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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $)
3 Months Ended 7 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Square Butte PPA [Member]      
Power Purchase Agreements [Line Items]      
PPA Counterparty Debt Outstanding $ 390,900,000us-gaap_LongTermContractForPurchaseOfElectricPowerAmountOfLongTermDebtOrLeaseObligationOutstanding
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
   
Annual Debt Service 45,000,000us-gaap_LongTermContractForPurchaseOfElectricPowerAnnualMinimumDebtServicePaymentRequired
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
   
Power Purchased under Long-term Contracts 19,200,000us-gaap_UtilitiesOperatingExpensePurchasedPowerUnderLongTermContracts
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
16,800,000us-gaap_UtilitiesOperatingExpensePurchasedPowerUnderLongTermContracts
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
 
Interest Expense 2,500,000us-gaap_LongTermContractForPurchaseOfElectricPowerInterestIncludedInContractCharges
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
2,500,000us-gaap_LongTermContractForPurchaseOfElectricPowerInterestIncludedInContractCharges
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
 
Square Butte PPA [Member] | Square Butte Coal-fired Unit [Member]      
Power Purchase Agreements [Line Items]      
Generating Unit Capacity (MW) 455ale_GeneratingCapacityMw
/ ale_GeneratingFacilityAxisAxis
= ale_SquareButteCoalFiredUnitMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
   
Output Entitlement 50.00%us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
/ ale_GeneratingFacilityAxisAxis
= ale_SquareButteCoalFiredUnitMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
   
Square Butte PPA [Member] | Minnkota Sales Agreement [Member] | Square Butte Coal-fired Unit [Member]      
Power Purchase Agreements [Line Items]      
Output Entitlement 28.00%us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
/ ale_GeneratingFacilityAxisAxis
= ale_SquareButteCoalFiredUnitMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
/ ale_LongtermContractforSaleofElectricPowerAxis
= ale_MinnkotaSalesAgreementMember
  23.00%us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased
/ ale_GeneratingFacilityAxisAxis
= ale_SquareButteCoalFiredUnitMember
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_SquareButtePpaMember
/ ale_LongtermContractforSaleofElectricPowerAxis
= ale_MinnkotaSalesAgreementMember
Minnkota PPA [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 50ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_MinnkotaPpaMember
   
Oliver Wind I PPA [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 50ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_OliverWindIPpaMember
   
Fixed Capacity Charges 0ale_LongTermContractForPurchaseOfElectricPowerFixedCapacityCarges
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_OliverWindIPpaMember
   
Oliver Wind II PPA [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 48ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_OliverWindIiPpaMember
   
Fixed Capacity Charges $ 0ale_LongTermContractForPurchaseOfElectricPowerFixedCapacityCarges
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_OliverWindIiPpaMember
   
Manitoba Hydro PPA (expires May 2020) [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 50ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_ManitobaHydroPPAexpiresMay2020Member
   
Manitoba Hydro PPA (expires April 2022) [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MWh) 1,000,000ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMwh
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_ManitobaHydroPpaExpiresApril2022Member
   
Manitoba Hydro PPA (expires 2035) [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 250ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_ManitobaHydroPpaExpires2035Member
   
Term of Contract (Years) 15ale_LongTermContractForPurchaseOfElectricPowerTermOfContractYears
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_ManitobaHydroPpaExpires2035Member
   
Manitoba Hydro PPA (expires 2040) [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 133ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_ManitobaHydroPPAexpires2040Member
   
Term of Contract (Years) 20ale_LongTermContractForPurchaseOfElectricPowerTermOfContractYears
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_ManitobaHydroPPAexpires2040Member
   
Great River Energy PPA (expires May 2020) [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 50ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_GreatRiverEnergyPPAexpires2020Member
   
Great River Energy Capacity Only PPA (expires May 2020) [Member]      
Power Purchase Agreements [Line Items]      
Output Being Purchased (MW) 50ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw
/ us-gaap_LongTermContractForPurchaseOfElectricPowerAxis
= ale_GreatRiverEnergyCapacityOnlyPPAexpires2020Member