Commitments and Contingencies Disclosure [Abstract] Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Domain] Guarantee Entity and Type [Domain] [Domain] for Guarantee Entity and Type [Axis] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] BNI Energy Reclamation Liability [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. 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. Town Center District Capital Improvement Bonds [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Letters of Credit [Member] Financial Standby Letter of Credit [Member] Surety Bonds [Member] Surety Bond [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] 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. Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted 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. 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. Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Long-term Contract for Sale of Energy [Axis] Long-term Contract for Sale of Energy [Axis] Long-term Contract for Sale of Energy [Axis] 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] 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] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Square Butte [Member] Square Butte [Member] Square Butte [Member] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Power Purchase Agreement - Energy and Capacity [Axis] Long-term Contract for Purchase of Electric Power [Axis] Power Purchase Agreement - Energy and Capacity [Domain] Long-term Contract for Purchase of Electric Power [Domain] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] Generating Facility [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota 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] 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] Minnkota Power PSA [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] PPA Counterparty Total Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Cost of Power Purchased Utilities Operating Expense, Purchased Power under Long-term Contracts Expected Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges MW Sold Under Short-term PSAs MW Sold Under Short-term PSAs MW Sold Under Short-term PSAs Earnings Per Share [Abstract] Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Income Tax Disclosure [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Income Tax Expense (Benefit) [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Total Current Income Tax Expense Current Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Total Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Total Income Tax Expense (Benefit) Income Tax Expense (Benefit) Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Income Before Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Taxes Computed at Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (Decrease) in Income Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount 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. U.S. Water Services Sale of Stock Basis Difference Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Share-Based Compensation Effective Income Tax Rate Reconciliation, Tax Expense (Benefit), Share-based Payment Arrangement, Amount Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Gross Unrecognized Tax Benefits Unrecognized Tax Benefits Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Statement of Financial Position [Abstract] Accounts Receivable [Abstract] Accounts Receivable, after Allowance for Credit Loss, Current [Abstract] Accounts Receivable, Allowance Accounts Receivable, Allowance for Credit Loss, Current Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value Common Stock, No Par Value Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Outstanding Common Stock, Shares, Outstanding Common Stock, Shares Issued Common Stock, Shares, Issued Statement of Comprehensive Income [Abstract] Unrealized Gain (Loss) on Securities, Income Tax Expense Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax ALLETE Clean Energy Asset Acquisition [Abstract] ALLETE Clean Energy Asset Acquisition [Abstract] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Caddo [Member] Caddo [Member] Caddo [Member] Business Segments [Axis] Segments [Axis] Business Segments [Domain] Segments [Domain] Business Segments [Line Items] Segment Reporting Information [Line Items] Cash Paid for Assets Acquired Cash Paid for Assets Acquired Cash Paid for Assets Acquire Generating Capacity Under Development (MW) Generating Capacity Under Development (MW) Generating Capacity Under Development (MW) Debt Disclosure [Abstract] Financial Covenants [Table] Schedule of Long-term Debt Instruments [Table] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Maximum [Member] Maximum [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] 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. 