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Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2014 MW Years | Mar. 31, 2015 MW
Square Butte PPA [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
PPA Counterparty Debt Outstanding | $ 402.9 us-gaap_LongTermContractForPurchaseOfElectricPowerAmountOfLongTermDebtOrLeaseObligationOutstanding / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember |  |  |  |
Annual Debt Service | 45 us-gaap_LongTermContractForPurchaseOfElectricPowerAnnualMinimumDebtServicePaymentRequired / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember |  |  |  |
Power Purchased under Long-term Contracts | 70.1 us-gaap_UtilitiesOperatingExpensePurchasedPowerUnderLongTermContracts / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember | 71.1 us-gaap_UtilitiesOperatingExpensePurchasedPowerUnderLongTermContracts / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember | 67.1 us-gaap_UtilitiesOperatingExpensePurchasedPowerUnderLongTermContracts / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember |  |
Interest Expense | 10.5 us-gaap_LongTermContractForPurchaseOfElectricPowerInterestIncludedInContractCharges / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember | 10.5 us-gaap_LongTermContractForPurchaseOfElectricPowerInterestIncludedInContractCharges / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember | 11.1 us-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) | 455.0 ale_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 |  |  |  |
Minnkota PPA [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) | 50 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_MinnkotaPpaMember |  |  |  |
Oliver Wind I PPA [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) | 50 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_OliverWindIPpaMember |  |  |  |
Fixed Capacity Charges | 0 ale_LongTermContractForPurchaseOfElectricPowerFixedCapacityCarges / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_OliverWindIPpaMember |  |  |  |
Oliver Wind II PPA [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) | 48 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_OliverWindIiPpaMember |  |  |  |
Fixed Capacity Charges | $ 0 ale_LongTermContractForPurchaseOfElectricPowerFixedCapacityCarges / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_OliverWindIiPpaMember |  |  |  |
Manitoba Hydro PPA (expires May 2020) [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) | 50 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ManitobaHydroPPAexpiresMay2020Member |  |  |  |
Manitoba Hydro PPA (expires April 2022) [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MWh) | 1,000,000 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMwh / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ManitobaHydroPpaExpiresApril2022Member |  |  |  |
Manitoba Hydro PPA (expires 2035) [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) | 250 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ManitobaHydroPpaExpires2035Member |  |  |  |
Term of Contract (Years) | 15 ale_LongTermContractForPurchaseOfElectricPowerTermOfContractYears / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ManitobaHydroPpaExpires2035Member |  |  |  |
Manitoba Hydro PPA (expires 2040) [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) |  |  |  | 133 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ManitobaHydroPPAexpires2040Member |
Term of Contract (Years) |  |  |  | 20 ale_LongTermContractForPurchaseOfElectricPowerTermOfContractYears / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ManitobaHydroPPAexpires2040Member |
Great River Energy PPA (expires 2020) [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) | 50 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_GreatRiverEnergyPPAexpires2020Member |  |  |  |
Great River Energy Capacity Only PPA (expires 2020) [Member] | Subsequent Event [Member] |  |  |  |  |
Power Purchase Agreements [Line Items] |  |  |  |  |
Output Being Purchased (MW) |  |  |  |  | 50 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_GreatRiverEnergyCapacityOnlyPPAexpires2020Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

X
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- Definition Share of plant output being purchased in megawatts 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 No definition available. + Details
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- Definition Share of plant output being purchased in megawatthours 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 No definition available. + Details
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- Definition Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. + References No definition available. + Details
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- Definition Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service. + 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
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- Definition Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service. If the utility is obligated to pay certain minimum amounts which cover debt service requirements whether or not the plant is operating, the amount can be disclosed here. + 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
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- 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
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