Payments for Purchase of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Supplemental Cash Flow Information [Abstract] Supplemental Cash Flow Information [Abstract] Customer [Domain] Customer [Domain] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] State Current State and Local Tax Expense (Benefit) Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Net Income Attributable to ALLETE Net Income Attributable ALLETE Net Income (Loss) Attributable to ALLETE Net Income (Loss) Attributable to Parent Town Center District Capital Improvement Bonds [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. Consolidated Entities [Axis] Consolidated Entities [Axis] Forced Outage Costs Forced Outage Costs Forced Outage Costs Entity Emerging Growth Company Entity Emerging Growth Company Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. Operating Lease, Right-of-Use Asset Operating Lease, Right-of-Use Asset Transmission Services - Utility Transmission Services - Utility Transmission Services - Utility Unrecorded Unconditional Purchase Obligation [Table] Unrecorded Unconditional Purchase Obligation [Table] Component of Other Income (Expense) - Other [Domain] Component of Other Income (Expense) - Other [Domain] [Domain] for Component of Other Income (Expense) - Other [Axis] Interest Cost Defined Benefit Plan, Interest Cost ALLETE Properties Development and Maintenance Obligations [Member] ALLETE Properties Performance and Maintenance Obligations [Member] Performance and maintenance obligations to governmental entities to construct improvements in various projects. Debt Drawn Through Accordion Feature Debt Drawn Through Accordion Feature Debt Drawn Through Accordion Feature Statistical Measurement [Domain] Statistical Measurement [Domain] Entity Address, Address Line One Entity Address, Address Line One Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Square Butte [Member] Square Butte [Member] Square Butte [Member] Other [Member] Other [Member] Regulated Operations Other Revenue Recognized From Other Sources Immaterial For Disaggregation Other [Member] ALLETE Clean Energy Other [Member] ALLETE Clean Energy Other Revenue Recognized From Other Sources Immaterial For Disaggregation [Member] Entity Filer Category Entity Filer Category Regulatory Agency [Domain] Regulatory Agency [Domain] Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Authorized Return on Equity Public Utilities, Approved Return on Equity, Percentage Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Generating Capacity Subject to Tax Equity Financing Generating Capacity Subject to Tax Equity Financing Generating Capacity Subject to Tax Equity Financing Other Non-Current Assets Total Other Non-Current Assets Other Assets, Noncurrent Components of Net Periodic Benefit Cost (Credit) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Expected Period for Costs of Compliance Expected Period for Costs of Compliance Expected Period for Costs of Compliance Equity Method Investment [Axis] Investment, Name [Axis] Expected Return on Plan Assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Business Segments [Axis] Segments [Axis] Proceeds from Non-controlling Interest in Subsidiaries Proceeds from Noncontrolling Interests Retirement Benefits [Abstract] Retirement Benefits [Abstract] Other Income (Expense) - Other [Line Items] Other Income (Expense) - Other [Line Items] [Line Items] for Other Income (Expense) - Other [Table] Non-Controlling Interest in Subsidiaries Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Amount Other Income [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] Document Fiscal Year Focus Document Fiscal Year Focus PPA Counterparty Total Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Subsequent Event Type [Domain] Subsequent Event Type [Domain] Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Inventories – Net [Table Text Block] Schedule of Inventory, Current [Table Text Block] Other [Member] Other Income (Expense) - Other [Member] Other Income (Expense) - Other [Member] Long-term Contract [Member] Long-term Contract [Member] Long-term Contract Revenue Stream [Member] Minnkota Power PSA [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Generating Capacity to be Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Nobles 2 [Member] Nobles 2 [Member] Nobles 2 [Member] Defined Benefit Pension and Other Postretirement Benefit Plans Liability, Defined Benefit Plan, Noncurrent Assets [Abstract] Assets [Abstract] Amortization of Net Loss Defined Benefit Plan, Amortization of Gain (Loss) Dairyland Power Cooperative Dairyland Power Cooperative [Member] Dairyland Power Cooperative ALLETE Properties Real Estate Investments, Net PPACA Income Tax Deferral [Member] Medicare Part D Regulatory Asset [Member] Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Distributions to Non-controlling Interest Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Entity Address, City or Town Entity