3.19.3.a.u2 html 710 697 1 true 246 0 false 11 false false R1.htm 0001000 - Document - Document and Entity Information Document Sheet http://www.allete.com/role/DocumentAndEntityInformationDocument Document and Entity Information Document Cover 1 false false R2.htm 1001000 - Statement - Consolidated Balance Sheet Sheet http://www.allete.com/role/ConsolidatedBalanceSheet Consolidated Balance Sheet Statements 2 false false R3.htm 1001501 - Statement - Consolidated Balance Sheet Parentheticals Sheet http://www.allete.com/role/ConsolidatedBalanceSheetParentheticals Consolidated Balance Sheet Parentheticals Statements 3 false false R4.htm 1002000 - Statement - Consolidated Statement of Income Sheet http://www.allete.com/role/ConsolidatedStatementOfIncome Consolidated Statement of Income Statements 4 false false R5.htm 1003000 - Statement - Consolidated Statement of Comprehensive Income Sheet http://www.allete.com/role/ConsolidatedStatementOfComprehensiveIncome Consolidated Statement of Comprehensive Income Statements 5 false false R6.htm 1003501 - Statement - Consolidated Statement of Comprehensive Income Parentheticals Sheet http://www.allete.com/role/ConsolidatedStatementOfComprehensiveIncomeParentheticals Consolidated Statement of Comprehensive Income Parentheticals Statements 6 false false R7.htm 1004000 - Statement - Consolidated Statement of Cash Flows Sheet http://www.allete.com/role/ConsolidatedStatementOfCashFlows Consolidated Statement of Cash Flows Statements 7 false false R8.htm 1005000 - Statement - Consolidated Statement of Equity Sheet http://www.allete.com/role/ConsolidatedStatementOfEquity Consolidated Statement of Equity Statements 8 false false R9.htm 2101100 - Disclosure - Operations and Significant Accounting Policies Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPolicies Operations and Significant Accounting Policies Notes 9 false false R10.htm 2102100 - Disclosure - Property, Plant and Equipment Sheet http://www.allete.com/role/PropertyPlantAndEquipment Property, Plant and Equipment Notes 10 false false R11.htm 2103100 - Disclosure - Jointly-Owned Facilities and Assets Sheet http://www.allete.com/role/JointlyOwnedFacilitiesAndAssets Jointly-Owned Facilities and Assets Notes 11 false false R12.htm 2104100 - Disclosure - Regulatory Matters Sheet http://www.allete.com/role/RegulatoryMatters Regulatory Matters Notes 12 false false R13.htm 2105100 - Disclosure - Equity Investments Sheet http://www.allete.com/role/EquityInvestments Equity Investments Notes 13 false false R14.htm 2107100 - Disclosure - Goodwill and Intangible Assets Sheet http://www.allete.com/role/GoodwillAndIntangibleAssets Goodwill and Intangible Assets Notes 14 false false R15.htm 2110100 - Disclosure - Fair Value Sheet http://www.allete.com/role/FairValue Fair Value Notes 15 false false R16.htm 2111100 - Disclosure - Short-Term and Long-Term Debt Sheet http://www.allete.com/role/ShortTermAndLongTermDebt Short-Term and Long-Term Debt Notes 16 false false R17.htm 2112100 - Disclosure - Commitments, Guarantees and Contingencies Sheet http://www.allete.com/role/CommitmentsGuaranteesAndContingencies Commitments, Guarantees and Contingencies Notes 17 false false R18.htm 2113100 - Disclosure - Common Stock and Earnings Per Share Sheet http://www.allete.com/role/CommonStockAndEarningsPerShare Common Stock and Earnings Per Share Notes 18 false false R19.htm 2114100 - Disclosure - Income Tax Expense Sheet http://www.allete.com/role/IncomeTaxExpense Income Tax Expense Notes 19 false false R20.htm 2118100 - Disclosure - Pension and Other Postretirement Benefit Plans Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlans Pension and Other Postretirement Benefit Plans Notes 20 false false R21.htm 2119100 - Disclosure - Employee Stock and Incentive Plans Sheet http://www.allete.com/role/EmployeeStockAndIncentivePlans Employee Stock and Incentive Plans Notes 21 false false R22.htm 2121100 - Disclosure - Business Segments Business Segments (Notes) Notes