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Acquisitions - Wind Development Project (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Nov. 20, 2014 | Dec. 31, 2014 MW | Dec. 31, 2014 MW | Dec. 31, 2013 | Nov. 20, 2014
Assets [Abstract] |  |  |  |  |  |  |  |
Construction in Progress, Gross |  | $ 48.2 us-gaap_ConstructionInProgressGross | [1] | $ 48.2 us-gaap_ConstructionInProgressGross | [1] | 0 us-gaap_ConstructionInProgressGross | [1] |
Liabilities [Abstract] |  |  |  |  |  |  |  |
Deposits |  | 54.3 us-gaap_Deposits | [2] | 54.3 us-gaap_Deposits | [2] | 0 us-gaap_Deposits | [2] |
Wind Development Project [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  | Nov. 20, 2014 |  |  |  |  |  |
Payments to Acquire Businesses, Gross | 27.0 us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |  |  |  |
Generating Capacity (MW) |  | 107.0 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  | 107.0 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |
Contractual Sales Price | 200 ale_ContractualSalesPrice / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |  |  |  |
Assets [Abstract] |  |  |  |  |  |  |  |
Cash and Cash Equivalents | 3.6 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |  |  |  | 3.6 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember
Construction in Progress, Gross | 23.4 us-gaap_ConstructionInProgressGross / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember | 48.2 us-gaap_ConstructionInProgressGross / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  | 48.2 us-gaap_ConstructionInProgressGross / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  | 23.4 us-gaap_ConstructionInProgressGross / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember
Goodwill | 0 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |  |  |  | 0 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember
Liabilities [Abstract] |  |  |  |  |  |  |  |
Total Liabilities and Non-Controlling Interest Assumed | 0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |  |  |  | 0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember
Business Combination, Acquisition Related Cost |  |  |  | not material |  |  |  |
Business Acquisition, Pro Forma Information, Description |  |  |  | not material |  | not material |  |
Deposits |  | $ 54.3 us-gaap_Deposits / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  | $ 54.3 us-gaap_Deposits / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |
[1] | Construction Costs for Development Project acquired/incurred due to ALLETE Clean Energy’s acquisition on November 20, 2014 of a project to develop and construct a wind energy facility in 2015. (See Other Current Liabilities table and Note 7. Acquisitions.)
[2] | Construction Deposits Received for Development Project are due to ALLETE Clean Energy’s project to develop and construct a wind energy facility in 2015. (See Prepayment and Other Current Assets table and Note 7. Acquisitions.)

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