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. Income Statement [Abstract] Operating Revenues [Abstract] Revenues [Abstract] Contracts with Customers - Utility Regulated Operating Revenue Contracts with Customers - Non-utility Unregulated Operating Revenue Other - Non-utility 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. Total Operating Revenue Revenues Operating Expenses [Abstract] Operating Expenses [Abstract] Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Transmission Services - Utility Transmission Services - Utility Transmission Services - Utility Cost of Sales - Non-utility Cost of Goods and Services Sold Operating and Maintenance Operating and Maintenance Operating and Maintenance Expenses Depreciation and Amortization Depreciation, Depletion and Amortization Taxes Other than Income Taxes Taxes, Other Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Other Income (Expense) [Abstract] Other Nonoperating Income (Expense) [Abstract] Interest Expense Interest Expense Equity Earnings Income (Loss) from Equity Method Investments Gain on Sale of U.S. Water Services Gain (Loss) on Disposition of Business Other Other Nonoperating Income (Expense) Total Other Income (Expense) Nonoperating Income (Expense) Income Before Income Taxes Income Tax Expense (Benefit) Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net Loss Attributable to Non-Controlling Interest Net Income (Loss) Attributable to Noncontrolling Interest Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent Earnings Per Share [Abstract] Basic (Shares) Weighted Average Number of Shares Outstanding, Basic Diluted (Shares) Weighted Average Number of Shares Outstanding, Diluted Basic Earnings Per Share of Common Stock Earnings Per Share, Basic Diluted Earnings Per Share of Common Stock Earnings Per Share, Diluted Other Income (Expense) - Other [Abstract] Other Income (Expense) - Other [Abstract] Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Component of Other Income (Expense) - Other [Axis] Component of Other Income (Expense) - Other [Axis] Component of Other Income (Expense) - Other [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 Non-Service Credit [Member] Pension and Other Postretirement Benefit Plans Non-Service Credit [Member] Pension and Other Postretirement Benefit Plans Non-Service Credit [Member] Interest and Investment Income (Loss) [Member] Interest and Investment Earnings [Member] Interest and Investment Earnings [Member] AFUDC - Equity [Member] AFUDC - Equity [Member] AFUDC - Equity [Member] Gain on Land Sales [Member] Gain (Loss) on Land Sales [Member] Gain (Loss) on Land Sales [Member] Other [Member] Other Income (Expense) - Other [Member] Other Income (Expense) - Other [Member] Other Income (Expense) - Other [Line Items] Other Income (Expense) - Other [Line Items] [Line Items] for Other Income (Expense) - Other [Table] Total Other Income Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Minimum [Member] Minimum [Member] SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Superior, WI [Member] Superior, WI [Member] Superior, WI Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Expected Period for Costs of Compliance Expected Period for Costs of Compliance Expected Period for Costs of Compliance Estimated Costs of Compliance (Accrued) Accrual for Environmental Loss Contingencies Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] 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] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Table] Document Information [Table] Document and Entity Information, Document [Axis] Document Information, Document [Axis] Document [Domain] Document [Domain] Document and Entity Information [Line Items] Document Information [Line Items] Document Type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name State of Incorporation Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Name of Exchange Security Exchange Name Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Organization, Consolidation and Presentation of Financial Statements [Abstract] 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 Cash, Cash Equivalents and Restricted Cash [Abstract] Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract] Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Restricted Cash included in Prepayments and Other Restricted Cash, Current Restricted Cash, Non-Current Restricted Cash, Noncurrent Total Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Inventories – Net [Abstract] Inventory, Net [Abstract] Fuel Energy Related Inventory, Coal Materials and Supplies Inventory, Supplies, Net of Reserves Total Inventories – Net Inventory, Net Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Contract Assets Contract with Customer, Asset, Net, Noncurrent Operating Lease, Right-of-Use Asset Operating Lease, Right-of-Use Asset ALLETE Properties Real Estate Investments, Net Other Postretirement Benefit Plans Other Postretirement Benefit Plans Other Postretirement Benefit Plans Other Other Assets, Miscellaneous, Noncurrent Total Other Non-Current Assets Other Assets, Noncurrent Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] 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. Operating Lease Liabilities Operating Lease, Liability, Current Other Other Accrued Liabilities, Current Total Other Current Liabilities Other Liabilities, Current Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent 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. Operating Lease Liabilities Operating Lease, Liability, Noncurrent Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Other Liabilities, Noncurrent Schedule of Short-Term and Long-Term Debt [Table Text Block] Schedule of Debt [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Net Income Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Gain (Loss) on Securities Net of Income Tax Expense of $(0.1) and $– Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1 and $0.1 Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Total Other Comprehensive Income (Loss) Other Comprehensive Income (Loss), Net of Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Net Loss Attributable to Non-Controlling Interest Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Total Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Statement of Cash Flows [Abstract] Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] AFUDC – Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Realized and Unrealized (Gain) Loss on Investments and Property, Plant and Equipment Gain (Loss) on Disposition of Other Assets Depreciation Expense Depreciation Amortization of PSAs Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Expense (Benefit) Share-Based and ESOP Compensation Expense Share-based Payment Arrangement, Noncash Expense Defined Benefit Pension and Postretirement Benefit Expense Pension and Other Postretirement Benefits Cost (Reversal of Cost) Payments / Provision for Interim Rate Refund Provision for Interim Rate Refund Provision for Interim Rate Refund Payments / Provision for Tax Reform Refund Provision for Tax Reform Refund Provision for Tax Reform Refund Bad Debt Expense Accounts Receivable, Credit Loss Expense (Reversal) Gain on Sale of U.S. Water Services Changes in Operating Assets and Liabilities [Abstract] Increase (Decrease) in Operating Capital [Abstract] Accounts Receivable Increase (Decrease) in Accounts and Other Receivables Inventories Increase (Decrease) in Inventories Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Accounts Payable Increase (Decrease) in Accounts Payable Other Current Liabilities Increase (Decrease) in Other Current Liabilities Cash Contributions to Defined Benefit Pension Plans Payment for Pension Benefits Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Investing Activities [Abstract] Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Payments for Equity Investments Payments to Acquire Equity Method Investments Return of Capital From Equity Investments Proceeds from Equity Method Investment, Distribution, Return of Capital Proceeds from Sale of U.S. Water Services - Net of Transaction Costs and Cash Retained Proceeds from Divestiture of Businesses Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Payments for (Proceeds from) Other Investing Activities Payments for (Proceeds from) Other Investing Activities Cash from (for) Investing Activities Net Cash Provided by (Used in) Investing Activities Financing Activities [Abstract] Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from Issuance of Common Stock Proceeds from Issuance of Long-term Debt Proceeds from Issuance of Long-term Debt Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition-Related Contingent Consideration Payments Payment for Contingent Consideration Liability, Financing Activities Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Other Financing Activities Proceeds from (Payments for) Other Financing Activities Cash from Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Retirement Benefits [Abstract] Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Net Income (Loss) Attributable to ALLETE Average Common Shares Earnings Per Share Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Net Income (Loss) Available to Common Stockholders, Diluted Average Common Shares Earnings Per Share Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Non-Controlling Interest Details [Table] Non-Controlling Interest Details [Table] Non-Controlling Interest Details [Table] Glen Ullin Energy Center [Member] Glen Ullin Energy Center [Member] Glen Ullin Energy Center [Member] South Peak [Member] South Peak [Member] South Peak [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Non-Controlling Interest Details [Line Items] Non-Controlling Interest Details [Line Items] [Line Items] for Non-Controlling Interest Details [Table] Generating Capacity Subject to Tax Equity Financing Generating Capacity Subject to Tax Equity Financing Generating Capacity Subject to Tax Equity Financing Proceeds from Noncontrolling Interests Proceeds from Noncontrolling Interests Unrecorded Unconditional Purchase Obligation [Table] Coal, Rail and Shipping Contracts [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Coal, Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Coal, Rail and Shipping Contracts [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Fair Value Disclosures [Abstract] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Statement of Stockholders' Equity [Abstract] Consolidated Statement of Shareholders' Equity [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock [Member] Common Stock [Member] AOCI Attributable to Parent [Member] AOCI Attributable to Parent [Member] Retained Earnings [Member] Retained Earnings [Member] Noncontrolling Interest [Member] Noncontrolling Interest [Member] Consolidated Statement of Shareholders' Equity [Line Items] Statement [Line Items] Consolidated Statement of Shareholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Balance, Beginning Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Common Stock Issued Stock Issued During Period, Value, New Issues Unrealized Gain (Loss) on Debt Securities Defined Benefit Pension and Other Postretirement Plans Common Stock Dividends Balance, End of Period Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Equity Method Investments and Joint Ventures [Abstract] Investment in Nobles 2 [Table] Schedule of Equity Method Investments [Table] Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Resource Package [Member] Resource Package [Member] Resource Package [Member] Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] MPUC [Member] MPUC [Member] Minnesota Public Utilities Commission [Member] Power Purchase Agreements [Axis] Power Purchase Agreements [Domain] Tenaska PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] Equity Method Investment [Axis] Investment, Name [Axis] Equity Method Investment [Domain] Investment, Name [Domain] Nobles 2 [Member] Nobles 2 [Member] Nobles 2 [Member] Investment in Nobles 2 [Line Items] Schedule of Equity Method Investments [Line Items] Ownership Percentage Equity Method Investment, Ownership Percentage Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) 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. Equity Investment Balance Equity Method Investments Cash Investments 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. Regulated Operations [Abstract] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount Long-term Debt Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value Segment Reporting [Abstract] Regulated Operations [Member] Regulated Operations [Member] Regulated Operations [Member] U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] Corporate and Other [Member] Corporate and Other [Member] Revenue Streams [Axis] Contract with Customer, Sales Channel [Axis] Revenue Streams [Domain] Contract with Customer, Sales Channel [Domain] Residential [Member] Residential [Member] Revenue Recognized From Residential Customers Commercial [Member] Commercial [Member] Revenue Recognized From Commercial Customers Municipal [Member] Municipal [Member] Municipal Sales Channel Industrial [Member] Industrial [Member] Industrial Sales Channel [Member] Other Power Suppliers [Member] Other Power Suppliers [Member] Other Power Suppliers Sales Channel [Member] Other [Member] Other [Member] Regulated Operations Other Revenue Recognized From Other Sources Immaterial For Disaggregation Long-term PSA [Member] Long-term PSA [Member] Revenue Recognized From Long-term PSAs Other [Member] ALLETE Clean Energy Other [Member] ALLETE Clean Energy Other Revenue Recognized From Other Sources Immaterial For Disaggregation [Member] Point-in-Time [Member] Point-in-Time [Member] Revenue Recognized From Point-in-Time Sales Contract [Member] Contract [Member] Revenue Recognized by U.S. Water From Contract Sales Capital Project [Member] Capital Project [Member] Revenue Recognized From Capital Projects Long-term Contract [Member] Long-term Contract [Member] Long-term Contract Revenue Stream [Member] Other [Member] Corporate & Other - Other [Member] Revenue Recognized by Corporate & Other From Other Sources Immaterial for Disaggregation Number of Reportable Segments Number of Reportable Segments Number of Operating Segments Number of Operating Segments Land in Minnesota (Acres) Area of Land Operating Revenue Revenue from Contract with Customer, Excluding Assessed Tax Operating Revenue, Other Than Customer Revenue Revenue, Other Than Customer Revenue Non-cash Revenue Recognized From Amortization of PSAs Assumed Through Acquisitions Operating Revenue Gain on Sale of U.S. Water Services, After-tax Gain on Sale of Affiliate, After-tax Gain on Sale of Affiliate, After-tax Assets Assets Supplemental Cash Flow Information [Abstract] Cash Paid for Interest – Net of Amounts Capitalized Interest Paid, Excluding Capitalized Interest, Operating Activities Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] 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 Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations AFUDC–Equity Recurring Fair Value Measures [Table] Fair Value, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Domain] Recurring Fair Value Measures [Member] Fair Value, Recurring [Member] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Recurring Fair Value Measures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Investments [Abstract] Investments [Abstract] Available-for-sale – Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Available-for-sale – Corporate and Governmental Debt Securities (b) Debt Securities, Available-for-sale, Noncurrent Cash Equivalents Other Restricted Assets, Noncurrent Total Fair Value of Assets Assets, Fair Value Disclosure Available-for-sale Securities, Debt Maturities, within One Year, Fair Value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value 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). 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). 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). Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] 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. Total Fair Value of Liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Sale of U.S. Water Services [Abstract] Sale of U.S. Water Services [Abstract] 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 Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Accounts Receivable (Less Allowance of $1.2 and $0.9) Accounts Receivable, after Allowance for Credit Loss, Current Inventories – Net Prepayments and Other Other Assets, Current Total Current Assets Assets, Current Property, Plant and Equipment – Net Property, Plant and Equipment, Net Regulatory Assets Regulatory Assets, Noncurrent Equity Investments Other Non-Current Assets Total Assets Liabilities and Shareholders' Equity [Abstract] Liabilities and Equity [Abstract] Liabilities [Abstract] Current Liabilities [Abstract] Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest Payable, Current Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Other Total Current Liabilities Liabilities, Current Long-Term Debt Long-term Debt, Excluding Current Maturities Deferred Income Taxes Deferred Income Taxes and Other Tax Liabilities, Noncurrent Regulatory Liabilities Regulatory Liability, Noncurrent Defined Benefit Pension and Other Postretirement Benefit Plans Liability, Defined Benefit Plan, Noncurrent Other Non-Current Liabilities Total Liabilities Liabilities Commitments, Guarantees and Contingencies (Note 6) Commitments and Contingencies Shareholders’ Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 51.8 and 51.7 Shares Issued and Outstanding Common Stock, Value, Issued Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained Earnings Retained Earnings (Accumulated Deficit) Total ALLETE Equity Stockholders' Equity Attributable to Parent Non-Controlling Interest in Subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Total Equity Total Liabilities and Equity Liabilities and Equity Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Asset [Axis] Total Regulatory Asset [Domain] Regulatory Asset [Domain] Pension and Other Postretirement Plan Costs [Member] Pension and Other Postretirement Plans Costs [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] 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). Other Other Regulatory Assets (Liabilities) [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Transmission Formula Rates [Member] Transmission Formula Rates [Member] Transmission Formula Rates Provision for Tax Reform Refund [Domain] Provision for Tax Reform Refund [Domain] Provision for Tax Reform Refund [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Plant Removal Obligations [Member] Removal Costs [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] Fuel Adjustment Clause [Member] Fuel Adjustment Clause [Member] Amount expected to be refunded to customers for the over-collection of fuel adjustment clause recoveries. Conservation Improvement Program [Member] CIP Financial Incentive [Member] Conservation Improvement Program [Member] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Regulatory Assets Currently Earning a Return Types of Net Regulatory Assets Earning Returns Non-Current Regulatory Assets Current Regulatory Liabilities Regulatory Liability, Current Non-Current Regulatory Liabilities Total Regulatory Liabilities Regulatory Liabilities Components of Net Periodic Benefit Expense (Income) [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] 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] Inventories – Net [Table Text Block] Schedule of Inventory, Current [Table Text Block] Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Other Income [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Supplemental Statement of Cash Flows Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Regulatory Matters [Table] Public Utilities General Disclosures [Table] Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Rate Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Minnesota Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] 2020 Minnesota General Rate Case [Member] 2020 Minnesota General Rate Review [Member] 2020 Minnesota General Rate Review [Member] Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Customer Class [Domain] Customer Class [Domain] [Domain] for Customer Class [Axis] description of customer types or classes Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Revenue from Cost Recovery Riders Regulated Operating Revenue, Other Approved Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Requested Rate Increase Percent Public Utilities, Requested Rate Increase (Decrease), Percentage Interim Rate Increase Public Utilities, Interim Rate Increase (Decrease), Amount Requested Return on Equity Public Utilities, Requested Return on Equity, Percentage Requested Equity Capital Structure Public Utilities, Requested Equity Capital Structure, Percentage Interim Rates Collected to Date Interim Rates Collected to Date Interim Rates Collected to Date Proposed Ongoing Customer Rate Increase Proposed Ongoing Customer Rate Increase Proposed Ongoing Customer Rate Increase Public Utilities, Interim Rate Increase (Decrease), Percentage Public Utilities, Interim Rate Increase (Decrease), Percentage Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] ALLETE Bonds 2.50% Due August 2030 [Member] ALLETE Bonds 2.50% Due August 2030 [Member] ALLETE Bonds 2.50% Due August 2030 [Member] Unsecured Term Loan Due in 2021 [Member] Unsecured Term Loan Due in 2021 [Member] Unsecured Term Loan Due in 2021 [Member] Unsecured Term Loan Due April 2021 [Member] Unsecured Term Loan Due April 2021 [Member] Unsecured Term Loan Due April 2021 [Member] ALLETE Bonds 3.30% Due August 2050 [Member] ALLETE Bonds 3.30% Due August 2050 [Member] ALLETE Bonds 3.30% Due August 2050 [Member] 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. 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. Short-Term Debt - Total Debt, Current Long-Term Debt - Principal Long-term Debt, Gross 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. Long-Term Debt - Total 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. Total Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense Total Debt - Total Debt, Long-term and Short-term, Combined Amount Letters of Credit Outstanding, Amount Letters of Credit Outstanding, Amount Line of Credit, Current Line of Credit, Current Unsecured Term Loan, Amount Unsecured Term Loan, Amount Unsecured Term Loan, Amount Proceeds from Issuance of First Mortgage Bond Proceeds from Issuance of First Mortgage Bond Interest Rate Debt Instrument, Interest Rate, Stated Percentage Proceeds from Issuance of Unsecured Debt Proceeds from Issuance of Unsecured Debt Debt Instrument, Unused Borrowing Capacity, Amount Debt Instrument, Unused Borrowing Capacity, Amount Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate Debt Instrument, Interest Rate Floor Debt Instrument, Interest Rate Floor Debt Instrument, Interest Rate Floor Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] Inventories – Net [Policy Text Block] Inventory, Policy [Policy Text Block] New Accounting Standards [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Equity Method Investments [Policy Text Block] Equity Method Investments [Policy Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Environmental Accruals [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Income Tax [Policy Text Block] Income Tax, Policy [Policy Text Block] Uncertain Tax Positions [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Loss Contingencies [Table] Loss Contingencies [Table] FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder Pension and Other Postretirement Benefit Plans [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Pension [Member] Pension Plan [Member] Other Postretirement [Member] Other Postretirement Benefits Plan [Member] Pension and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Components of Net Periodic Benefit Cost (Credit) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Service Cost Defined Benefit Plan, Service Cost Interest Cost Defined Benefit Plan, Interest Cost Expected Return on Plan Assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Amortization of Prior Service Credits Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of Net Loss Defined Benefit Plan, Amortization of Gain (Loss) Net Periodic Benefit Cost (Credit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Employer Contributions to Defined Benefit Plans Defined Benefit Plan, Plan Assets, Contributions by Employer Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Fair Value [Text Block] Fair Value Disclosures [Text Block] ALLETE, Inc. [Member] ALLETE, Inc. [Member] -- None. No documentation exists for this element. -- Natural Gas-fired [Member] Natural Gas Processing Plant [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] 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] Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Jointly Owned Electricity Generation Plant [Member] Jointly Owned Electricity Generation Plant [Member] Total Project Costs Incurred to Date Capital Cost Spent to Date Capital cost spent to date for project. Generating Capacity to be Counterparty Owned (MW) 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 Generating Capacity to be Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Expected Output Entitlement (Percent) 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. Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Investment in ATC [Table] ATC [Member] ATC [Member] ATC [Member] Investment in ATC [Line Items] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Balance as of December 31, 2019 Equity in ATC Earnings Distributed ATC Earnings Proceeds from Equity Method Investment, Distribution 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 Equity Investment Balance as of March 31, 2020 Authorized Return on Equity 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. Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Manitoba Hydro PPA [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Name of transmission investment. Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] 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. 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. Transmission Line Length (Miles) Transmission Line Length (Miles) Length of a transmission line in miles. Transmission Line Capacity (kV) Transmission Line Capacity (kV) Capacity of a transmission line in kilovolts. Total Project Cost in the U.S. Project Costs Recovered from Counterparty Capital Costs Spent To Date, Recovered Capital Costs Spent To Date, Recovered