Address, City or Town Retained Earnings [Member] Retained Earnings [Member] ALLETE Bonds 2.79% Due September 2031 ALLETE Bonds 2.79% Due September 2031 [Member] ALLETE Bonds 2.79% Due September 2031 Debt Instrument [Axis] Debt Instrument [Axis] Pension and Other Postretirement Plan Costs [Member] Pension and Other Postretirement Plans Costs [Member] Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Operating and Maintenance Operating and Maintenance Operating and Maintenance Expenses Requested Equity Capital Structure Public Utilities, Requested Equity Capital Structure, Percentage Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Cash Contributions to Defined Benefit Pension Plans Payment for Pension Benefits Long-term PSA [Member] Long-term PSA [Member] Revenue Recognized From Long-term PSAs Amortization of Prior Service Credits Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Distributed Earnings Proceeds from Equity Method Investment, Distribution Industrial [Member] Industrial [Member] Industrial Sales Channel [Member] Revenue Streams [Axis] Contract with Customer, Sales Channel [Axis] Increase (Decrease) in Income Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Defined Benefit Pension and Postretirement Benefit Expense Pension and Other Postretirement Benefits Cost (Reversal of Cost) Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Entity Interactive Data Current Entity Interactive Data Current Power Purchase Agreements [Domain] Power Purchase Agreement - Energy and Capacity [Domain] Long-term Contract for Purchase of Electric Power [Domain] Number of Operating Segments Number of Operating Segments Steam Services Agreement Steam Services Agreement [Member] Steam Services Agreement Basic Earnings Per Share of Common Stock Earnings Per Share Earnings Per Share, Basic Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense of $0.1, $0.1, $0.4 and $0.2 Defined Benefit Pension and Other Postretirement Plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Ongoing Customer Rate Increase Proposed Ongoing Customer Rate Increase Proposed Ongoing Customer Rate Increase 2021 Integrated Resource Plan 2021 Integrated Resource Plan [Member] 2021 Integrated Resource Plan Regulatory Agency [Axis] Regulatory Agency [Axis] Document Type Document Type Total Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense Unsecured Term Loan, Amount Unsecured Term Loan, Amount Unsecured Term Loan, Amount Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Entity Current Reporting Status Entity Current Reporting Status Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Requested Return on Equity Public Utilities, Requested Return on Equity, Percentage Regulatory Liability [Axis] Regulatory Liability [Axis] Other Income (Expense) - Other [Abstract] Other Income (Expense) - Other [Abstract] Other Income (Expense) - Other [Abstract] Bad Debt Expense Accounts Receivable, Credit Loss Expense (Reversal) Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Coal, Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Long-Term Debt - Principal Long-term Debt, Gross Superior, WI [Member] Superior, WI [Member] Superior, WI Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Short-Term and Long-Term Debt - Carrying Amount Long-term Debt Total Fair Value of Assets Assets, Fair Value Disclosure Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Investing Activities [Abstract] Net Cash Provided by (Used in) Investing Activities [Abstract] Document Transition Report Document Transition Report Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Boswell Units 1 and 2 Net Plant and Equipment [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Total Liabilities and Equity Liabilities and Equity Revenue from Cost Recovery Riders and Other Regulatory Programs Regulated Operating Revenue, Other Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Other Other Accrued Liabilities, Current Outstanding Customer Accounts Receivable Accounts Receivable, after Allowance for Credit Loss Diamond Spring Diamond Spring [Member] Diamond Spring [Member] Customer Deposits Contract with Customer, Liability, Current Share-Based Compensation Effective Income Tax Rate Reconciliation, Tax Expense (Benefit), Share-based Payment Arrangement, Amount Long-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Long-Term Debt Excluding Current Maturities The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of long-term debt excluding current maturities. Non-Controlling Interest Details [Line Items] Non-Controlling Interest Details [Line Items] [Line Items] for Non-Controlling Interest Details [Table] Accounts Receivable Increase (Decrease) in Accounts and Other Receivables Non-Controlling Interest Details [Table] Non-Controlling Interest Details [Table] Non-Controlling Interest Details [Table] Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Liabilities and Equity [Abstract] Liabilities and Equity [Abstract] Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Accounts Payable Increase (Decrease) in Accounts Payable Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Operating Expenses [Abstract] Operating Expenses [Abstract] Ownership Percentage Equity Method Investment, Ownership Percentage Total Regulatory Liabilities Regulatory Liabilities Standby Letters of Credit [Member] Standby Letters of Credit [Member] Minimum [Member] Minimum [Member] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Regulated Operations [Member] Regulated Operations [Member] Regulated Operations [Member] Total Fair Value of Liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure Net Loss Attributable to Non-Controlling Interest Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest PSAs Power Purchase Agreement - Current Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability. Gross Unrecognized Tax Benefits Unrecognized Tax Benefits PSAs Power Purchase Agreement - Noncurrent Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent liability. Authorized Return on Equity, Including Incentive Adder Equity Method Investment, Approved Return on Common Equity, Including Incentive Adder The approved percentage return on common equity, including an incentive adder for participation in a regional transmission organization dedicated to the utility plant in the investee accounted for under the equity method of accounting. Collateral Guarantor Obligations, Collateral Held Directly or by Third Parties, Amount Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Contracts with Customers - Non-utility Unregulated Operating Revenue Environmental Accruals [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Component of Other Income (Expense) - Other [Axis] Component of Other Income (Expense) - Other [Axis] Component of Other Income (Expense) - Other [Axis] Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Trading Symbol Trading Symbol Current Liabilities [Abstract] Liabilities, Current [Abstract] Gain on Sale after tax Gain on Sale after tax Gain on Sale after tax Common Stock Issued Stock Issued During Period, Value, New Issues Deferred Income Taxes Deferred Income Taxes and Other Tax Liabilities, Noncurrent Basin Electric Power Cooperative Basin Electric Power Cooperative [Member] Basin Electric Power Cooperative Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Line of Credit [Member] Line of Credit [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Dividends on Common Stock Common Stock Dividends Payments of Ordinary Dividends, Common Stock Commercial [Member] Commercial [Member] Revenue Recognized From Commercial Customers Actual Ratio of Indebtedness to Total Capitalization Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Accounting Policies [Abstract] Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] Number of Reportable Segments Number of Reportable Segments Regulatory Assets Currently Earning a Return Types of Net Regulatory Assets Earning Returns Other - Non-utility Amortization of PSAs Amortization of Power Purchase Agreements The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method. Federal Current Federal Tax Expense (Benefit) Letters of Credit Outstanding, Amount Letters of Credit Outstanding, Amount Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Construction of Wind Energy Facilities Other Inventory, Gross Common Stock Without Par Value, 80.0 Shares Authorized, 52.6 and 52.1 Shares Issued and Outstanding Common Stock, Value, Issued Total Other Expense Nonoperating Income (Expense) Inventories – Net [Abstract] Inventory, Net [Abstract] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Change in Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Other Postretirement [Member] Other Postretirement Benefits Plan [Member] Commitments, Guarantees and Contingencies (Note 6) Commitments and Contingencies Subsequent Event [Table] Subsequent Event [Table] Short-Term Debt - Principal Debt, Current, Gross Amount of short-term debt and current maturities of long-term debt before deduction of unamortized discount or premium and unamortized debt issuance costs. Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Fair Value, by Balance Sheet Grouping [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Anticipated Additional Generating Capacity (MW) Anticipated Additional Generating Capacity (MW) Anticipated Additional Generating Capacity (MW) Total Debt - Principal Debt, Long-Term and Short-Term, Combined Amount, Gross Represents the aggregate of total long-term debt, including current maturities and short-term debt, before deduction of unamortized discount or premium and unamortized debt issuance expense. Fuel Energy Related Inventory, Coal Accounts Receivable, Allowance Accounts Receivable, Allowance for Credit Loss, Current Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Document Period End Date Document Period End Date Estimated Market Value - Reservoir Land to be Sold Estimated Market Value - Reservoir Land to be Sold Estimated Market Value - Reservoir Land to be Sold Total Operating Expenses Costs and Expenses Other Power Suppliers [Member] Other Power Suppliers [Member] Other Power Suppliers Sales Channel [Member] Entity Registrant Name Entity Registrant Name Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] Total Debt - Total Debt, Long-term and Short-term, Combined Amount Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the third fiscal year through the fifth fiscal year following the latest fiscal year (maturing in three years to less than five years). Fuel Adjustment Clause (Member) Regulatory Clause Revenues, under-recovered [Member] Operating Revenue, Other Than Customer Revenue Revenue, Other Than Customer Revenue Non-cash Revenue Recognized From Amortization of PSAs Assumed Through Acquisitions ALLETE, Inc. [Member] ALLETE, Inc. [Member] -- None. No documentation exists for this element. -- Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Short-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Debt, Current The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of short-term debt or the current maturities of long-term debt. Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Operating Revenue Revenue from Contract with Customer, Excluding Assessed Tax Square Butte PPA [Member] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Electric Services Agreement Electric Services Agreement [Member] Electric Services Agreement Current Income Tax Expense (Benefit) [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Materials and Supplies Inventory, Supplies, Net of Reserves Common Stock [Member] Common Stock [Member] Total Project Costs Incurred to Date Capital Cost Spent to Date Capital cost spent to date for project. Other Income (Expense) [Abstract] Other Nonoperating Income (Expense) [Abstract] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Operating Income Operating Income (Loss) Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Regulatory Liabilities Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Cost Recovery Riders [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] Regulatory Asset [Axis] Regulatory Asset [Axis] Operating Lease Liabilities Operating Lease, Liability, Current Estimated Annual Reduction to Revenue Due to Idled Customers Keetac and Verso Estimated Annual Reduction to Revenue Due to Idled Customers Estimated Annual Reduction to Revenue Due to Idled Customers net of associated expense savings such as fuel costs for Keetac and Verso. Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] North Dakota Investment Tax Credits [Member] North Dakota Investment Tax Credits Subject to Refund [Member] North Dakota Investment Tax Credits Subject to Refund [Member] Consolidated Statement of Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Share-Based and ESOP Compensation Expense Share-based Payment Arrangement, Noncash Expense Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Interim Rate Increase Public Utilities, Interim Rate Increase (Decrease), Amount Depreciation Expense Depreciation Provision for Tax Reform Refund Provision for Tax Reform Refund Provision for Tax Reform Refund Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Document [Domain] Document [Domain] Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] Equity Method Investments [Policy Text Block] Equity Method Investments [Policy Text Block] Rate Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Other Total Other Income Other Nonoperating Income (Expense) Short-Term and Long-Term Debt - Fair Value Long-term Debt, Fair Value Available-for-sale - Equity Securities Equity Securities, FV-NI, Noncurrent Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] BNI Energy BNI Energy [Member] BNI Energy Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing in after the fifth fiscal year following the latest fiscal year (maturing in five years or more). Other Investing Activities Payments for (Proceeds from) Other Investing Activities Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Other Total Other Current Liabilities Other Liabilities, Current Pension and Other Postretirement Benefit Plans [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] 2020 Minnesota General Rate Case [Member] 2020 Minnesota General Rate Review [Member] 2020 Minnesota General Rate Review [Member] Employer Contributions to Defined Benefit Plans Defined Benefit Plan, Plan Assets, Contributions by Employer Maximum [Member] Maximum [Member] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Cost of Power Purchased Utilities Operating Expense, Purchased Power under Long-term Contracts Total Liabilities Liabilities Legal Entity [Axis] Legal Entity [Axis] 2022 Minnesota General Rate Review 2022 Minnesota General Rate Review [Member] 2022 Minnesota General Rate Review Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Financing Activities [Abstract] Net Cash Provided by (Used in) Financing Activities [Abstract] Total ALLETE Equity Stockholders' Equity Attributable to Parent Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] Common Stock, Shares Issued Common Stock, Shares, Issued Equity Investments [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Supplemental Statement of Cash Flows Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Other Other Assets, Miscellaneous, Noncurrent ATC [Member] ATC [Member] ATC [Member] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Annual Assessment Community Development District Obligation, Annual Assessment Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. Guarantee Entity and Type [Domain] Guarantee Entity and Type [Domain] [Domain] for Guarantee Entity and Type [Axis] Operating Lease Liabilities Operating Lease, Liability, Noncurrent Measurement Frequency [Domain] Measurement Frequency [Domain] Total Current Income Tax Expense Current Income Tax Expense (Benefit) Interest Expense Interest Expense Estimated Capital Expenditures, Including Past Expenditures Estimated Capital Expenditures, Including Past Expenditures The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. Deferred Income Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Net Periodic Benefit Cost (Credit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Public Utilities Reserve for Interim Rates Public Utilities Reserve for Interim Rates Public Utilities Reserve for Interim Rates City Area Code City Area Code Retained Earnings Retained Earnings (Accumulated Deficit) ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Equity Method Investments (Table Text Block) Equity Method Investments [Table Text Block] Service Agreement Type [Axis] Service Agreement Type [Axis] Service Agreement Type Cash, Cash Equivalents and Restricted Cash [Abstract] Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract] Income Statement [Abstract] Income Statement [Abstract] Equity Investments Equity Investment Balance as of December 31, 2020 Equity Investment Balance as of September 30, 2021 Equity Investment Balance Equity Method Investments Amendment Flag Amendment Flag Interest and Investment Income (Loss) [Member] Interest and Investment Earnings [Member] Interest and Investment Earnings [Member] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Income Tax Benefit Total Income Tax Expense (Benefit) Income Tax Expense (Benefit) Total Equity Balance, Beginning Balance Balance, End of Period Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Accounts Receivable (Less Allowance of $2.1 and $2.5) Accounts Receivable, after Allowance for Credit Loss, Current Conservation Improvement Program [Member] CIP Financial Incentive [Member] CIP Financial Incentive [Member] Conservation Improvement Program [Member] Other (Member) Other Regulatory Assets (Liabilities) [Member] Proceeds from Issuance of Short-Term and Long-Term Debt Proceeds from Issuance of Long-term Debt Equity Components [Axis] Equity Components [Axis] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Total Current Assets Assets, Current Pension and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Contract Assets Contract with Customer, Asset, after Allowance for Credit Loss, Noncurrent Inventories – Net Total Inventories – Net Inventory, Net Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Entity File Number Entity File Number Federal Deferred Federal Income Tax Expense (Benefit) Recurring Fair Value Measures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Equity Method Investments and Joint Ventures [Abstract] Equity Method Investments and Joint Ventures [Abstract] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Administrative Law Judge Recommended Refund Administrative Law Judge Recommended Refund Administrative Law Judge Recommended Refund Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Other Postretirement Benefit Plans Other Postretirement Benefit Plans Other Postretirement Benefit Plans Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] State of Incorporation Entity Incorporation, State or Country Code Entity Small Business Entity Small Business Resource Package [Member] Resource Package [Member] Resource Package [Member] Equity Component [Domain] Equity Component [Domain] Other Receivables Other Receivables Cash Equivalents Other Restricted Assets, Noncurrent Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Investment in ATC [Table] Investment in Nobles 2 [Table] Schedule of Equity Method Investments [Table] Investments [Abstract] Investments [Abstract] Consolidated Statement of Equity [Line Items] Statement [Line Items] Other Current Liabilities Increase (Decrease) in Other Current Liabilities Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Restricted Cash included in Other Non-Current Assets Restricted Cash, Non-Current Restricted Cash, Noncurrent Glen Ullin Energy Center [Member] Glen Ullin Energy Center [Member] Glen Ullin Energy