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Acquisitions - Chanarambie/Viking (Details) (Chanarambie/Viking [Member], Subsequent Event [Member], USD $) In Millions, unless otherwise specified | 0 Months Ended |
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Apr. 15, 2015 | Apr. 15, 2015 MW
Business Acquisition [Line Items] |  |
Business Acquisition, Effective Date of Acquisition | Apr. 15, 2015 |
Business Acquisition, Name of Acquired Entity | Chanarambie/Viking |
Payments to Acquire Businesses, Gross | $ 47.5 us-gaap_PaymentsToAcquireBusinessesGross |
Generating Capacity (MW) | 97.5 ale_GeneratingCapacityMw | 97.5 ale_GeneratingCapacityMw
Chanarambie/Viking PPA (expires 2018) [Member] |  |
Business Acquisition [Line Items] |  |
Generating Capacity (MW) | 12 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_ChanarambieVikingMember / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ChanarambieVikingPPAexpires2018Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember | 12 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_ChanarambieVikingMember / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ChanarambieVikingPPAexpires2018Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Chanarambie/Viking PPA (expires 2023) [Member] |  |
Business Acquisition [Line Items] |  |
Generating Capacity (MW) | 85.5 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_ChanarambieVikingMember / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ChanarambieVikingPPAexpires2023Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember | 85.5 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_ChanarambieVikingMember / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_ChanarambieVikingPPAexpires2023Member / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e845-128460 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Acquisition Date -URI http://asc.fasb.org/extlink&oid=6919109 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=34752592&loc=d3e848-128460 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463 + Details
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- Definition The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585 + Details
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