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Acquisitions - Wind Development Project (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 3 Months Ended | 0 Months Ended |
--- | --- | --- | --- | ---
Nov. 20, 2014 | Dec. 31, 2014 | Apr. 15, 2015 | Mar. 31, 2015
Assets [Abstract] |  |  |  |  |  |
Construction Costs for Development Project |  | $ 48.2 us-gaap_ConstructionInProgressGross | [1] |  | $ 48.4 us-gaap_ConstructionInProgressGross | [1]
Liabilities [Abstract] |  |  |  |  |  |
Customer Deposits Received for Development Project |  | 54.3 us-gaap_CustomerAdvancesCurrent | [2] |  | 54.3 us-gaap_CustomerAdvancesCurrent | [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 |
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 |  |  |  |  |
Construction Costs for Development Project | 23.4 us-gaap_ConstructionInProgressGross / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember | 48.2 us-gaap_ConstructionInProgressGross / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |
Goodwill | 0 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |  |
Liabilities [Abstract] |  |  |  |  |  |
Customer Deposits Received for Development Project |  | 54.3 us-gaap_CustomerAdvancesCurrent / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  | 54.3 us-gaap_CustomerAdvancesCurrent / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities | 0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember |  |  |  |  |
Subsequent Event [Member] | Wind Development Project [Member] |  |  |  |  |  |
Liabilities [Abstract] |  |  |  |  |  |
Increase (Decrease) in Customer Advances |  |  |  | $ 50 us-gaap_IncreaseDecreaseInCustomerAdvances / us-gaap_BusinessAcquisitionAxis = ale_WindDevelopmentProjectMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |  |
[1] | Construction Costs for Development Project relate to ALLETE Clean Energy’s acquisition in November 2014 of a project to develop and construct a wind energy facility in 2015. (See Other Current Liabilities table and Note 4. Acquisitions.)
[2] | Construction Deposits Received for Development Project relate to ALLETE Clean Energy’s project to develop and construct a wind energy facility in 2015. (See Prepayments and Other Current Assets table and Note 4. Acquisitions.)

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