Line of Credit, Current Line of Credit, Current Other Total Other Income Other Nonoperating Income (Expense) Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Town Center District Capital Improvement Bonds [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. Transmission Services - Utility Transmission Services - Utility Transmission Services - Utility Business Segments [Axis] Segments [Axis] Component of Other Income (Expense) - Other [Domain] Component of Other Income (Expense) - Other [Domain] [Domain] for Component of Other Income (Expense) - Other [Axis] Pension and Other Postretirement Benefit Plans [Text Block] Retirement Benefits [Text Block] 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. Cash Equivalents Other Restricted Assets, Noncurrent Level 3 [Member] Fair Value, Inputs, Level 3 [Member] 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 Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Square Butte [Member] Square Butte [Member] Square Butte [Member] Other [Member] Other [Member] Regulated Operations Other Revenue Recognized From Other Sources Immaterial For Disaggregation Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Other [Member] ALLETE Clean Energy Other [Member] ALLETE Clean Energy Other Revenue Recognized From Other Sources Immaterial For Disaggregation [Member] Statistical Measurement [Domain] Statistical Measurement [Domain] Liabilities and Equity [Abstract] Liabilities and Equity [Abstract] Line of Credit [Member] Line of Credit [Member] Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Generating Capacity Subject to Tax Equity Financing Generating Capacity Subject to Tax Equity Financing Generating Capacity Subject to Tax Equity Financing Expected Period for Costs of Compliance Expected Period for Costs of Compliance Expected Period for Costs of Compliance ALLETE Bonds 3.30% Due August 2050 [Member] ALLETE Bonds 3.30% Due August 2050 [Member] ALLETE Bonds 3.30% Due August 2050 [Member] Current Fiscal Year End Date Current Fiscal Year End Date Debt Instrument, Unused Borrowing Capacity, Amount Debt Instrument, Unused Borrowing Capacity, Amount Other Income (Expense) - Other [Line Items] Other Income (Expense) - Other [Line Items] [Line Items] for Other Income (Expense) - Other [Table] Accounts Receivable Increase (Decrease) in Accounts and Other Receivables Proceeds from Issuance of Long-term Debt Proceeds from Issuance of Long-term Debt Operating Income Operating Income (Loss) Entity Address, Postal Zip Code Entity Address, Postal Zip Code Total Current Liabilities Liabilities, Current 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] Expected Output Entitlement (Percent) Expected Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Commitments, Guarantees and Contingencies (Note 6) Commitments and Contingencies Available-for-sale – Corporate and Governmental Debt Securities (b) Debt Securities, Available-for-sale, Noncurrent Assets [Abstract] Assets [Abstract] Equity Method Investment [Axis] Investment, Name [Axis] Accrued Taxes Taxes Payable, Current Long-term Contract for Sale of Energy [Axis] Long-term Contract for Sale of Energy [Axis] Long-term Contract for Sale of Energy [Axis] Entity Shell Company Entity Shell Company Income Taxes Computed at Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Accounts Payable Accounts Payable, Current Gain on Land Sales [Member] Gain (Loss) on Land Sales [Member] Gain (Loss) on Land Sales [Member] Document Type Document Type Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Other [Member] Other Income (Expense) - Other [Member] Other Income (Expense) - Other [Member] Document and Entity Information, Document [Axis] Document Information, Document [Axis] Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Retained Earnings Retained Earnings (Accumulated Deficit) Operating Lease, Right-of-Use Asset Operating Lease, Right-of-Use Asset Cash, Cash Equivalents and Restricted Cash [Abstract] Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract] 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] Unsecured Term Loan Due April 2021 [Member] Unsecured Term Loan Due April 2021 [Member] Unsecured Term Loan Due April 2021 [Member] Other Other Accrued Liabilities, Noncurrent 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] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Debt Instrument, Interest Rate Floor Debt Instrument, Interest Rate Floor Debt Instrument, Interest Rate Floor State Current State and Local Tax Expense (Benefit) Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Output Being Purchased (MW) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. 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). Income Tax [Policy Text Block] Income Tax, Policy [Policy Text Block] Regulatory Liabilities Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Payments for Equity Investments Cash Investments Payments to Acquire Equity Method Investments Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Document Quarterly Report Document Quarterly Report Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Other Non-Current Liabilities Total Other Non-Current Liabilities Other Liabilities, Noncurrent Payments for (Proceeds from) Other Investing Activities Payments for (Proceeds from) Other Investing Activities Operating and Maintenance Operating and Maintenance Operating and Maintenance Expenses Inventories – Net Total Inventories – Net Inventory, Net Operating Revenue Revenue from Contract with Customer, Excluding Assessed Tax Measurement Frequency [Domain] Measurement Frequency [Domain] Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Regulatory Liability [Domain] Regulatory Liability [Domain] Entity File Number Entity File Number Long-term PSA [Member] Long-term PSA [Member] Revenue Recognized From Long-term PSAs Industrial [Member] Industrial [Member] Industrial Sales Channel [Member] Geographical [Domain] Geographical [Domain] Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Consolidated Statement of Shareholders' Equity [Line Items] Statement [Line Items] Increase (Decrease) in Income Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] Document Fiscal Period Focus Document Fiscal Period Focus Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Accrued Interest Interest Payable, Current Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Consolidated Statement of Shareholders' Equity [Table] Statement [Table] Pension and Other Postretirement Benefit Plans [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Interest Cost Defined Benefit Plan, Interest Cost Statistical Measurement [Axis] Statistical Measurement [Axis] Common Stock, Par Value Common Stock, No Par Value Ongoing Customer Rate Increase Proposed Ongoing Customer Rate Increase Proposed Ongoing Customer Rate Increase Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Unrecorded Unconditional Purchase Obligation [Table] Unrecorded Unconditional Purchase Obligation [Table] Consolidated Statement of Shareholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Unsecured Term Loan, Amount Unsecured Term Loan, Amount Unsecured Term Loan, Amount Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Total Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Proceeds from Issuance of Common Stock Proceeds from Issuance of Common Stock Pension and Other Postretirement Plan Costs [Member] Pension and Other Postretirement Plans Costs [Member] U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Transmission [Axis] Transmission [Axis] Information about each transmission investment. Other Income (Expense) - Other [Abstract] Other Income (Expense) - Other [Abstract] Other Income (Expense) - Other [Abstract] Public Utilities, Interim Rate Increase (Decrease), Percentage Public Utilities, Interim Rate Increase (Decrease), Percentage Amendment Flag Amendment Flag ALLETE Bonds 2.50% Due August 2030 [Member] ALLETE Bonds 2.50% Due August 2030 [Member] ALLETE Bonds 2.50% Due August 2030 [Member] Other Other Accrued Liabilities, Current Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Superior, WI [Member] Superior, WI [Member] Superior, WI Equity Method Investments Equity Method Investments [Table Text Block] MW Sold Under Short-term PSAs MW Sold Under Short-term PSAs MW Sold Under Short-term PSAs Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Gain on Sale of U.S. Water Services, After-tax Gain on Sale of Affiliate, After-tax Gain on Sale of Affiliate, After-tax Measurement Frequency [Axis] Measurement Frequency [Axis] Pension and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges Surety Bonds [Member] Surety Bond [Member] Expected Return on Plan Assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Equity Method Investments [Policy Text Block] Equity Method Investments [Policy Text Block] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Revenue Streams [Domain] Contract with Customer, Sales Channel [Domain] Geographical [Axis] Geographical [Axis] PPA Counterparty Total Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Local Phone Number Local Phone Number 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] Business Segments [Domain] Segments [Domain] Non-Controlling Interest Details [Table] Non-Controlling Interest Details [Table] Non-Controlling Interest Details [Table] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Minnesota Power Short-term PSAs in 2020 and 2021 [Member] Minnesota Power Short-term PSAs in 2020 and 2021 [Member] Minnesota Power Short-term PSAs in 2020 and 2021 [Member] Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment ALLETE Clean Energy Asset Acquisition [Abstract] ALLETE Clean Energy Asset Acquisition [Abstract] ALLETE Clean Energy Asset Acquisition [Abstract] Accounts Receivable, Allowance Accounts Receivable, Allowance for Credit Loss, Current Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Financing Activities [Abstract] Net Cash Provided by (Used in) Financing Activities [Abstract] Federal Current Federal Tax Expense (Benefit) Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Taxes Other than Income Taxes Taxes, Other Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Entity Current Reporting Status Entity Current Reporting Status Regulated Operations [Member] Regulated Operations [Member] Regulated Operations [Member] Other Non-Current Assets Total Other Non-Current Assets Other Assets, Noncurrent Equity Investments [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Common Stock Issued Stock Issued During Period, Value, New Issues 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. 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. Total Assets Assets Assets 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. Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Component of Other Income (Expense) - Other [Axis] Component of Other Income (Expense) - Other [Axis] Component of Other Income (Expense) - Other [Axis] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Amortization of Prior Service Credits Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Interim Rate Increase Public Utilities, Interim Rate Increase (Decrease), Amount Subsequent Event Type [Axis] Subsequent Event Type [Axis] Unrealized Gain on Securities Net of Income Tax Expense of $0.1, $0.1, $– and $0.1 Unrealized Gain (Loss) on Debt Securities OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Entity Small Business Entity Small Business Unrealized Gain (Loss) on Securities, Income Tax Expense OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax Interest Expense Interest Expense Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Depreciation and Amortization Depreciation, Depletion and Amortization Entity Filer Category Entity Filer Category Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Regulatory Assets Currently Earning a Return Types of Net Regulatory Assets Earning Returns 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. Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] Total Liabilities Liabilities Entity Address, City or Town Entity Address, City or Town 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. Proceeds from Issuance of Unsecured Debt Proceeds from Issuance of Unsecured Debt Retirement Plan Type [Axis] Retirement Plan Type [Axis] Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manitoba Hydro PPA [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Regulated Operations [Abstract] Regulated Operations [Abstract] 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. Inventories Increase (Decrease) in Inventories 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. ALLETE Properties Real Estate Investments, Net Recurring Fair Value Measures [Table] Fair Value, Recurring and Nonrecurring [Table] Document Fiscal Year Focus Document Fiscal Year Focus Accounts Payable Increase (Decrease) in Accounts Payable Regulatory Agency [Axis] Regulatory Agency [Axis] Debt Instrument [Axis] Debt Instrument [Axis] Other Income [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Other Power Suppliers [Member] Other Power Suppliers [Member] Other Power Suppliers Sales Channel [Member] Coal, Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Other Other Assets, Miscellaneous, Noncurrent Common Stock, Shares Outstanding Common Stock, Shares, Outstanding Equity Investments Equity Investment Balance as of December 31, 2019 Equity Investment Balance as of June 30, 2020 Equity Investment Balance Equity Method Investments Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] 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). State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Contracts with Customers - Utility Regulated Operating Revenue Income Before Income Taxes Income Before Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest 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. -- Subsequent Event [Member] Subsequent Event [Member] 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. Square Butte PPA [Member] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Letters of Credit Outstanding, Amount Letters of Credit Outstanding, Amount Common Stock, Shares Authorized Common Stock, Shares Authorized Total Debt - Total Debt, Long-term and Short-term, Combined Amount Federal Deferred Federal Income Tax Expense (Benefit) Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Document Period End Date Document Period End Date Uncertain Tax Positions [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Entity Registrant Name Entity Registrant Name Income Statement [Abstract] Income Statement [Abstract] AOCI Attributable to Parent [Member] AOCI Attributable to Parent [Member] Total Project Costs Incurred to Date Capital Cost Spent to Date Capital cost spent to date for project. Operating Lease Liabilities Operating Lease, Liability, Noncurrent Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Cash Paid for Assets Acquired Cash Paid for Assets Acquired Cash Paid for Assets Acquire Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $–, $–, $0.1 and $0.1 Defined Benefit Pension and Other Postretirement Plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Cost Recovery Riders [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] North Dakota Investment Tax Credits [Member] North Dakota Investment Tax Credits Subject to Refund [Member] North Dakota Investment Tax Credits Subject to Refund [Member] Current Assets [Abstract] Assets, Current [Abstract] Share-Based Compensation Effective Income Tax Rate Reconciliation, Tax Expense (Benefit), Share-based Payment Arrangement, Amount Coal, Rail and Shipping Contracts [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Payments / 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] Consolidated Entities [Axis] Consolidated Entities [Axis] Total Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] 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] Rate Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Generating Capacity Under Development (MW) Generating Capacity Under Development (MW) Generating Capacity Under Development (MW) Counterparty Name [Domain] Counterparty Name [Domain] Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Document and Entity Information [Line Items] Document Information [Line Items] Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] Entity Tax Identification Number Entity Tax Identification Number 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). 2020 Minnesota General Rate Case [Member] 2020 Minnesota General Rate Review [Member] 2020 Minnesota General Rate Review [Member] Equity Earnings Equity in ATC Earnings (a) Income (Loss) from Equity Method Investments Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Trading Symbol Trading Symbol Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Power Purchase Agreements [Axis] Power Purchase Agreement - Energy and Capacity [Axis] Long-term Contract for Purchase of Electric Power [Axis] Income Taxes [Member] Deferred Income Tax Charge [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 Inventories – Net [Table Text Block] Schedule of Inventory, Current [Table Text Block] Share-Based and ESOP Compensation Expense Share-based Payment Arrangement, Noncash Expense Legal Entity [Axis] Legal Entity [Axis] Other Financing Activities Proceeds from (Payments for) Other Financing Activities Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Total Regulatory Asset [Domain] Regulatory Asset [Domain] Distributed ATC Earnings Proceeds from Equity Method Investment, Distribution ATC [Member] ATC [Member] ATC [Member] Entity [Domain] Entity [Domain] Document [Domain] Document [Domain] 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. Revenue Streams [Axis] Contract with Customer, Sales Channel [Axis] Short-Term and Long-Term Debt - Carrying Amount Long-term Debt Guarantee Entity and Type [Domain] Guarantee Entity and Type [Domain] [Domain] for Guarantee Entity and Type [Axis] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Conservation Improvement Program Goal, In 3 Years Conservation Improvement Program Goal, In 3 Years Conservation Improvement Program Goal, In 3 Years Cash Contributions to Defined Benefit Pension Plans Payment for Pension Benefits Service Cost Defined Benefit Plan, Service Cost Asset Retirement Obligation Asset Retirement Obligations, Noncurrent Estimated Capital Expenditures, Including Past Expenditures Total Project Cost in the U.S. 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. City Area Code City Area Code Cost of Sales - Non-utility Cost of Goods and Services Sold Provision for Interim Rate Refund Public Utilities Reserve for Interim Rates Public Utilities Reserve for Interim Rates Public Utilities Reserve for Interim Rates Total Operating Expenses Costs and Expenses Equity Components [Axis] Equity Components [Axis] Title of 12(b) Security Title of 12(b) Security Other Other Regulatory Assets (Liabilities) [Member] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Minimum [Member] Minimum [Member] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Consolidated Entities [Domain] Consolidated Entities [Domain] Operating Lease Liabilities Operating Lease, Liability, Current Unsecured Term Loan Due in 2021 [Member] Unsecured Term Loan Due in 2021 [Member] Unsecured Term Loan Due in 2021 [Member] Contracts with Customers - Non-utility Unregulated Operating Revenue Supplemental Statement of Cash Flows Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Net Loss Attributable to Non-Controlling Interest Net Income (Loss) Attributable to Noncontrolling Interest Equity Method Investments and Joint Ventures [Abstract] Equity Method Investments and Joint Ventures [Abstract] Contract [Member] Contract [Member] Revenue Recognized by U.S. Water From Contract Sales Amortization of Net Loss Defined Benefit Plan, Amortization of Gain (Loss) Contract Assets Contract with Customer, Asset, after Allowance for Credit Loss, Noncurrent Realized and Unrealized (Gain) Loss on Investments and Property, Plant and Equipment Gain (Loss) on Disposition of Other Assets Net Periodic Benefit Cost (Credit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Operating Revenues [Abstract] Revenues [Abstract] Fair Value [Text Block] Fair Value Disclosures [Text Block] Interest and Investment Income (Loss) [Member] Interest and Investment Earnings [Member] Interest and Investment Earnings [Member] Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Regulatory Assets Non-Current Regulatory Assets Regulatory Assets, Noncurrent Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Conservation Improvement Program Goal, Next Fiscal Year Conservation Improvement Program Goal, Next Fiscal Year Conservation Improvement Program Goal, Next Fiscal Year Conservation Improvement Program [Member] CIP Financial Incentive [Member] Conservation Improvement Program [Member] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Expected Additional Investment in 2020 Equity Method Investment, Expected Additional Investment Additional investments expected to be made during the current fiscal year but after the filing date, not already separately disclosed as part of current period/year-to-date investments or as a subsequent event. Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Power Purchase Agreements [Domain] Power Purchase Agreement - Energy and Capacity [Domain] Long-term Contract for Purchase of Electric Power [Domain] AFUDC – Equity AFUDC–Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Estimated Investment Amount Estimated Investment Amount Estimated Investment Amount Current Regulatory Liabilities Regulatory Liability, Current Number of Reportable Segments Number of Reportable Segments Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Regulatory Assets and Liabilities Disclosure [Abstract] Regulatory Assets and Liabilities Disclosure [Abstract] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Defined Benefit Pension and Postretirement Benefit Expense Pension and Other Postretirement Benefits Cost (Reversal of Cost) Other Income (Expense) [Abstract] Other Nonoperating Income (Expense) [Abstract] Investment in ATC [Table] Investment in Nobles 2 [Table] Schedule of Equity Method Investments [Table] Dividends on Common Stock Common Stock Dividends Payments of Ordinary Dividends, Common Stock Other Postretirement Benefit Plans Other Postretirement Benefit Plans Other Postretirement Benefit Plans Investments [Abstract] Investments [Abstract] Accounts Receivable (Less Allowance of $1.3 and $0.9) Accounts Receivable, after Allowance for Credit Loss, Current Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Output Being Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased Resource Package [Member] Resource Package [Member] Resource Package [Member] Restricted Cash, Non-Current Restricted Cash, Noncurrent Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Capital Project [Member] Capital Project [Member] Revenue Recognized From Capital Projects Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] 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 Glen Ullin Energy Center [Member] Glen Ullin Energy Center [Member] Glen Ullin Energy Center [Member] Deferred Income Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Coal, Rail and Shipping Contracts [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] 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] Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Prepayments and Other Other Assets, Current Additional Equity in Earnings During the Period Additional Equity in Earnings During the Period Additional Equity in Earnings During the Period Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Current Income Tax Expense (Benefit) [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Components of Net Periodic Benefit Cost (Credit) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Total Fair Value of Assets Assets, Fair Value Disclosure Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Entity Central Index Key Entity Central Index Key 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] Entity Emerging Growth Company Entity Emerging Growth Company Asset Carrying Value Asset Carrying Value - Undiscounted Cash Flow Analysis Asset Carrying Value - Undiscounted Cash Flow Analysis Diluted (Shares) Average Common Shares Weighted Average Number of Shares Outstanding, Diluted Basic Earnings Per Share of Common Stock Earnings Per Share Earnings Per Share, Basic Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Regulatory Matters [Table] Public Utilities General Disclosures [Table] Environmental Accruals [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Number of Operating Segments Number of Operating Segments Debt Disclosure [Abstract] Debt Disclosure [Abstract] Defined Benefit Pension and Other Postretirement Benefit Plans Liability, Defined Benefit Plan, Noncurrent Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations Cash from (for) Financing Activities Net Cash Provided by (Used in) Financing Activities Proceeds from Noncontrolling Interests Proceeds from Noncontrolling Interests Repayments of Long-Term Debt Repayments of Long-term Debt Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Gross Unrecognized Tax Benefits Unrecognized Tax Benefits Natural Gas-fired [Member] Natural Gas Processing Plant [Member] Counterparty Name [Axis] Counterparty Name [Axis] Common Stock Without Par Value, 80.0 Shares Authorized, 51.9 and 51.7 Shares Issued and Outstanding Common Stock, Value, Issued Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Restricted Cash included in Prepayments and Other Restricted Cash, Current Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Short-Term Debt - Total Debt, Current Plant Removal Obligations [Member] Removal Costs [Member] Retirement Benefits [Abstract] Retirement Benefits [Abstract] Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Schedule of Long-term Debt Instruments [Table] Financial Covenants [Table] Schedule of Long-term Debt Instruments [Table] Inventories – Net [Abstract] Inventory, Net [Abstract] Other [Member] Corporate & Other - Other [Member] Revenue Recognized by Corporate & Other From Other Sources Immaterial for Disaggregation Non-Controlling Interest in Subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Investing Activities [Abstract] Net Cash Provided by (Used in) Investing Activities [Abstract] Common Stock, Shares Issued Common Stock, Shares, Issued Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Transmission [Line Items] Transmission [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Transmission Formula Rates [Member] Transmission Formula Rates [Member] Transmission Formula Rates Long-Term Debt Long-Term Debt - Total Long-term Debt, Excluding Current Maturities Segment Reporting [Abstract] Segment Reporting [Abstract] Total Regulatory Liabilities Regulatory Liabilities Fuel Energy Related Inventory, Coal Debt Instrument [Line Items] Debt Instrument [Line Items] Revenue from Cost Recovery Riders Regulated Operating Revenue, Other Transmission Line Capacity (kV) Transmission Line Capacity (kV) Capacity of a transmission line in kilovolts. Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Total ALLETE Equity Stockholders' Equity Attributable to Parent 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] Land in Minnesota (Acres) Area of Land Total Liabilities and Equity Liabilities and Equity Loss Contingencies [Line Items] Loss Contingencies [Line Items] Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Requested Rate Increase Percent Public Utilities, Requested Rate Increase (Decrease), Percentage Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] South Peak [Member] South Peak [Member] South Peak [Member] Cost of Power Purchased Utilities Operating Expense, Purchased Power under Long-term Contracts Total Current Income Tax Expense Current Income Tax Expense (Benefit) Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Requested Return on Equity Public Utilities, Requested Return on Equity, Percentage 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] Transmission [Domain] Transmission [Domain] Name of transmission investment. Requested Equity Capital Structure Public Utilities, Requested Equity Capital Structure, Percentage Equity Component [Domain] Equity Component [Domain] BNI Energy Reclamation Liability [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. Liabilities [Abstract] Liabilities [Abstract] Entity Address, Address Line One Entity Address, Address Line One Proceeds from Issuance of First Mortgage Bond Proceeds from Issuance of First Mortgage Bond Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Proceeds from Sale of U.S. Water Services - Net of Transaction Costs and Cash Retained Approximate Proceeds from Divestiture of Business Approximate Proceeds from Divestiture of Business Property, Plant and Equipment – Net Property, Plant and Equipment, Net Net Income Attributable to ALLETE Net Income (Loss) Attributable to ALLETE Net Income (Loss) Attributable to Parent Entity Address, State or Province Entity Address, State or Province Caddo [Member] Caddo [Member] Caddo [Member] Changes in Operating Assets and Liabilities [Abstract] Increase (Decrease) in Operating Capital [Abstract] Other Total Other Current Liabilities Other Liabilities, Current Equity Method Investment [Domain] Investment, Name [Domain] Total Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax Regulatory Asset [Axis] Regulatory Asset [Axis] Total Current Assets Assets, Current Corporate and Other [Member] Corporate and Other [Member] Operating Expenses [Abstract] Operating Expenses [Abstract] Net Loss Attributable to Non-Controlling Interest Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] Earnings Per Share [Abstract] Earnings Per Share [Abstract] Total Project Costs To Date Total Project Costs To Date Total Project Costs To Date Gain on Sale of U.S. Water Services Gain on Sale of U.S. Water Services Gain (Loss) on Disposition of Business Payments / Provision for Interim Rate Refund Provision (Payment) for Interim Rate Refund Provision (Payment) for Interim Rate Refund Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Entity Interactive Data Current Entity Interactive Data Current Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Materials and Supplies Inventory, Supplies, Net of Reserves 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 State of Incorporation Entity Incorporation, State or Country Code Net Income Net Income (Loss) Available to Common Stockholders, Diluted Available-for-sale – Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Employer Contributions to Defined Benefit Plans Defined Benefit Plan, Plan Assets, Contributions by Employer Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Supplemental Cash Flow Information [Abstract] Supplemental Cash Flow Information [Abstract] Customer Class [Domain] Customer Class [Domain] [Domain] for Customer Class [Axis] description of customer types or classes Letters of Credit [Member] Financial Standby Letter of Credit [Member] Common Stock [Member] Common Stock [Member] Return of Capital From Equity Investments Proceeds from Equity Method Investment, Distribution, Return of Capital Jointly Owned Electricity Generation Plant [Member] Jointly Owned Electricity Generation Plant [Member] 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. Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Residential [Member] Residential [Member] Revenue Recognized From Residential Customers Investment in ATC [Line Items] Investment in Nobles 2 [Line Items] Schedule of Equity Method Investments [Line Items] Interest Rate Debt Instrument, Interest Rate, Stated Percentage Bad Debt Expense Accounts Receivable, Credit Loss Expense (Reversal) Recognition of Right-of-use Assets and Lease Liabilities Recognition of Right-of-use Assets and Lease Liabilities Non Cash Impact From the Recognition of Right-of-use Assets and Lease Liabilities Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] 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 Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Total Other Income (Expense) Nonoperating Income (Expense) Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] U.S. Water Services Sale of Stock Basis Difference Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Generating Capacity to be Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Basic (Shares) Average Common Shares Weighted Average Number of Shares Outstanding, Basic Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate 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). Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Tenaska PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] State Deferred State and Local Income Tax Expense (Benefit) Loss Contingencies [Table] Loss Contingencies [Table] Municipal [Member] Municipal [Member] Municipal Sales Channel Ownership Percentage Equity Method Investment, Ownership Percentage Cash Paid for Interest – Net of Amounts Capitalized Interest Paid, Excluding Capitalized Interest, Operating Activities Other Current Liabilities Increase (Decrease) in Other Current Liabilities Total Equity Balance, Beginning Balance Balance, End of Period Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Point-in-Time [Member] Point-in-Time [Member] Revenue Recognized From Point-in-Time Sales Short-Term and Long-Term Debt - Fair Value Long-term Debt, Fair Value Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] Total Operating Revenue Operating Revenue Revenues Inventories – Net [Policy Text Block] Inventory, Policy [Policy Text Block] Income Tax Expense (Benefit) Total Income Tax Expense (Benefit) Income Tax Expense (Benefit) 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. Proceeds from Sale of U.S. Water Services - Net of Transaction Costs and Cash Retained Proceeds from Divestiture of Businesses Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Total Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Total Fair Value of Liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure Estimated Generation from Projects Under Development Estimated Generation from Projects Under Development Estimated Generation from Projects Under Development Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Diluted Earnings Per Share of Common Stock Earnings Per Share Earnings Per Share, Diluted 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 Schedule of Short-Term and Long-Term Debt [Table Text Block] Schedule of Debt [Table Text Block] Accounts Receivable [Abstract] Accounts Receivable, after Allowance for Credit Loss, Current [Abstract] Estimated Costs of Compliance (Accrued) Accrual for Environmental Loss Contingencies Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Long-Term Debt - Principal Long-term Debt, Gross Schedule of Long-term Debt Instruments [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Depreciation Expense Depreciation FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Sale of U.S. Water Services [Abstract] Sale of U.S. Water Services [Abstract] Sale of U.S. Water Services [Abstract] Deferred Income Tax Benefit Total Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Deferred Income Taxes Deferred Income Taxes and Other Tax Liabilities, Noncurrent Maximum [Member] Maximum [Member] Name of Exchange Security Exchange Name FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Noncontrolling Interest [Member] Noncontrolling Interest [Member] Document and Entity Information [Table] Document Information [Table] Approved Return on Common Equity Authorized Return on Equity Public Utilities, Approved Return on Equity, Percentage Retained Earnings [Member] Retained Earnings [Member] Acquisition-Related Contingent Consideration Payments Payment for Contingent Consideration Liability, Financing Activities Components of Net Periodic Benefit Expense (Income) [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Recurring Fair Value Measures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Cash from (for) Investing Activities Net Cash Provided by (Used in) Investing Activities Generating Facility [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Document Transition Report Document Transition Report Other Postretirement [Member] Other Postretirement Benefits Plan [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Pension [Member] Pension Plan [Member] Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] 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. Long-term Contract for Sale of Energy [Domain] Long-term Contract for Sale of Energy [Domain] [Domain] for Long-term Contract for Sale of Energy [Axis] Current Liabilities [Abstract] Liabilities, Current [Abstract] Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Transmission Line Length (Miles) Transmission Line Length (Miles) Length of a transmission line in miles. Business Segments [Line Items] Segment Reporting Information [Line Items] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Recurring Fair Value Measures [Member] Fair Value, Recurring [Member] Minnesota Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Conservation Improvement Program Goal, In 2 Years Conservation Improvement Program Goal, In 2 Years Conservation Improvement Program Goal, In 2 Years 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. Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount New Accounting Standards [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Regulatory Agency [Domain] Regulatory Agency [Domain]