Center [Member] Available-for-sale – Corporate and Governmental Debt Securities (b) Debt Securities, Available-for-sale, Noncurrent Retirement Plan Type [Domain] Retirement Plan Type [Domain] Liabilities [Abstract] Liabilities [Abstract] Pension and Other Postretirement Benefit Plans Non-Service Credit [Member] Pension and Other Postretirement Benefit Plans Non-Service Credit [Member] Pension and Other Postretirement Benefit Plans Non-Service Credit [Member] Minnesota Power Land Sales Minnesota Power Land Sales [Member] Minnesota Power Land Sales Counterparty Name [Domain] Counterparty Name [Domain] Debt Disclosure [Abstract] Debt Disclosure [Abstract] Entity Address, State or Province Entity Address, State or Province Income Before Income Taxes Income Before Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Document and Entity Information [Table] Document Information [Table] Segment Reporting [Abstract] Segment Reporting [Abstract] Entity [Domain] Entity [Domain] Power Sales Agreements - Energy and Capacity [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Line of Credit, Current Line of Credit, Current Customer [Axis] Customer [Axis] Pension and Other Postretirement Benefit Plans [Text Block] Retirement Benefits [Text Block] Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Entity Shell Company Entity Shell Company Local Phone Number Local Phone Number Fuel Adjustment Clause Filing [Member] Fuel Adjustment Clause Filing [Member] Fuel Adjustment Clause Filing Proceeds from Issuance of Common Stock Proceeds from Issuance of Common Stock Total Assets Assets Assets Other Other Accrued Liabilities, Noncurrent Unsecured Term Loan Due in 2022 Unsecured Term Loan Due in 2022 [Member] Unsecured Term Loan Due in 2022 Equity Method Investment [Domain] Investment, Name [Domain] Common Stock, Shares Authorized Common Stock, Shares Authorized Cash Paid for Interest – Net of Amounts Capitalized Interest Paid, Excluding Capitalized Interest, Operating Activities Net Income Attributable to ALLETE Net Income (Loss) Available to Common Stockholders, Diluted Geographical [Domain] Geographical [Domain] Non-controlling Interest [Member] Noncontrolling Interest [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] COVID-19 Related Deferred Accounting [Member] COVID-19 Related Deferred Accounting [Member] COVID-19 Related Deferred Accounting Interest Rate Debt Instrument, Interest Rate, Stated Percentage Net Loss Attributable to Non-Controlling Interest Net Income (Loss) Attributable to Noncontrolling Interest Proceeds from Sale of Available-for-sale Securities Cash Proceeds from Sale of Available-for-sale Securities Cash Proceeds from Sale of Available-for-sale Securities Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Uncertain Tax Positions [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Measurement Frequency [Axis] Measurement Frequency [Axis] Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Schedule of Short-Term and Long-Term Debt [Table Text Block] Schedule of Debt [Table Text Block] Recurring Fair Value Measures [Table] Fair Value, Recurring and Nonrecurring [Table] Schedule of Long-term Debt Instruments [Table] Financial Covenants [Table] Schedule of Long-term Debt Instruments [Table] Entity Tax Identification Number Entity Tax Identification Number Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Subsequent Event [Line Items] Subsequent Event [Line Items] Geographical [Axis] Geographical [Axis] Corporate and Other [Member] Corporate and Other [Member] Diluted Earnings Per Share of Common Stock Earnings Per Share Earnings Per Share, Diluted Basic (Shares) Average Common Shares Weighted Average Number of Shares Outstanding, Basic Other [Member] Corporate & Other - Other [Member] Revenue Recognized by Corporate & Other From Other Sources Immaterial for Disaggregation Business Segments [Domain] Segments [Domain] Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Verso Corporation (Member) Verso Corporation [Member] Verso Corporation Requested Rate Increase Percent Public Utilities, Requested Rate Increase (Decrease), Percentage Service Cost Defined Benefit Plan, Service Cost Coal, Rail and Shipping Contracts [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Power Purchase Agreements [Axis] Power Purchase Agreement - Energy and Capacity [Axis] Long-term Contract for Purchase of Electric Power [Axis] SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Total Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Transmission Formula Rates [Member] Transmission Formula Rates [Member] Transmission Formula Rates Regulatory Liability [Domain] Regulatory Liability [Domain] Cash from Financing Activities Net Cash Provided by (Used in) Financing Activities Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] Equity Earnings Equity in ATC Earnings Income (Loss) from Equity Method Investments Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Gain on Investments and Property, Plant and Equipment Gain (Loss) on Disposition of Other Assets Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Fair Value [Text Block] Fair Value Disclosures [Text Block] Accounts Payable Accounts Payable, Current Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] AFUDC – Equity AFUDC–Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Depreciation and Amortization Depreciation, Depletion and Amortization South Peak [Member] South Peak [Member] South Peak [Member] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Common Stock, Par Value Common Stock, No Par Value Surety Bonds [Member] Surety Bond [Member] Power Sales Agreements - Energy and Capacity [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Accumulated Other Comprehensive Loss [Member] AOCI Attributable to Parent [Member] Inventories Increase (Decrease) in Inventories Common Stock, Shares Outstanding Common Stock, Shares, Outstanding BNI Energy Reclamation Liability [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. Service Agreement Type [Domain] Service Agreement Type [Domain] Service Agreement Type [Domain] Total Regulatory Asset [Domain] Regulatory Asset [Domain] Cost of Sales - Non-utility Cost of Goods and Services Sold Operating Revenues [Abstract] Revenues [Abstract] Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Accounts Receivable [Abstract] Accounts Receivable, after Allowance for Credit Loss, Current [Abstract] Consolidated Entities [Domain] Consolidated Entities [Domain] State Deferred State and Local Income Tax Expense (Benefit) Unsecured Debt, Total Drawn Unsecured Debt, Total Drawn on Loan Unsecured Debt, Total Drawn on Loan Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Property, Plant and Equipment – Net Property, Plant and Equipment, Net Other Non-Current Liabilities Total Other Non-Current Liabilities Other Liabilities, Noncurrent Provision for Interim Rate Refund Provision (Payment) for Interim Rate Refund Provision (Payment) for Interim Rate Refund Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Debt Instrument [Line Items] Debt Instrument [Line Items] Repayments of Short-Term and Long-Term Debt Repayments of Long-term Debt Earnings Per Share [Abstract] Earnings Per Share [Abstract] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Document Quarterly Report Document Quarterly Report Amortization of the Remeasurement of Deferred Income Taxes Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Total Operating Revenue Operating Revenue Revenues Customer Class [Domain] Customer Class [Domain] [Domain] for Customer Class [Axis] description of customer types or classes Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Long-Term Debt Long-Term Debt - Total Long-term Debt, Excluding Current Maturities Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Regulatory Differences – Excess Deferred Tax Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. Investment in ATC [Line Items] Investment in Nobles 2 [Line Items] Schedule of Equity Method Investments [Line Items] Regulatory Assets and Liabilities Disclosure [Abstract] Regulatory Assets and Liabilities Disclosure [Abstract] Document Fiscal Period Focus Document Fiscal Period Focus Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] Net Income Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Residential [Member] Residential [Member] Revenue Recognized From Residential Customers Short-Term Debt - Total Debt, Current Restricted Cash included in Prepayments and Other Restricted Cash, Current Coal, Rail and Shipping Contracts [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Counterparty Name [Axis] Counterparty Name [Axis] Regulatory Assets Non-Current Regulatory Assets Regulatory Assets, Noncurrent Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Natural Gas-fired [Member] Natural Gas Processing Plant [Member] Fuel Adjustment Clause [Member] Fuel Adjustment Clause [Member] Amount expected to be refunded to customers for the over-collection of fuel adjustment clause recoveries. Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Accrued Taxes Taxes Payable, Current Current Assets [Abstract] Assets, Current [Abstract] Other Financing Activities Proceeds from (Payments for) Other Financing Activities Entity Central Index Key Entity Central Index Key Income Tax [Policy Text Block] Income Tax, Policy [Policy Text Block] Name of Exchange Security Exchange Name Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Revenue Streams [Domain] Contract with Customer, Sales Channel [Domain] Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the first fiscal year through the third fiscal year following the latest fiscal year (maturing in more than one year to less than three years). Diluted (Shares) Average Common Shares Weighted Average Number of Shares Outstanding, Diluted Tenaska PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] Total Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax Recurring Fair Value Measures [Member] Fair Value, Recurring [Member] Municipal [Member] Municipal [Member] Municipal Sales Channel Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Accrued Interest Interest Payable, Current Plant Removal Obligations [Member] Removal Costs [Member] Public Utilities, Interim Rate Increase (Decrease), Percentage Public Utilities, Interim Rate Increase (Decrease), Percentage South Shore Energy South Shore Energy [Member] South Shore Energy Current Fiscal Year End Date Current Fiscal Year End Date Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Components of Net Periodic Benefit Expense (Income) [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent Deferred Compensation Deferred Compensation, Fair Value Disclosure This element represents the portion of the balance sheet assertion, Deferred Compensation, valued at fair value by the entity. Payments for Equity Method Investments Cash Investments Payments to Acquire Equity Method Investments Land in Minnesota (Acres) Area of Land Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities Changes in Operating Assets and Liabilities [Abstract] Increase (Decrease) in Operating Capital [Abstract] Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges CIP Consolidated Filing [Member] CIP Consolidated Filing [Member] CIP Consolidated Filing Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) AFUDC - Equity [Member] AFUDC - Equity [Member] AFUDC - Equity [Member] MPUC [Member] MPUC [Member] Minnesota Public Utilities Commission [Member] ALLETE South Wind [Member] ALLETE South Wind [Member] ALLETE South Wind Total Current Liabilities Liabilities, Current Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Fuel Adjustment Clause Public Utilities, Fuel Adjustment Clause Public Utilities, Fuel Adjustment Clause Regulatory Matters [Table] Public Utilities General Disclosures [Table] Business Segments [Line Items] Segment Reporting Information [Line Items] Entity Address, Postal Zip Code Entity Address, Postal Zip Code Approximate Sales Price Approximate Sales Price Approximate Sales Price Taxes Other than Income Taxes Taxes, Other Document and Entity Information, Document [Axis] Document Information, Document [Axis] Document and Entity Information [Line Items] Document Information [Line Items] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Title of 12(b) Security Title of 12(b) Security Current Regulatory Liabilities Regulatory Liability, Current Consolidated Statement of Equity [Table] Statement [Table] Prepayments and Other Other Assets, Current Estimated Costs of Compliance (Accrued) Accrual for Environmental Loss Contingencies Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Development Costs Sold Development Costs Sold Development Costs Sold Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Statistical Measurement [Axis] Statistical Measurement [Axis] Income Taxes [Member] Deferred Income Tax Charge [Member] Generating Facility [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Proceeds from Issuance of First Mortgage Bond Proceeds from Issuance of First Mortgage Bond Income Taxes Computed at Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Subsequent Event [Member] Subsequent Event [Member] Renewable Generation Renewable Generation [Member] Renewable Generation Approximate Refund for Reversal of FERC ROE Order, ALLETE's Proportion Approximate Refund for Reversal of FERC ROE Order, ALLETE's Proportionate Share Approximate Refund for Reversal of FERC ROE Order, ALLETE's Proportionate Share Contracts with Customers - Utility Regulated Operating Revenue Non-Controlling Interest in Subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Inventories – Net [Policy Text Block] Inventory, Policy [Policy Text Block] Pension [Member] Pension Plan [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Expected Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Ownership Percentage of Benefited Property Community Development District Obligation, Ownership of Property Benefited Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. Available-for-sale Securities, Debt Maturities, within One Year, Fair Value Debt Securities, Available-for-Sale, Fair Value, Maturity, Allocated and Single Maturity Date, Year One Regulated Operations [Abstract] Regulated Operations [Abstract] Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Schedule of Regulatory Assets and Liabilities [Line Items] Schedule of Regulatory Assets and Liabilities [Line Items] Schedule of Regulatory Assets and Liabilities [Line Items] Approximate Refund for Reversal of FERC ROE Order Approximate Refund for Reversal of FERC ROE Order Approximate Refund for Reversal of FERC ROE Order Minnesota Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] Required Ratio of Indebtedness to Total Capitalization Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Deferred Income Tax Benefit Total Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit)