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Regulatory Matters - Boswell Mercury Emissions Reduction Plan (Details) (Boswell Unit 4 [Member], USD $) In Millions, unless otherwise specified | Mar. 31, 2015
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Boswell Unit 4 [Member] |
Schedule of Utility Generating Facilities [Line Items] |
Estimated Capital Expenditures | $ 260 ale_EstimatedCapitalExpendituresIncludingPastExpenditures / ale_GeneratingFacilityAxisAxis = ale_BoswellUnit4Member
Capital Cost Spent to Date | $ 162 ale_CapitalCostSpentToDate / ale_GeneratingFacilityAxisAxis = ale_BoswellUnit4Member

X
- Definition Capital cost spent to date for project. + References No definition available. + Details
Name: | ale_CapitalCostSpentToDate
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. + References No definition available. + Details
Name: | ale_EstimatedCapitalExpendituresIncludingPastExpenditures
Namespace Prefix: | ale_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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