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Acquisitions - Storm Lake I (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 17, 2014 | Dec. 31, 2014 MW | Dec. 31, 2014 MW | Dec. 31, 2013
Liabilities [Abstract] |  |  |  |  |  |  |  |
Power Purchase Agreements |  |  | 110.7 ale_PowerPurchaseAgreementNoncurrent | [1] | 110.7 ale_PowerPurchaseAgreementNoncurrent | [1] | $ 0 ale_PowerPurchaseAgreementNoncurrent | [1]
Power Purchase Agreements - Current |  |  | 19.4 ale_PowerPurchaseAgreementCurrent | [1] | 19.4 ale_PowerPurchaseAgreementCurrent | [1] | 0 ale_PowerPurchaseAgreementCurrent | [1]
Storm Lake I [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  |  | Dec. 17, 2014 |  |  |  |  |
Business Acquisition, Name of Acquired Entity |  |  |  |  | Storm Lake I |  |  |
Payments to Acquire Businesses, Gross | 15.0 us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Generating Capacity (MW) |  |  | 108.0 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  | 108.0 ale_GeneratingCapacityMw / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |
Assets [Abstract] |  |  |  |  |  |  |  |
Cash and Cash Equivalents | 0.4 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Other Current Assets | 4.7 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Property, Plant and Equipment - Net | 47.3 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Other Non-Current Assets | 11.4 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember | [2] |  |  |  |  |  |
Total Assets Acquired | 63.8 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Liabilities [Abstract] |  |  |  |  |  |  |  |
Other Current Liabilities | 8.2 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember | [3] |  |  |  |  |  |
Power Purchase Agreements | 23.5 ale_PowerPurchaseAgreementNoncurrent / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Other Non-Current Liabilities | 17.0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Total Liabilities and Non-Controlling Interest Assumed | 48.7 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Net Identifiable Assets Acquired | 15.1 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Restricted Cash and Investments, Noncurrent | 0.4 us-gaap_RestrictedCashAndInvestmentsNoncurrent / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Goodwill Description | immaterial |  |  |  |  |  |  |
Power Purchase Agreements - Current | $ 7.5 ale_PowerPurchaseAgreementCurrent / us-gaap_BusinessAcquisitionAxis = ale_StormLakeIMember |  |  |  |  |  |  |
Business Combination, Acquisition Related Cost |  |  |  |  | not material |  |  |
Business Acquisition, Pro Forma Information, Description |  |  |  |  | not material |  |  |
[1] | Power Purchase Agreements were acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions in 2014. (See Note 7. Acquisitions.)
[2] | Included in Other Non-Current Assets was $0.4 million of restricted cash and an immaterial amount of goodwill; for tax purposes, the purchase price allocation resulted in no allocation to goodwill.
[3] | Other Current Liabilities included $7.5 million related to the current liabilities portion of the Power Purchase Agreements.

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