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Commitments, Guarantees and Contingencies - Hydro Operations (Details) (Hydro Project [Member], USD $) In Millions, unless otherwise specified | Sep. 30, 2014
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Hydro Project [Member] |
Hydro Operations [Line Items] |
Estimated Capital Expenditures, Including Past Expenditures | $ 90
Capital Cost Spent to Date | $ 80

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_HydroOperationsLineItems
Namespace Prefix: | ale_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration