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Acquisitions - U.S. Water Services (Details) (USD $) In Millions, unless otherwise specified | 0 Months Ended | 3 Months Ended |
--- | --- | --- | ---
Feb. 10, 2015 | Mar. 31, 2015 | Dec. 31, 2014
Liabilities [Abstract] |  |  |  |  |  |
Restricted Cash |  |  | $ 5.6 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue | [1] | $ 2.7 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue | [1]
U.S. Water Services [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  |  | Feb. 10, 2015 |  |  |
Business Acquisition, Name of Acquired Entity |  |  | U.S. Water Services |  |  |
Business Acquisition, Consideration Transferred | 202.3 us-gaap_BusinessCombinationConsiderationTransferred1 / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Payments to Acquire Businesses, Gross | 166.6 us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Business Acquisition, Percentage of Voting Interests Acquired | 87.00%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Business Acquisition, Contingent Consideration, Liability | 35.7 us-gaap_BusinessCombinationContingentConsiderationLiability / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Business Acquisition, Percent of Results of Operations Reflected in Income Statement |  |  | 100.00%ale_BusinessCombinationPercentofResultsofOperationsReflectedinIncomeStatement / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |
Business Acquisition, Effective Percentage of Business Acquired | 100.00%ale_BusinessCombinationEffectivePercentageofBusinessAcquired / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Assets [Abstract] |  |  |  |  |  |
Cash and Cash Equivalents | 0.9 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Accounts Receivable | 16.8 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Inventories | 13.4 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember | [2] |  |  |  |
Other Current Assets | 5.3 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember | [3] |  |  |  |
Property, Plant and Equipment | 10.6 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Goodwill | 127.1 us-gaap_Goodwill / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember | [4] |  |  |  |
Intangible Assets | 83.0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember | [5] |  |  |  |
Other Non-Current Assets | 0.2 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Total Assets Acquired | 257.3 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Liabilities [Abstract] |  |  |  |  |  |
Other Current Liabilities | 18.7 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Other Non-Current Liabilities | 36.3 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Total Liabilities and Non-Controlling Interest Assumed | 55.0 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Net Identifiable Assets Acquired | 202.3 us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Fair Value Adjustments for Work in Process and Finished Goods | 2.7 ale_BusinesscombinationFairValueAdjustmentforWorkinProcessandFinishedGoods / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Fair Value of Sales Backlog | 1.6 ale_BusinessCombinationFairValueofSalesBacklog / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Restricted Cash | 2.1 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Business Acquisition, Goodwill, Expected Tax Deductible Amount | 3.2 us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |  |  |
Business Acquisition, Acquisition Related Costs |  |  | $ 3.0 us-gaap_BusinessCombinationAcquisitionRelatedCosts / us-gaap_BusinessAcquisitionAxis = ale_U.S.WaterServicesMember |  |  |
[1] | Restricted Cash related to ALLETE Clean Energy’s wind energy facilities operating expense and capital distribution reserve requirements and cash pledged as collateral by U.S. Water Services for stand-by letters of credit.
[2] | Included in Inventories was $2.7 million of fair value adjustments relating to work in process and finished goods inventories which will be recognized as Cost of Sales within one year from the acquisition date.
[3] | Included in Other Current Assets was $1.6 million relating to the fair value of sales backlog. Sales backlog will be recognized as Cost of Sales within one year from the acquisition date. Also included in Other Current Assets was restricted cash of $2.1 million relating to cash pledged as collateral for stand-by letters of credit.
[4] | Goodwill is largely attributable to strategic opportunities for growing U.S. Water Services and the benefits of the existing workforce. Goodwill of $3.2 million is deductible for tax purposes.
[5] | Intangible Assets include customer relationships, patents, non-compete agreements and trademarks and trade names. (See Note 5. Goodwill and Intangible Assets.)

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