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Acquisitions - ACE Agreement (Details) (ACE Agreement [Member], USD $)
In Millions, unless otherwise specified
 0 Months Ended
Dec. 31, 2014
Dec. 31, 2014
MW
ACE Agreement [Member]
    
Business Acquisition [Line Items]    
Member's Interest Purchase Agreement $ 47.5ale_MembersInterestPurchaseAgreement
/ us-gaap_BusinessAcquisitionAxis
= ale_ACEAgreementMember		  
Generating Capacity (MW) 97.5ale_GeneratingCapacityMw
/ us-gaap_BusinessAcquisitionAxis
= ale_ACEAgreementMember		 97.5ale_GeneratingCapacityMw
/ us-gaap_BusinessAcquisitionAxis
= ale_ACEAgreementMember