting Standards Update 2017-07 [Member] - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2017 | Jun. 30, 2017
Operating and Maintenance [Member] |  |
Reclassification of Prior Income Statement [Line Items] |  |
Prior Period Reclassification Amount | $ 1.0 | $ 2.1
Other Income (Expense) [Member] |  |
Reclassification of Prior Income Statement [Line Items] |  |
Prior Period Reclassification Amount | $ 1.0 | $ 2.0

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- Definition [Line Items] for Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period. + References No definition available. + Details
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Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of a reclassification adjustment made to prior period financial statement amounts. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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