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Operations and Significant Accounting Policies - Investments and Other (Details) | 12 Months Ended
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Dec. 31, 2013
Long-term Contract for Purchase of Electric Power [Line Items] |
Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement (Percent) | 20.00%
Square Butte PPA [Member] |
Long-term Contract for Purchase of Electric Power [Line Items] |
Output Purchased (MW) | 227.5
Square Butte Coal-fired Unit [Member] | Square Butte PPA [Member] |
Long-term Contract for Purchase of Electric Power [Line Items] |
Output Purchased (Percent) | 50.00%

X
- Definition Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased + References No definition available. + Details
Name: | ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputPurchasedMw
Namespace Prefix: | ale_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | duration

X
- Definition The minimum amount of cash collected on the contract price in order to fully recognize profit on the sale of real estate upon closing (company policy). + References No definition available. + Details
Name: | ale_RealEstateSalesRevenueRecognitionMinimumContractPriceCashCollectionsRequirement
Namespace Prefix: | ale_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

X
- Definition Share of plant output being purchased under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.D) -URI http://asc.fasb.org/extlink&oid=6499423&loc=d3e659339-123030 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section D + Details
Name: | us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration