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Acquisitions - Armenia Mountain (Details) (Armenia Mountain [Member], Subsequent Event [Member], USD $) In Millions, unless otherwise specified | 0 Months Ended |
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Apr. 30, 2015 | Apr. 30, 2015 MW
Armenia Mountain [Member] | Subsequent Event [Member] |  |
Business Acquisition [Line Items] |  |
Member's Interest Purchase Agreement, Name of Business to be Acquired | Armenia Mountain |
Member's Interest Purchase Agreement, Consideration | $ 108.0 ale_MembersInterestPurchaseAgreement / us-gaap_BusinessAcquisitionAxis = ale_ArmeniaMountainMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Member's Interest Purchase Agreement, Percent Ownership to be Acquired | 100.00%ale_MembersInterestPurchaseAgreementPercentOwnershiptobeAcquired / us-gaap_BusinessAcquisitionAxis = ale_ArmeniaMountainMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Generating Capacity (MW) | 100.5 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_ArmeniaMountainMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember | 100.5 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_ArmeniaMountainMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition Capacity of a generating facility, unit, or project in megawatts. + References No definition available. + Details
Name: | ale_GeneratingCapacityMw
Namespace Prefix: | ale_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | instant

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- Definition Member's Interest Purchase Agreement + References No definition available. + Details
Name: | ale_MembersInterestPurchaseAgreement
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Member's Interest Purchase Agreement, Name of Business to be Acquired + References No definition available. + Details
Name: | ale_MembersInterestPurchaseAgreementNameofBusinesstobeAcquired
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Member's Interest Purchase Agreement, Percent Ownership to be Acquired + References No definition available. + Details
Name: | ale_MembersInterestPurchaseAgreementPercentOwnershiptobeAcquired
Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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