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Operations and Significant Accounting Policies - Reclassification of Prior Income Statement (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2017 USD ($)
Operating Revenue to Operating Revenue – Utility [Member] |
Reclassification of Prior Income Statement [Line Items] |
Prior Period Reclassification Amount | $ 281.6
Operating Revenue – Non-utility [Member] |
Reclassification of Prior Income Statement [Line Items] |
Prior Period Reclassification Amount | 84.0
Cost of Sales to Fuel, Purchased Power and Gas – Utility [Member] |
Reclassification of Prior Income Statement [Line Items] |
Prior Period Reclassification Amount | $ 3.6

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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
Name: | ale_ScheduleofPriorPeriodReclassificationAdjustmentsLineItems
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
Name: | us-gaap_PriorPeriodReclassificationAdjustment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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

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