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Operations and Significant Accounting Policies (Details) (USD $) | 12 Months Ended | 0 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Feb. 10, 2015
Subsequent Events [Line Items] |  |  |  |  |  |
Business Combination, Reason for Business Combination | The acquisitions below are consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth. |  |  |  |  |
Investments and Other [Abstract] |  |  |  |  |  |
Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement (Percent) | 20.00%ale_RealEstateSalesRevenueRecognitionMinimumContractPriceCashCollectionsRequirement |  |  |  |  |
Amortization of Power Purchase Agreements | $ 12,700,000 ale_AmortizationofPowerPurchaseAgreements |  | $ 0 ale_AmortizationofPowerPurchaseAgreements |  | $ 0 ale_AmortizationofPowerPurchaseAgreements |
Trade Accounts Receivable [Abstract] |  |  |  |  |  |
Billed | 85,500,000 us-gaap_AccountsReceivableGrossCurrent |  | 78,700,000 us-gaap_AccountsReceivableGrossCurrent |  |  |
Unbilled | 18,600,000 us-gaap_UnbilledReceivablesCurrent |  | 18,700,000 us-gaap_UnbilledReceivablesCurrent |  |  |
Less: Allowance for Doubtful Accounts | 1,100,000 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent |  | 1,100,000 us-gaap_AllowanceForDoubtfulAccountsReceivableCurrent |  |  |
Total Accounts Receivable - Net | 103,000,000 us-gaap_AccountsReceivableNetCurrent |  | 96,300,000 us-gaap_AccountsReceivableNetCurrent |  |  |
Inventories [Abstract] |  |  |  |  |  |
Fuel | 29,000,000 us-gaap_EnergyRelatedInventoryCoal | [1] | 13,100,000 us-gaap_EnergyRelatedInventoryCoal | [1] |  |
Materials and Supplies | 51,500,000 us-gaap_InventoryRawMaterialsAndSuppliesNetOfReserves |  | 46,200,000 us-gaap_InventoryRawMaterialsAndSuppliesNetOfReserves |  |  |
Total Inventories | 80,500,000 us-gaap_InventoryNet |  | 59,300,000 us-gaap_InventoryNet |  |  |
Impairement of Long-Lived Assets [Abstract] |  |  |  |  |  |
Land Impairment | 0 us-gaap_ImpairmentOfRealEstate |  | 0 us-gaap_ImpairmentOfRealEstate |  | 0 us-gaap_ImpairmentOfRealEstate |
Prepayments and Other Current Assets [Abstract] |  |  |  |  |  |
Deferred Fuel Adjustment Clause | 16,300,000 us-gaap_DeferredFuelCost |  | 23,000,000 us-gaap_DeferredFuelCost |  |  |
Construction Costs for Development Project | 48,200,000 us-gaap_ConstructionInProgressGross | [2] | 0 us-gaap_ConstructionInProgressGross | [2] |  |
Restricted Cash | 2,700,000 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue | [3] | 0 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue | [3] |  |
Other | 14,800,000 us-gaap_OtherAssetsMiscellaneousCurrent |  | 12,100,000 us-gaap_OtherAssetsMiscellaneousCurrent |  |  |
Total Prepayments and Other Current Assets | 82,000,000 us-gaap_PrepaidExpenseAndOtherAssetsCurrent |  | 35,100,000 us-gaap_PrepaidExpenseAndOtherAssetsCurrent |  |  |
Other Non-Current Assets [Abstract] |  |  |  |  |  |
Restricted Cash | 5,300,000 us-gaap_RestrictedCashAndCashEquivalentsNoncurrent |  | 5,400,000 us-gaap_RestrictedCashAndCashEquivalentsNoncurrent |  |  |
Other Current Liabilities [Abstract] |  |  |  |  |  |
Customer Deposits | 19,700,000 us-gaap_CustomerDepositsCurrent |  | 26,000,000 us-gaap_CustomerDepositsCurrent |  |  |
Power Purchase Agreements | 19,400,000 ale_PowerPurchaseAgreementCurrent | [4] | 0 ale_PowerPurchaseAgreementCurrent | [4] |  |
Construction Deposits Received for Development Project | 54,300,000 us-gaap_Deposits | [5] | 0 us-gaap_Deposits | [5] |  |
Other | 27,400,000 us-gaap_OtherAccruedLiabilitiesCurrent |  | 26,600,000 us-gaap_OtherAccruedLiabilitiesCurrent |  |  |
Total Other Current Liabilities | 120,800,000 us-gaap_OtherLiabilitiesCurrent |  | 52,600,000 us-gaap_OtherLiabilitiesCurrent |  |  |
Other Non-Current Liabilities [Abstract] |  |  |  |  |  |
Asset Retirement Obligation | 109,200,000 us-gaap_AssetRetirementObligation | [6] | 81,800,000 us-gaap_AssetRetirementObligation | [6] | 77,900,000 us-gaap_AssetRetirementObligation |
Power Purchase Agreements | 110,700,000 ale_PowerPurchaseAgreementNoncurrent | [4] | 0 ale_PowerPurchaseAgreementNoncurrent | [4] |  |
Other | 45,100,000 us-gaap_OtherAccruedLiabilitiesNoncurrent |  | 45,400,000 us-gaap_OtherAccruedLiabilitiesNoncurrent |  |  |
Total Other Non-Current Liabilities | 265,000,000 us-gaap_OtherLiabilitiesNoncurrent |  | 127,200,000 us-gaap_OtherLiabilitiesNoncurrent |  |  |
Income Taxes [Abstract] |  |  |  |  |  |
More Likely Than Not Definition | 50.00%ale_Morelikelythannotdefined |  |  |  |  |
Square Butte PPA [Member] |  |  |  |  |  |
Long-term Contract for Purchase of Electric Power [Line Items] |  |  |  |  |  |
Output Purchased (MW) | 227.5 ale_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputPurchasedMw / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember |  |  |  |  |
Square Butte Coal-fired Unit [Member] | Square Butte PPA [Member] |  |  |  |  |  |
Long-term Contract for Purchase of Electric Power [Line Items] |  |  |  |  |  |
Output Purchased (Percent) | 50.00%us-gaap_LongTermContractForPurchaseOfElectricPowerShareOfPlantOutputBeingPurchased / ale_GeneratingFacilityAxisAxis = ale_SquareButteCoalFiredUnitMember / us-gaap_LongTermContractForPurchaseOfElectricPowerAxis = ale_SquareButtePpaMember |  |  |  |  |
Municipal Customers [Member] |  |  |  |  |  |
Customer Information [Line Items] |  |  |  |  |  |
Number of Customers | 16 ale_NumberOfCustomers / ale_CustomerClassAxis = ale_MunicipalCustomersMember |  |  |  |  |
Retail Customers [Member] |  |  |  |  |  |
Customer Information [Line Items] |  |  |  |  |  |
Number of Customers | 144,000 ale_NumberOfCustomers / ale_CustomerClassAxis = ale_RetailMember |  |  |  |  |
Electric [Member] |  |  |  |  |  |
Customer Information [Line Items] |  |  |  |  |  |
Number of Customers | 15,000 ale_NumberOfCustomers / ale_CustomerClassAxis = ale_ElectricMember |  |  |  |  |
Natural Gas [Member] |  |  |  |  |  |
Customer Information [Line Items] |  |  |  |  |  |
Number of Customers | 12,000 ale_NumberOfCustomers / ale_CustomerClassAxis = ale_NaturalGasMember |  |  |  |  |
Water [Member] |  |  |  |  |  |
Customer Information [Line Items] |  |  |  |  |  |
Number of Customers | 10,000 ale_NumberOfCustomers / ale_CustomerClassAxis = ale_WaterMember |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |
Subsequent Events [Line Items] |  |  |  |  |  |
Business Acquisition, Effective Date of Acquisition |  |  |  |  |  | Feb. 10, 2015
Business Acquisition, Name of Acquired Entity |  |  |  |  |  | U.S. Water Services
Payments to Acquire Businesses, Gross |  |  |  |  |  | $ 168,000,000 us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Business Acquisition, Acquired Business Number of Customers |  |  |  |  |  | 3,600 ale_BusinessAcquisitionAcquiredBusinessNumberofCustomers / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Acquired Business, Customer Retention |  |  |  |  |  | 90.00%ale_CustomerRetentionPercentage / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Business Combination, Reason for Business Combination |  |  |  |  |  | The acquisition of U.S. Water Services is consistent with ALLETE’s stated strategy of investing in energy infrastructure and related services to complement its core regulated utility, balance exposure to business cycles and changing demand, and provide potential long-term earnings growth.
Investments and Other [Member] |  |  |  |  |  |
Investments and Other [Abstract] |  |  |  |  |  |
Land in Minnesota (in acres) | 5,000 us-gaap_AreaOfLand / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  |  |  |  |
[1] | Fuel inventory was lower at December 31, 2013 primarily due to the timing of coal shipments.
[2] | 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.)
[3] | Restricted Cash related to ALLETE Clean Energy’s wind energy facilities operating expense and capital distribution reserve requirements.
[4] | Power Purchase Agreements were acquired in conjunction with the ALLETE Clean Energy wind energy facilities acquisitions in 2014. (See Note 7. Acquisitions.)
[5] | 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.)
[6] | The increase in 2014 is primarily related to BNI Coal for coal mining expansion and ALLETE Clean Energy due to wind energy facilities acquisitions.

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