http://www.allete.com/role/BusinessSegmentsBusinessSegmentsNotes Business Segments Business Segments (Notes) Notes 22 false false R23.htm 2122100 - Disclosure - Quarterly Financial Data (Unaudited) Sheet http://www.allete.com/role/QuarterlyFinancialDataUnaudited Quarterly Financial Data (Unaudited) Notes 23 false false R24.htm 2123100 - Schedule - Schedule II Sheet http://www.allete.com/role/ScheduleIi Schedule II Uncategorized 24 false false R25.htm 2201201 - Disclosure - Operations and Significant Accounting Policies (Policies) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesPolicies Operations and Significant Accounting Policies (Policies) Notes 25 false false R26.htm 2301302 - Disclosure - Operations and Significant Accounting Policies (Tables) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesTables Operations and Significant Accounting Policies (Tables) Tables http://www.allete.com/role/OperationsAndSignificantAccountingPolicies 26 false false R27.htm 2302301 - Disclosure - Property, Plant and Equipment (Tables) Sheet http://www.allete.com/role/PropertyPlantAndEquipmentTables Property, Plant and Equipment (Tables) Tables http://www.allete.com/role/PropertyPlantAndEquipment 27 false false R28.htm 2303301 - Disclosure - Jointly-Owned Facilities and Assets (Tables) Sheet http://www.allete.com/role/JointlyOwnedFacilitiesAndAssetsTables Jointly-Owned Facilities and Assets (Tables) Tables http://www.allete.com/role/JointlyOwnedFacilitiesAndAssets 28 false false R29.htm 2304301 - Disclosure - Regulatory Matters (Tables) Sheet http://www.allete.com/role/RegulatoryMattersTables Regulatory Matters (Tables) Tables http://www.allete.com/role/RegulatoryMatters 29 false false R30.htm 2305301 - Disclosure - Equity Investments (Tables) Sheet http://www.allete.com/role/EquityInvestmentsTables Equity Investments (Tables) Tables http://www.allete.com/role/EquityInvestments 30 false false R31.htm 2307301 - Disclosure - Goodwill and Intangible Assets (Tables) Sheet http://www.allete.com/role/GoodwillAndIntangibleAssetsTables Goodwill and Intangible Assets (Tables) Tables http://www.allete.com/role/GoodwillAndIntangibleAssets 31 false false R32.htm 2310301 - Disclosure - Fair Value (Tables) Sheet http://www.allete.com/role/FairValueTables Fair Value (Tables) Tables http://www.allete.com/role/FairValue 32 false false R33.htm 2311301 - Disclosure - Short-Term and Long-Term Debt (Tables) Sheet http://www.allete.com/role/ShortTermAndLongTermDebtTables Short-Term and Long-Term Debt (Tables) Tables http://www.allete.com/role/ShortTermAndLongTermDebt 33 false false R34.htm 2312301 - Disclosure - Commitments, Guarantees and Contingencies (Tables) Sheet http://www.allete.com/role/CommitmentsGuaranteesAndContingenciesTables Commitments, Guarantees and Contingencies (Tables) Tables http://www.allete.com/role/CommitmentsGuaranteesAndContingencies 34 false false R35.htm 2313301 - Disclosure - Common Stock and Earnings Per Share (Tables) Sheet http://www.allete.com/role/CommonStockAndEarningsPerShareTables Common Stock and Earnings Per Share (Tables) Tables http://www.allete.com/role/CommonStockAndEarningsPerShare 35 false false R36.htm 2314301 - Disclosure - Income Tax Expense (Tables) Sheet http://www.allete.com/role/IncomeTaxExpenseTables Income Tax Expense (Tables) Tables http://www.allete.com/role/IncomeTaxExpense 36 false false R37.htm 2318301 - Disclosure - Pension and Other Postretirement Benefit Plans (Tables) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansTables Pension and Other Postretirement Benefit Plans (Tables) Tables http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlans 37 false false R38.htm 2319301 - Disclosure - Employee Stock and Incentive Plans (Tables) Sheet http://www.allete.com/role/EmployeeStockAndIncentivePlansTables Employee Stock and Incentive Plans (Tables) Tables http://www.allete.com/role/EmployeeStockAndIncentivePlans 38 false false R39.htm 2321301 - Disclosure - Business Segments Business Segment (Tables) Sheet http://www.allete.com/role/BusinessSegmentsBusinessSegmentTables Business Segments Business Segment (Tables) Tables http://www.allete.com/role/BusinessSegmentsBusinessSegmentsNotes 39 false false R40.htm 2322301 - Disclosure - Quarterly Financial Data (Unaudited) (Tables) Sheet http://www.allete.com/role/QuarterlyFinancialDataUnauditedTables Quarterly Financial Data (Unaudited) (Tables) Tables http://www.allete.com/role/QuarterlyFinancialDataUnaudited 40 false false R41.htm 2401403 - Disclosure - Operations and Significant Accounting Policies - Business Segments (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesBusinessSegmentsDetails Operations and Significant Accounting Policies - Business Segments (Details) Details 41 false false R42.htm 2401404 - Disclosure - Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesSupplementalStatementOfCashFlowInformationDetails Operations and Significant Accounting Policies - Supplemental Statement of Cash Flow Information (Details) Details 42 false false R43.htm 2401405 - Disclosure - Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesBalanceSheetAndIncomeStatementDisclosuresDetails Operations and Significant Accounting Policies - Balance Sheet and Income Statement Disclosures (Details) Details 43 false false R44.htm 2401406 - Disclosure - Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesRevenueNatureOfRevenueStreamsDetails Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Details) Details 44 false false R45.htm 2401407 - Disclosure - Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Continued) (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesRevenueNatureOfRevenueStreamsContinuedDetails Operations and Significant Accounting Policies - Revenue - Nature of Revenue Streams (Continued) (Details) Details 45 false false R46.htm 2401408 - Disclosure - Operations and Significant Accounting Policies - Concentration of Credit Risk (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesConcentrationOfCreditRiskDetails Operations and Significant Accounting Policies - Concentration of Credit Risk (Details) Details 46 false false R47.htm 2401409 - Disclosure - Operations and Significant Accounting Policies - Revenue - Assets Recognized From the Costs to Obtain a Contract with a Customer (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesRevenueAssetsRecognizedFromCostsToObtainContractWithCustomerDetails Operations and Significant Accounting Policies - Revenue - Assets Recognized From the Costs to Obtain a Contract with a Customer (Details) Details 47 false false R48.htm 2401410 - Disclosure - Operations and Significant Accounting Policies - Leases (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesLeasesDetails Operations and Significant Accounting Policies - Leases (Details) Details http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesTables 48 false false R49.htm 2401411 - Disclosure - Operations and Significant Accounting Policies - Lease Maturities (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesLeaseMaturitiesDetails Operations and Significant Accounting Policies - Lease Maturities (Details) Details 49 false false R50.htm 2401412 - Disclosure - Operations and Significant Accounting Policies - Other Income (Expense) - Other (Details) Sheet http://www.allete.com/role/OperationsAndSignificantAccountingPoliciesOtherIncomeExpenseOtherDetails Operations and Significant Accounting Policies - Other Income (Expense) - Other (Details) Details 50 false false R51.htm 2402402 - Disclosure - Property, Plant and Equipment (Details) Sheet http://www.allete.com/role/PropertyPlantAndEquipmentDetails Property, Plant and Equipment (Details) Details http://www.allete.com/role/PropertyPlantAndEquipmentTables 51 false false R52.htm 2403402 - Disclosure - Jointly-Owned Facilities and Assets (Details) Sheet http://www.allete.com/role/JointlyOwnedFacilitiesAndAssetsDetails Jointly-Owned Facilities and Assets (Details) Details http://www.allete.com/role/JointlyOwnedFacilitiesAndAssetsTables 52 false false R53.htm 2404402 - Disclosure - Regulatory Matters - Electric Rates (Details) Sheet http://www.allete.com/role/RegulatoryMattersElectricRatesDetails Regulatory Matters - Electric Rates (Details) Details 53 false false R54.htm 2404403 - Disclosure - Regulatory Matters - Integrated Resource Plan (Details) Sheet http://www.allete.com/role/RegulatoryMattersIntegratedResourcePlanDetails Regulatory Matters - Integrated Resource Plan (Details) Details 54 false false R55.htm 2404404 - Disclosure - Regulatory Matters - Conservation Improvement Program (CIP) (Details) Sheet http://www.allete.com/role/RegulatoryMattersConservationImprovementProgramCipDetails Regulatory Matters - Conservation Improvement Program (CIP) (Details) Details 55 false false R56.htm 2404405 - Disclosure - Regulatory Matters - MISO Return on Equity Complaints (Details) Sheet http://www.allete.com/role/RegulatoryMattersMisoReturnOnEquityComplaintsDetails Regulatory Matters - MISO Return on Equity Complaints (Details) Details 56 false false R57.htm 2404406 - Disclosure - Regulatory Matters - Minnesota Solar Energy Standard (Details) Sheet http://www.allete.com/role/RegulatoryMattersMinnesotaSolarEnergyStandardDetails Regulatory Matters - Minnesota Solar Energy Standard (Details) Details 57 false false R58.htm 2404407 - Disclosure - Regulatory Matters - Regulatory Assets and Liabilities (Details) Sheet http://www.allete.com/role/RegulatoryMattersRegulatoryAssetsAndLiabilitiesDetails Regulatory Matters - Regulatory Assets and Liabilities (Details) Details 58 false false R59.htm 2405402 - Disclosure - Equity Investments (Details) Sheet http://www.allete.com/role/EquityInvestmentsDetails Equity Investments (Details) Details http://www.allete.com/role/EquityInvestmentsTables 59 false false R60.htm 2405403 - Disclosure - Equity Investments Equity Investment in Nobles 2 (Details) Sheet http://www.allete.com/role/EquityInvestmentsEquityInvestmentInNobles2Details Equity Investments Equity Investment in Nobles 2 (Details) Details 60 false false R61.htm 2407402 - Disclosure - Goodwill and Intangible Assets Goodwill (Details) Sheet http://www.allete.com/role/GoodwillAndIntangibleAssetsGoodwillDetails Goodwill and Intangible Assets Goodwill (Details) Details 61 false false R62.htm 2407403 - Disclosure - Goodwill and Intangible Assets - Intangible Assets (Details) Sheet http://www.allete.com/role/GoodwillAndIntangibleAssetsIntangibleAssetsDetails Goodwill and Intangible Assets - Intangible Assets (Details) Details 62 false false R63.htm 2410402 - Disclosure - Fair Value - Recurring Fair Value Measures (Details) Sheet http://www.allete.com/role/FairValueRecurringFairValueMeasuresDetails Fair Value - Recurring Fair Value Measures (Details) Details 63 false false R64.htm 2410403 - Disclosure - Fair Value - Financial Instruments (Details) Sheet http://www.allete.com/role/FairValueFinancialInstrumentsDetails Fair Value - Financial Instruments (Details) Details 64 false false R65.htm 2410404 - Disclosure - Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) Sheet http://www.allete.com/role/FairValueAssetsAndLiabilitiesMeasuredAtFairValueOnNonrecurringBasisDetails Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) Details 65 false false R66.htm 2411402 - Disclosure - Short-Term and Long-Term Debt - Short-Term Debt (Details) Sheet http://www.allete.com/role/ShortTermAndLongTermDebtShortTermDebtDetails Short-Term and Long-Term Debt - Short-Term Debt (Details) Details 66 false false R67.htm 2411403 - Disclosure - Short-Term and Long-Term Debt - Long-Term Debt (Details) Sheet http://www.allete.com/role/ShortTermAndLongTermDebtLongTermDebtDetails Short-Term and Long-Term Debt - Long-Term Debt (Details) Details 67 false false R68.htm 2411404 - Disclosure - Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) Sheet http://www.allete.com/role/ShortTermAndLongTermDebtScheduleOfLongTermDebtDetails Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) Details 68 false false R69.htm 2411405 - Disclosure - Short-Term and Long-Term Debt - Financial Covenants (Details) Sheet http://www.allete.com/role/ShortTermAndLongTermDebtFinancialCovenantsDetails Short-Term and Long-Term Debt - Financial Covenants (Details) Details 69 false false R70.htm 2412402 - Disclosure - Commitments, Guarantees and Contingencies Minimum Payments for Certain Long-Term Commitments (Details) Sheet http://www.allete.com/role/CommitmentsGuaranteesAndContingenciesMinimumPaymentsForCertainLongTermCommitmentsDetails Commitments, Guarantees and Contingencies Minimum Payments for Certain Long-Term Commitments (Details) Details 70 false false R71.htm 2412403 - Disclosure - Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) Sheet http://www.allete.com/role/CommitmentsGuaranteesAndContingenciesPowerPurchaseAgreementsDetails Commitments, Guarantees and Contingencies - Power Purchase Agreements (Details) Details 71 false false R72.htm 2412404 - Disclosure - Commitments, Guarantees and Contingencies - Transmission (Details) Sheet http://www.allete.com/role/CommitmentsGuaranteesAndContingenciesTransmissionDetails Commitments, Guarantees and Contingencies - Transmission (Details) Details 72 false false R73.htm 2412405 - Disclosure - Commitments, Guarantees and Contingencies - Environmental Matters (Details) Sheet http://www.allete.com/role/CommitmentsGuaranteesAndContingenciesEnvironmentalMattersDetails Commitments, Guarantees and Contingencies - Environmental Matters (Details) Details 73 false false R74.htm 2412406 - Disclosure - Commitments, Guarantees and Contingencies - Other Matters (Details) Sheet http://www.allete.com/role/CommitmentsGuaranteesAndContingenciesOtherMattersDetails Commitments, Guarantees and Contingencies - Other Matters (Details) Details 74 false false R75.htm 2413402 - Disclosure - Common Stock and Earnings Per Share - Summary of Common Stock (Details) Sheet http://www.allete.com/role/CommonStockAndEarningsPerShareSummaryOfCommonStockDetails Common Stock and Earnings Per Share - Summary of Common Stock (Details) Details 75 false false R76.htm 2413403 - Disclosure - Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) Sheet http://www.allete.com/role/CommonStockAndEarningsPerShareReconciliationOfBasicAndDilutedEarningsPerShareDetails Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) Details 76 false false R77.htm 2414402 - Disclosure - Income Tax Expense (Details) Sheet http://www.allete.com/role/IncomeTaxExpenseDetails Income Tax Expense (Details) Details http://www.allete.com/role/IncomeTaxExpenseTables 77 false false R78.htm 2414403 - Disclosure - Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) Sheet http://www.allete.com/role/IncomeTaxExpenseReconciliationOfTaxesFromFederalStatutoryRateToTotalIncomeTaxExpenseDetails Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) Details 78 false false R79.htm 2414404 - Disclosure - Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) Sheet http://www.allete.com/role/IncomeTaxExpenseDeferredIncomeTaxAssetsAndLiabilitiesDetails Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) Details 79 false false R80.htm 2414405 - Disclosure - Income Tax Expense - NOL and Tax Credit Carryforwards (Details) Sheet http://www.allete.com/role/IncomeTaxExpenseNolAndTaxCreditCarryforwardsDetails Income Tax Expense - NOL and Tax Credit Carryforwards (Details) Details 80 false false R81.htm 2414406 - Disclosure - Income Tax Expense - Gross Unrecognized Income Tax Benefits (Details) Sheet http://www.allete.com/role/IncomeTaxExpenseGrossUnrecognizedIncomeTaxBenefitsDetails Income Tax Expense - Gross Unrecognized Income Tax Benefits (Details) Details 81 false false R82.htm 2418402 - Disclosure - Pension and Other Postretirement Benefit Plans - Contributions (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansContributionsDetails Pension and Other Postretirement Benefit Plans - Contributions (Details) Details 82 false false R83.htm 2418403 - Disclosure - Pension and Other Postretirement Benefit Plans - Obligation and Funded Status (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansObligationAndFundedStatusDetails Pension and Other Postretirement Benefit Plans - Obligation and Funded Status (Details) Details 83 false false R84.htm 2418404 - Disclosure - Pension and Other Postretirement Benefit Plans - Reconciliation of Net Pension Amounts Recognized in Consolidated Balance Sheet (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansReconciliationOfNetPensionAmountsRecognizedInConsolidatedBalanceSheetDetails Pension and Other Postretirement Benefit Plans - Reconciliation of Net Pension Amounts Recognized in Consolidated Balance Sheet (Details) Details 84 false false R85.htm 2418405 - Disclosure - Pension and Other Postretirement Benefit Plans - Components of Net Periodic Expense (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansComponentsOfNetPeriodicExpenseDetails Pension and Other Postretirement Benefit Plans - Components of Net Periodic Expense (Details) Details 85 false false R86.htm 2418406 - Disclosure - Pension and Other Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansOtherChangesInPlanAssetsAndBenefitObligationsRecognizedInOtherComprehensiveIncomeAndRegulatoryAssetsOrLiabilitiesDetails Pension and Other Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities (Details) Details 86 false false R87.htm 2418407 - Disclosure - Pension and Other Postretirement Benefit Plans - Information for Pension Plans with an Accumulated Benefit Obligation in Excess of Plan Assets (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansInformationForPensionPlansWithAccumulatedBenefitObligationInExcessOfPlanAssetsDetails Pension and Other Postretirement Benefit Plans - Information for Pension Plans with an Accumulated Benefit Obligation in Excess of Plan Assets (Details) Details 87 false false R88.htm 2418408 - Disclosure - Pension and Other Postretirement Benefit Plans - Unrecognized Postretirement Health and Life Costs (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansUnrecognizedPostretirementHealthAndLifeCostsDetails Pension and Other Postretirement Benefit Plans - Unrecognized Postretirement Health and Life Costs (Details) Details 88 false false R89.htm 2418409 - Disclosure - Pension and Other Postretirement Benefit Plans - Reconciliation of Net Postretirement Health and Life Amounts Recognized in Consolidated Balance Sheet (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansReconciliationOfNetPostretirementHealthAndLifeAmountsRecognizedInConsolidatedBalanceSheetDetails Pension and Other Postretirement Benefit Plans - Reconciliation of Net Postretirement Health and Life Amounts Recognized in Consolidated Balance Sheet (Details) Details 89 false false R90.htm 2418410 - Disclosure - Pension and Other Postretirement Benefit Plans - Estimated Future Benefit Payments (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansEstimatedFutureBenefitPaymentsDetails Pension and Other Postretirement Benefit Plans - Estimated Future Benefit Payments (Details) Details 90 false false R91.htm 2418411 - Disclosure - Pension and Other Postretirement Benefit Plans - Defined Benefit Costs Recorded in Regulatory Long-Term Assets or Liabilities and Accumulated Other Comprehensive Income Expected to be Recognized over Next Fiscal Year (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansDefinedBenefitCostsRecordedInRegulatoryLongTermAssetsOrLiabilitiesAndAccumulatedOtherComprehensiveIncomeExpectedToBeRecognizedOverNextFiscalYearDetails Pension and Other Postretirement Benefit Plans - Defined Benefit Costs Recorded in Regulatory Long-Term Assets or Liabilities and Accumulated Other Comprehensive Income Expected to be Recognized over Next Fiscal Year (Details) Details 91 false false R92.htm 2418412 - Disclosure - Pension and Other Postretirement Benefit Plans - Assumptions Used to Determine Benefit Obligation and Net Periodic Benefit Costs (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansAssumptionsUsedToDetermineBenefitObligationAndNetPeriodicBenefitCostsDetails Pension and Other Postretirement Benefit Plans - Assumptions Used to Determine Benefit Obligation and Net Periodic Benefit Costs (Details) Details 92 false false R93.htm 2418413 - Disclosure - Pension and Other Postretirement Benefit Plans - Sensitivity of a One Percent Change in Health Care Trend Rates (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansSensitivityOfOnePercentChangeInHealthCareTrendRatesDetails Pension and Other Postretirement Benefit Plans - Sensitivity of a One Percent Change in Health Care Trend Rates (Details) Details 93 false false R94.htm 2418414 - Disclosure - Pension and Other Postretirement Benefit Plans - Plan Asset Allocations (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansPlanAssetAllocationsDetails Pension and Other Postretirement Benefit Plans - Plan Asset Allocations (Details) Details 94 false false R95.htm 2418415 - Disclosure - Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansRecurringFairValueMeasuresDetails Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures (Details) Details 95 false false R96.htm 2418416 - Disclosure - Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures, Activity in Level 3 (Details) Sheet http://www.allete.com/role/PensionAndOtherPostretirementBenefitPlansRecurringFairValueMeasuresActivityInLevel3Details Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures, Activity in Level 3 (Details) Details 96 false false R97.htm 2419402 - Disclosure - Employee Stock and Incentive Plans - Employee Stock Ownership Plan (Details) Sheet http://www.allete.com/role/EmployeeStockAndIncentivePlansEmployeeStockOwnershipPlanDetails Employee Stock and Incentive Plans - Employee Stock Ownership Plan (Details) Details 97 false false R98.htm 2419403 - Disclosure - Employee Stock and Incentive Plans - Stock-Based Compensation (Details) Sheet http://www.allete.com/role/EmployeeStockAndIncentivePlansStockBasedCompensationDetails Employee Stock and Incentive Plans - Stock-Based Compensation (Details) Details 98 false false R99.htm 2419404 - Disclosure - Employee Stock and Incentive Plans - Share-Based Compensation Expense (Details) Sheet http://www.allete.com/role/EmployeeStockAndIncentivePlansShareBasedCompensationExpenseDetails Employee Stock and Incentive Plans - Share-Based Compensation Expense (Details) Details 99 false false R100.htm 2419405 - Disclosure - Employee Stock and Incentive Plans - Performance Shares and Restricted Stock Units (Details) Sheet http://www.allete.com/role/EmployeeStockAndIncentivePlansPerformanceSharesAndRestrictedStockUnitsDetails Employee Stock and Incentive Plans - Performance Shares and Restricted Stock Units (Details) Details 100 false false R101.htm 2421402 - Disclosure - Business Segments Business Segment (Details) Sheet http://www.allete.com/role/BusinessSegmentsBusinessSegmentDetails Business Segments Business Segment (Details) Details http://www.allete.com/role/BusinessSegmentsBusinessSegmentTables 101 false false R102.htm 2422402 - Disclosure - Quarterly Financial Data (Unaudited) (Details) Sheet http://www.allete.com/role/QuarterlyFinancialDataUnauditedDetails Quarterly Financial Data (Unaudited) (Details) Details http://www.allete.com/role/QuarterlyFinancialDataUnauditedTables 102 false false R103.htm 2423401 - Schedule - Schedule II (Details) Sheet http://www.allete.com/role/ScheduleIiDetails Schedule II (Details) Details 103 false false All Reports Book All Reports ale12312019-10k.htm ale-20191231.xsd ale-20191231_cal.xml ale-20191231_def.xml ale-20191231_lab.xml ale-20191231_pre.xml exhibit10e812-31x2019.htm exhibit10i1212-31x2019.htm exhibit10i1312-31x2019.htm exhibit2112-31x2019.htm exhibit2312-31x2019.htm exhibit31a12-31x2019.htm exhibit31b12-31x2019.htm exhibit3212-31x2019.htm exhibit4g.htm exhibit4h.htm exhibit9512-31x2019.htm exhibit9912-31x2019.htm chart-50a68630a1ad51e0b38.jpg http://fasb.org/srt/2019-01-31 http://xbrl.sec.gov/dei/2019-01-31 http://fasb.org/us-gaap/2019-01-31 true true