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Commitments, Guarantees and Contingencies - Environmental Matters (Details) (Boswell Unit 4 [Member], USD $) In Millions, unless otherwise specified | Jun. 30, 2013
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Minimum [Member] |
Schedule of Utility Generating Facilities [Line Items] |
Estimated Capital Expenditures | $ 350
Maximum [Member] |
Schedule of Utility Generating Facilities [Line Items] |
Estimated Capital Expenditures | $ 400

X
- Definition The estimated amount of certain future capital expenditures related to environmental compliance, regulatory matters, or other committments and/or contingencies. + References No definition available. + Details
Name: | ale_EstimatedCapitalExpenditures
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | ale_ScheduleOfUtilityGeneratingFacilitiesLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration