Granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Draws Outstanding Line of Credit, Current Plan Asset Target Allocations Defined Benefit Plan, Plan Assets, Target Allocation, Percentage Water [Member] Water [Member] Water Customer [Member] PPA Counterparty Annual Debt Service through 2024 Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Change in Benefit Obligation [Roll Forward] Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Expected Employer Contributions in 2020 Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year First Mortgage Bonds - 5.28% Series Due 2020 [Member] 5.28% Series Due 2020 [Member] First Mortgage Bonds, 5.28% Series Due 2020 [Member] Commitments, Guarantees and Contingencies [Text Block] Commitments and Contingencies Disclosure [Text Block] Related Party [Axis] Related Party [Axis] Forced Outage Costs Forced Outage Costs Forced Outage Costs Change in Plan Assets [Roll Forward] Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Invest Direct, Shares Stock Issued During Period, Shares, Dividend Reinvestment Plan 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 Long-Term Debt - Unamortized Debt Issuance Costs Debt Issuance Costs, Noncurrent, Net 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 Options and Stock Awards, Shares Stock Issued During Period, Shares, Share-based Compensation and ESOP, Gross Gross number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan, including employee stock ownership plans (ESOP). Shares issued could result from the issuance of restricted stock units, the exercise of stock options, stock issued under the ESOP, and/or other employee benefit plans. First Mortgage Bonds - 4.95% Series Due 2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] Square Butte [Member] Square Butte [Member] Square Butte [Member] Gross Unrecognized Income Tax Benefits [Abstract] Income Tax Uncertainties [Abstract] ALLETE Clean Energy 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] Liabilities and Equity [Abstract] Stock-based Compensation [Line Items] Share-Based Compensation Expense [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Annual Financing Payment Debt Instrument, Periodic Payment Security Exchange Name Security Exchange Name Approved Rate Increase - Amount Public Utilities, Approved Rate Increase (Decrease), Amount Fair Value, Beginning of Year Fair Value, End of Year Fair Value of Assets Defined Benefit Plan, Plan Assets, Amount Debt Instrument, Unused Borrowing Capacity, Amount Debt Instrument, Unused Borrowing Capacity, Amount U.S. Large-cap [Member] U.S. Large-Cap [Member] Equity Securities, U.S. Large-Cap [Member] Proceeds from Issuance of Long-Term Debt Proceeds from Issuance of Long-term Debt Total Current Liabilities Liabilities, Current Liabilities, Current Regulated Operations [Member] Regulated Operations [Member] [Member] Operation of entities that are regulated by governmental organizations for example, but not limited to, local, state, county, country, or other regulatory organizations. Accounts Receivable [Policy Text Block] Accounts Receivable [Policy Text Block] Plan Curtailments Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] Performance Shares [Member] Performance Shares [Member] As of January 1 As of December 31 Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Variable Rate [Domain] Variable Rate [Domain] 2024 Defined Benefit Plan, Expected Future Benefit Payment, Year Five Unsecured Senior Notes, Amount Unsecured Senior Notes, Amount Unsecured Senior Notes, Amount Assets [Abstract] Assets [Abstract] Assets [Abstract] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Investment, Name [Axis] Equity Method Investment [Axis] Investment, Name [Axis] Power Sales Agreement - Energy [Axis] Long-term Contract for Sale of Energy [Axis] Long-term Contract for Sale of Energy [Axis] Operating Lease, Payments Operating Lease, Payments Gain (Loss) on Land Sales [Member] Gain (Loss) on Land Sales [Member] Gain (Loss) on Land Sales [Member] Long-Term Debt Maturing in 2022 Long-Term Debt, Maturity, Year Three Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Estimated Minimum Annual Payments for Certain Long-Term Commitments [Table Text Block] Unrecorded Unconditional Purchase Obligations Disclosure [Table Text Block] Unsecured Term Loan Due April 2021 [Member] Unsecured Term Loan Due April 2021 [Member] Unsecured Term Loan Due April 2021 [Member] Provision (Payments) for Tax Reform Refund Provision (Payments) for Tax Reform Refund Provision (Payments) for Tax Reform Refund 2021 Lessee, Operating Lease, Liability, to be Paid, Year One Other Other Accrued Liabilities, Noncurrent Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) 2024 Lessee, Operating Lease, Liability, to be Paid, Year Four Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Unrecognized Compensation Cost Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Debt Instrument, Interest Rate Floor Debt Instrument, Interest Rate Floor Debt Instrument, Interest Rate Floor Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contracts (Cost-Based Formula Methodology for Entire Term) [Member] State Current State and Local Tax Expense (Benefit) Tax Credit Carryforwards Tax Credit Carryforward, Amount Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Regulatory Liabilities Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year Three First Mortgage Bonds - 2.50% Series Due 2030 ALLETE Bonds 2.50% Due August 2030 [Member] ALLETE Bonds 2.50% Due August 2030 Other Non-Current Liabilities Other Non-Current Liabilities Other Liabilities, Noncurrent Invest Direct, Equity Stock Issued During Period, Value, Dividend Reinvestment Plan Retail Customers [Member] Retail [Member] Document and Entity Information [Table] Document Information [Table] Deferred Tax Assets [Member] SEC Schedule, 12-09, Valuation Allowance, Deferred Tax Asset [Member] Unbilled Unbilled Receivables, Current Inventories – Net Total Inventories Inventory, Net Measurement Frequency [Domain] Measurement Frequency [Domain] Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Generation [Member] Electricity Generation Plant, Non-Nuclear [Member] Concentration Risk, Percentage Percent of Operating Revenue Concentration Risk, Percentage Industrial [Member] Industrial [Member] Industrial Sales Channel [Member] Fair Value of Plan Assets Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets Valuation and Qualifying Accounts and Reserves [Line Items] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] Consolidated Statement of Equity [Line Items] Statement [Line Items] Increase (Decrease) in Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] First Mortgage Bonds - 4.39% Series Due 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] Number of entities Number of entities Number of entities Class of Stock [Domain] Class of Stock [Domain] Environmental Matters [Line Items] Site Contingency [Line Items] Easements [Member] Easements [Member] Easements [Member] Consolidated Statement of Equity [Table] Statement [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Short-Term Debt - Unamortized Debt Issuance Costs Debt Issuance Costs, Current, Net Basin Power Sales Agreement (expires May 2028) [Member] Basin Power Sales Agreement (expires May 2028) [Member] Basin Power Sales Agreement (expires May 2028) [Member] Interest Cost Defined Benefit Plan, Interest Cost Statistical Measurement [Axis] Statistical Measurement [Axis] 2021 Integrated Resource Plan [Member] 2021 Integrated Resource Plan [Member] 2021 Integrated Resource Plan Expected Additional Investment in 2021 Equity Method Investment, Expected Additional Investment Next Fiscal Year Additional investments expected to be made in the next fiscal year in an equity method investment. Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] Fair Value, Nonrecurring [Member] Available-for-Sale Securities [Policy Text Block] Marketable Securities, Policy [Policy Text Block] Minimum Annual Payments for Certain Long-Term Commitments [Table] Unrecorded Unconditional Purchase Obligation [Table] Entity Small Business Entity Small Business Principles of Consolidation [Policy Text Block] Consolidation, Policy [Policy Text Block] Unsecured Term Loan, Amount Unsecured Term Loan, Amount Unsecured Term Loan, Amount Impairment of Real Estate Impairment of Real Estate Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Fair Value Hierarchy and NAV [Domain] Total [Domain] Fair Value Hierarchy and NAV [Domain] Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension and Other Postretirement Plans Costs [Member] Pension and Other Postretirement Plans Costs [Member] Amendment Flag Amendment Flag Transmission [Axis] Transmission [Axis] Information about each transmission investment. More-Likely-Than-Not Percentage MoreLikelyThanNotDefined The percentage guideline to determine more-likely-than-not status 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 Shares Available for Issuance Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Output Being Purchased - Electric Energy (MW) Long-Term Contract for Purchase of Electric Energy (MW) Long-Term Contract for Purchase of Electric Energy (MW) Other Other Accrued Liabilities, Current Superior, WI [Member] Superior, WI [Member] Superior, WI FERC-Approved Wholesale Rates [Member] FERC-Approved Wholesale Rates [Member] Wholesale rates approved by the Federal Energy Regulatory Commission (FERC). Gain on Sale of Affiliate, After-tax Gain on Sale of Affiliate, After-tax Gain on Sale of Affiliate, After-tax Measurement Frequency [Axis] Measurement Frequency [Axis] Long-Term Debt Equity Method Investment, Summarized Financial Information, Long-Term Debt Equity Method Investment, Summarized Financial Information, Long-Term Debt Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Revenue, Performance Obligation, Description of Payment Terms Revenue, Performance Obligation, Description of Payment Terms Conservation Improvement Program [Member] Conservation Improvement Program [Member] Conservation Improvement Program [Member] Surety Bonds [Member] Surety Bond [Member] Expected Return on Plan Assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Unsecured Term Loan Due December 2021 [Member] Unsecured Term Loan Due December 2021 [Member] Unsecured Term Loan Due December 2021 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] Leasing Agreements [Member] Capital Addition Purchase Commitments [Member] Derivatives [Policy Text Block] Derivatives, Policy [Policy Text Block] Contract with Customer, Sales Channel [Domain] Contract with Customer, Sales Channel [Domain] PPA Counterparty Total Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Diamond Spring [Member] Diamond Spring [Member] Diamond Spring [Member] Contract with Customer, Duration [Axis] Contract with Customer, Duration [Axis] Output Being Sold Electric - Capacity (MW) Long-term Contract for Sale of Electric Capacity (MW) Long-term Contract for Sale of Electric Capacity (MW) Non-Controlling Interest Details [Line Items] Non-Controlling Interest Details [Line Items] Non-Controlling Interest Details Deferred Income Tax Assets [Abstract] Deferred Tax Assets, Net of Valuation Allowance [Abstract] Non-Controlling Interest Details [Table] Non-Controlling Interest Details [Table] Non-Controlling Interest Details Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Property, Plant and Equipment, Type [Axis] Long-Lived Tangible Asset [Axis] Employee Stock Purchase Plan (ESPP) [Member] Employee Stock [Member] Restricted Stock Units [Table Text Block] Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block] Balance at Beginning of Period Balance at End of Period SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Accounts Receivable, Allowance Less: Allowance for Doubtful Accounts Accounts Receivable, Allowance for Credit Loss, Current Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Abstract] Income Statement Location [Domain] Income Statement Location [Domain] 2022 Lessee, Operating Lease, Liability, to be Paid, Year Two State [Member] State and Local Jurisdiction [Member] Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Leases Maturities [Abstract] Leases Maturities [Abstract] Leases Maturities [Abstract] Lease, Cost [Table Text Block] Lease, Cost [Table Text Block] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] 2023 Lessee, Operating Lease, Liability, to be Paid, Year Three Regulated Operations [Member] Regulated Operations [Member] Regulated Operations [Member] Other Non-Current Assets Total Other Non-Current Assets Other Assets, Noncurrent Plan Name [Axis] Plan Name [Axis] Concentration Risk [Table] Concentration Risk [Table] 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 Regulatory Assets for Benefit Obligations Deferred Tax Liabilities, Regulatory Assets for Benefit Obligations Deferred Tax Liabilities, Regulatory Assets for Benefit Obligations Obligation and Funded Status [Table Text Block] Schedule of Net Funded Status [Table Text Block] Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] As of January 1 As of December 31 Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Regulatory Clause Revenues, under-recovered [Member] Regulatory Clause Revenues, under-recovered [Member] 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 Cost Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Sale of Wind Energy Facility [Member] Sale of Wind Energy Facility [Member] Sale of Wind Energy Facility [Member] Estimated Future Benefit Payments [Abstract] Defined Benefit Plan, Expected Future Benefit Payment [Abstract] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Summary of Common Stock [Table Text Block] Schedule of Common Stock Outstanding Roll Forward [Table Text Block] Basin Electric Power Cooperative [Member] Basin Electric Power Cooperative [Member] Basin Electric Power Cooperative Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Regulatory Assets and Liabilities Currently Earning a Return Types of Net Regulatory Assets Earning Returns Commercial [Member] Commercial [Member] Revenue Recognized From Commercial Customers Total Liabilities Liabilities Approved Equity Ratio Public Utilities, Approved Equity Capital Structure, Percentage Finance Receivables – Long-Term [Member] SEC Schedule, 12-09, Allowance, Loss on Finance Receivable [Member] PPAs [Member] Power Purchase Agreement [Member] Power Purchase Agreement [Member] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Accumulated Benefit Obligation Defined Benefit Plan, Accumulated Benefit Obligation Less: Imputed Interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manitoba Hydro PPA (expires May 2035) [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] Minimum Payments Thereafter Unrecorded Unconditional Purchase Obligation, to be Paid, after Year Five Regulated Operations [Abstract] Regulated Operations [Abstract] Inventories Increase (Decrease) in Inventories Minimum Payments in 2022 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Two Regulatory Agency [Axis] Regulatory Agency [Axis] Other Income (Expense) - Other [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 Authorized Return on Equity Equity Method Investment, Approved Return on Common Equity The approved percentage return on common equity dedicated to the utility plant in the investee accounted for under the equity method of accounting. Public Utilities Reserve for Interim Rates (Recorded During Period) Public Utilities Reserve for Interim Rates (Recorded During Period) Public Utilities Reserve for Interim Rates (Recorded During Period) Other Other Assets, Miscellaneous, Noncurrent Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Jointly-Owned Facilities and Assets [Abstract] Jointly-Owned Facilities and Assets [Abstract] Jointly-Owned Facilities and Assets [Abstract] Common Stock, Shares Outstanding Balance, Shares Common Stock, Shares, Outstanding Equity Investments Equity Investment Beginning Balance Equity Investment Ending Balance Equity Method Investments 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). Income Taxes [Abstract] Income Tax Disclosure [Abstract] First Mortgage Bonds - 5.82% Series Due 2040 [Member] 5.82% Series Due 2040 [Member] First Mortgage Bonds, 5.82% Series Due 2040 [Member] Bank Lines of Credit [Member] Revolving Credit Facility [Member] Income Before Non-Controlling Interest and Income Taxes Income Before Non-Controlling Interest and Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement Square Butte PPA [Member] Square Butte PPA (expires 2026) [Member] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Common Stock, Shares Authorized Common Stock, Shares Authorized Property, Plant and Equipment in Service Property, Plant and Equipment, Gross Irrevocable Grantor Trust [Member] Trust for Benefit of Employees [Member] Trust for Benefit of Employees [Member] Federal Deferred Federal Income Tax Expense (Benefit) 2021 Defined Benefit Plan, Expected Future Benefit Payment, Year Two Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Funded Status, End of Year Total Net Pension Amounts Recognized in Consolidated Balance Sheet Total Net Postretirement Health and Life Amounts Recognized in Consolidated Balance Sheet Defined Benefit Plan, Funded (Unfunded) Status of Plan Guarantor Obligations by Nature [Axis] Guarantor Obligations, Nature [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] CIP Spending Minimum - Percentage CIP Spend Requirement Minnesota Next Generation Act of 2007 introduced a spend goal of 1.5% of net gross operating revenues from service provided in the state on energy CIPs each year. Transmission [Member] Jointly Owned Electricity Transmission and Distribution System [Member] Income Statement Data [Abstract] Income Statement [Abstract] Minimum Payments in 2025 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Five Accumulated Other Comprehensive Loss [Member] AOCI Attributable to Parent [Member] Notice Required to Terminate (Years) Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract (in years). Contract Serving Requirement Contract Serving Requirement Contract Serving Requirement Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] First Mortgage Bonds - 6.00% Series Due 2040 [Member] 6.00% Series Due 2040 [Member] First Mortgage Bonds, 6.00% Series Due 2040 [Member] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Estimated Annual Reduction to Revenue Due to Idled Customers Estimated Annual Reduction to Revenue Due to Idled Customers Estimated Annual Reduction to Revenue Due to Idled Customers net of associated expense savings such as fuel costs. North Dakota Investment Tax Credits [Member] North Dakota Investment Tax Credits Subject to Refund [Member] North Dakota Investment Tax Credits Subject to Refund [Member] Material Changes to Unrecognized Tax Benefits Expected During Next 12 Months Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit Income Tax Authority [Domain] Income Tax Authority [Domain] Minimum Annual Payments for Certain Long-Term Commitments [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Weighted Average Assumptions Used to Determine Benefit Obligation and Net Periodic Benefit Costs [Table Text Block] Defined Benefit Plan, Assumptions [Table Text Block] First Mortgage Bonds - 2.80% Series Due 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Unearned Grant Award Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Unearned Grant Award Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Shares of original grant unearned (not paid out), a reduction to non-vested shares balance, similar to a forfeiture. Performance Shares [Table Text Block] Schedule of Nonvested Performance-based Units Activity [Table Text Block] Counterparty Name [Domain] Counterparty Name [Domain] Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Consolidation Items [Domain] Consolidation Items [Domain] Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Abstract] 2020 Minnesota General Rate Review [Member] 2020 Minnesota General Rate Review [Member] 2020 Minnesota General Rate Review [Member] Actual Return on Plan Assets Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) Common Stock Reserved (Shares) Common Stock, Capital Shares Reserved for Future Issuance Transmission [Table] Public Utility, Property, Plant and Equipment [Table] Equity Earnings ALLETE's Equity in Net Income Income (Loss) from Equity Method Investments Actual Plan Asset Allocations Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage Power Sales Agreement - Capacity [Axis] Long-term Contract for Sale of Capacity [Axis] Long-term Contract for Sale of Capacity [Axis] Camp Ripley Financing [Member] Camp Ripley Financing [Member] Camp Ripley Financing [Member] 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] Oliver Wind [Member] Oliver Wind [Member] Oliver Wind [Member] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Change in Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Years 2025 - 2029 Defined Benefit Plan, Expected Future Benefit Payment, after Year Five for Next Five Years Inventories – Net [Table Text Block] Schedule of Inventory, Current [Table Text Block] Power Purchase Agreement - Energy [Axis] Long-term Contract for Purchase of Energy [Axis] Long-term Contract for Purchase of Energy [Axis] NOL and Tax Credit Carryforwards [Table Text Block] Summary of Tax Credit Carryforwards [Table Text Block] Entity [Domain] Entity [Domain] Residual Value of Leased Asset Residual Value of Leased Asset Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] First Mortgage Bonds - 5.10% Series Due 2025 [Member] 5.10% Series Due 2025 [Member] First Mortgage Bonds, 5.10% Series Due 2025 [Member] Quarterly Financial Data (Unaudited) [Text Block] Quarterly Financial Information [Text Block] ATC [Member] ATC [Member] ATC [Member] Estimated Useful Lives Property, Plant and Equipment, Useful Life Length of Notice Required to Terminate Contract Length of Notice Required to Terminate Contract Length of Notice Required to Terminate Contract Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Lessee, Lease, Description [Line Items] Lessee, Lease, Description [Line Items] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Cash Contributions to Defined Benefit Pension Plans Payment for Pension Benefits Other [Member] Other Regulatory Assets (Liabilities) [Member] First Mortgage Bonds - 3.69% Series Due 2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] Share-based Payment Arrangement [Abstract] Share-based Payment Arrangement [Abstract] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Operating Lease, Liability, Current Less: Current Lease Obligations Operating Lease, Liability, Current Ultimate Trend Rate Defined Benefit Plan, Ultimate Health Care Cost Trend Rate Unsecured Term Loan Due in 2021 [Member] Unsecured Term Loan Due in 2021 [Member] Unsecured Term Loan Due in 2021 [Member] Jointly-Owned Facilities and Assets [Table] Jointly Owned Utility Plant Interests [Table] Contract [Member] Contract [Member] Revenue Recognized by U.S. Water From Contract Sales Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Regulatory Assets Deferred Tax Liabilities, Regulatory Assets Fair Value [Text Block] Fair Value Disclosures [Text Block] Regulatory Assets Non-Current Regulatory Assets Regulatory Assets, Noncurrent Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] U.S. Water Services Sale of Stock Basis Difference Effective Income Tax rate Reconciliation, Stock Basis Difference Effective Income Tax rate Reconciliation, Stock Basis Difference Customer [Domain] Customer [Domain] Cumulative Effect, Period of Adoption, Adjustment Cumulative Effect, Period of Adoption, Adjustment [Member] CIP Financial Incentive [Member] CIP Financial Incentive [Member] Conservation Improvement Program [Member] Entity Registrant Name Entity Registrant Name Subsequent Event Type [Domain] Subsequent Event Type [Domain] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Operating Lease, Weighted Average Discount Rate, Percent Operating Lease, Weighted Average Discount Rate, Percent Concentration Risk [Line Items] Concentration Risk [Line Items] Assets, Noncurrent Assets, Noncurrent Basin Electric Cooperative PSA June 2023 - May 2025 [Member] Basin Electric Cooperative PSA June 2023 - May 2025 [Member] Basin Electric Cooperative PSA June 2023 - May 2025 [Member] AFUDC – Equity AFUDC–Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Fair Value Transfers [Policy Text Block] Fair Value Transfer, Policy [Policy Text Block] Unamortized Debt Issuance Costs Debt Issuance Costs, Net Estimated Investment Amount Estimated Investment Amount Estimated Investment Amount Current Regulatory Liability Regulatory Liability, Current Income Tax Expense [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] 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 Excise Taxes [Policy Text Block] Excise Tax Policy [Policy Text Block] Policy regarding the collection and recording of excise taxes. Equity Securities [Member] Equity Securities [Member] Entity Incorporation, State Entity Incorporation, State or Country Code Investments [Abstract] Investments [Abstract] Minnesota Power Output Entitlement (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 Resource Package [Member] Resource Package [Member] Resource Package [Member] Restricted Cash, Noncurrent Restricted Cash, Noncurrent Long-Term Finance Receivables [Policy Text Block] Financing Receivable, Held-for-investment [Policy Text Block] Contributions to RSOP, Shares Shares Issued, Shares, Share-based Payment Arrangement, before Forfeiture Entity Address, Postal Zip Code Entity Address, Postal Zip Code Output Being Sold Electric - Energy and Capacity (MW) Long-term Contract for Sale of Electric Energy and Capacity (MW) Long-term Contract for Sale of Electric Energy and Capacity (MW) Purchase Option Purchase Option, Asset Purchase option to acquire an asset. Total Deferred Income Tax Assets Deferred Tax Assets, Net of Valuation Allowance Natural Gas [Member] Natural Gas [Member] Natural Gas Customer [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Manitoba Hydro PPA (expires April 2022) [Member] Manitoba Hydro PPA (expires April 2022) [Member] Long-term contract with Manitoba Hydro for the purchase of electric power. Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] NOL Carryforwards Deferred Tax Assets, Operating Loss Carryforwards 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 2024-01-01 [Member] 2024-01-01 [Member] 2024-01-01 [Member] Prepayments and Other Other Assets, Current Equity [Abstract] Equity [Abstract] Total Fair Value of Assets Assets, Fair Value Disclosure Unsecured Senior Notes 2.65% Due 2025 Unsecured Senior Notes 2.65% Due 2025 [Member] Unsecured Senior Notes 2.65% Due 2025 Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Transmission [Member] Electric Transmission [Member] Long-term Contracts for Sale of Electric Power [Domain] Power Sales Agreement - Energy and Capacity [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Customer Concentration Risk [Member] Customer Concentration Risk [Member] Diluted (Shares) Average Common Shares Weighted Average Number of Shares Outstanding, Diluted Regulatory Matters [Table] Public Utilities General Disclosures [Table] Environmental Liabilities [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Operating Expenses – Other [Abstract] Other Income and Expenses [Abstract] Fuel Adjustment Clause Filing [Member] Fuel Adjustment Clause Filing [Member] Fuel Adjustment Clause Filing Number of Operating Segments Number of Operating Segments Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Debt Disclosure [Abstract] Debt Disclosure [Abstract] Fair Value Hierarchy Transfers, All Levels Fair Value Transfers Between Levels, All Levels The amount of transfers in and out of levels 1, 2, and 3 of the fair value hierarchy during the period. 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Outstanding Debt, Current Plant Removal Obligations [Member] Removal Costs [Member] Unrecognized Postretirement Health and Life Costs [Table Text Block] Schedule of Net Periodic Benefit Cost Not yet Recognized [Table Text Block] Other Liabilities [Abstract] Other Liabilities Disclosure [Abstract] Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Corporate & Other - Other [Member] Corporate & Other - Other [Member] Revenue Recognized by Corporate & Other From Other Sources Immaterial for Disaggregation Equity Method Investment, Nonconsolidated Investee [Domain] Equity Method Investment, Nonconsolidated Investee [Domain] Document [Domain] Document [Domain] Common Stock, Shares Issued Common Stock, Shares, Issued Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Other Income (Expense) - Other [Table] Operating Lease, Liability Total Lease Obligations Operating Lease, Liability Large Power Customers [Member] Large Power Customers [Member] Large Power Customer [Member] Large Power Customer [Member] Net Deferred Income Taxes Deferred Tax Liabilities, Net Long-Term Debt Net Long-Term Debt Long-term Debt, Excluding Current Maturities Segment Reporting [Abstract] Segment Reporting [Abstract] 2018 Wisconsin General Rate Case [Member] 2018 Wisconsin General Rate Case [Member] 2018 retail rate increase request filed with the PSCW [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Revenue from Cost Recovery Riders CIP Financial Incentive Regulated Operating Revenue, Other Total ALLETE Equity Stockholders' Equity Attributable to Parent Employer Contributions to Defined Benefit Pension Plans - Cash Defined Benefit Plan, Plan Assets, Cash Contributions by Employer Defined Benefit Plan, Plan Assets, Cash Contributions by Employer Land in Minnesota (Acres) Area of Land Other – Non-utility Revenue Not from Contract with Customer, Other Minimum Payments in 2024 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Four Entity Address, City Entity Address, City or Town Fair Value Hierarchy and NAV [Axis] Defined Benefit Plan, Fair Value of Plan Assets by Measurement [Axis] Fair Value Hierarchy and NAV [Axis] Restricted Stock Units [Member] Restricted Stock Units (RSUs) [Member] Trend Rate Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] 2025 Lessee, Operating Lease, Liability, to be Paid, Year Five Cost of Purchased Power Utilities Operating Expense, Purchased Power under Long-term Contracts Revenue from CIP Financial Incentive [Policy Text Block] Revenue Recognition for Alternative Revenue Programs, Policy [Policy Text Block] Requested Return on Equity Public Utilities, Requested Return on Equity, Percentage Silver Bay Power [Member] Silver Bay Power [Member] Silver Bay Power [Member] Long-term Contracts for Sale of Electric Power [Axis] Power Sales Agreement - Energy and Capacity [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Construction Work in Progress Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress Deferred Income Tax Asset Valuation Allowance Deferred Tax Assets, Valuation Allowance Liabilities [Abstract] Liabilities [Abstract] 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 WPPI Energy Ownership % Jointly-Owned Electric Facility Percentage Not Owned Jointly-Owned Electric Facility Percentage Not Owned Other Other Other Liabilities, Current Employee Stock Ownership Plan [Abstract] Share-based Payment Arrangement, Disclosure [Abstract] Investment, Name [Domain] Equity Method Investment [Domain] Investment, Name [Domain] Total Other Comprehensive Income (Loss) Other Comprehensive Income (Loss), Net of Tax Regulatory Asset [Axis] Regulatory Asset [Axis] Leases [Abstract] Leases [Abstract] Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 [Member] Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006 Due 2025 [Member] Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006 Due 2025 [Member] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Total Current Assets Total Current Assets Assets, Current Corporate and Other [Member] Corporate and Other [Member] Capitalized Contract Cost [Table] Capitalized Contract Cost [Table] Net Loss Attributable to Non-Controlling Interest Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest NOL Carryforwards Operating Loss Carryforwards Average Shares of Common Stock and Per Share Data [Abstract] Earnings Per Share [Abstract] Deferred Tax Assets and Liabilities [Table Text Block] Schedule of Deferred Tax Assets and Liabilities [Table Text Block] First Mortgage Bonds - 6.02% Series Due 2023 [Member] 6.02% Series Due 2023 [Member] First Mortgage Bonds, 6.02% Series Due 2023 [Member] Amortization of Prior Service Cost Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Annual Debt Service Period [Domain] Annual Debt Service Period [Domain] [Domain] for Annual Debt Service Period [Axis] Percentage of Total Regulated Utility kWh Sales Percentage of Total Regulated Utility kWh Sales Percentage of Total Regulated Utility kWh Sales Gain on Sale of U.S. Water Services Gain on Sale of U.S. Water Services Gain (Loss) on Disposition of Business ALLETE Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Document Fiscal Period Focus Document Fiscal Period Focus Schedule of Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities [Text Block] Square Butte [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] Alternative Programs [Member] Alternative Programs [Member] Alternative Programs Sales Channel [Member] Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Valuation and Qualifying Accounts and Reserves [Table] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] Basin [Member] Basin [Member] Basin [Member] Approximate Proceeds Expected or Received from Divestiture of Business Approximate Proceeds Expected or Received from Divestiture of Business Approximate Proceeds Expected or Received from Divestiture of Business Net Income (Loss) Available to Common Stockholders, Diluted Net Income (Loss) Available to Common Stockholders, Diluted Available-for-sale – Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Retail Electric Service [Member] Retail Electric Service [Member] Retail Electric Service [Member] Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Employer Contribution Defined Benefit Plan, Plan Assets, Contributions by Employer Equity Method Investment, Nonconsolidated Investee or Group of Investees Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member] Silver Bay Power Self-Generation [Member] Silver Bay Power Coal, Liquid and Gaseous Fuels-fired Units [Member] Silver Bay Power Coal, Liquid and Gaseous Fuels-fired Units [Member] Entity Filer Category Entity Filer Category Common Stock [Member] Common Stock [Member] Product and Service [Domain] Product and Service [Domain] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Return of Capital from Equity Investments Proceeds from Equity Method Investment, Distribution, Return of Capital Proceeds from Equity Method Investment, Distribution, Return of Capital Jointly-Owned Electricity Generation Plant [Member] Jointly Owned Electricity Generation Plant [Member] Non-controlling interest Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Amount Non-Controlling Interest in Subsidiaries Consolidation, Subsidiaries or Other Investments, Consolidated Entities, Policy [Policy Text Block] Operating Revenue [Abstract] Regulated Operating Revenue [Abstract] Lease Contractual Term [Axis] Lease Contractual Term [Axis] Minimum Payments in 2021 Unrecorded Unconditional Purchase Obligation, to be Paid, Year One Other Power Supplier Customers [Member] Other Power Supplier Customers [Member] Other Power Supplier Customers Consolidated Operating Revenue [Member] Revenue Benchmark [Member] Regulatory Liabilities Deferred Tax Assets, Regulatory Liabilities Deferred Tax Assets, Regulatory Liabilities Equity Investments [Line Items] Schedule of Equity Method Investments [Line Items] Interest Rate Debt Instrument, Interest Rate, Stated Percentage Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Information for Pension Plans with an Accumulated Benefit Obligation in Excess of Plan Assets [Table Text Block] Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Table Text Block] Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Oliver Wind II PPA [Member] Oliver Wind II PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Total Other Expense Nonoperating Income (Expense) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] Operating Expenses [Abstract] Costs and Expenses [Abstract] Unamortized Investment Tax Credits Accumulated Deferred Investment Tax Credit Power Purchase Agreement - Capacity [Axis] Long-term Contract for Purchase of Electric Capacity [Axis] Long-term Contract for Purchase of Electric Capacity [Axis] Nobles 2 PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] State Deferred State and Local Income Tax Expense (Benefit) Long-Term Debt Maturing in 2024 Long-Term Debt, Maturity, Year Five Expected Long-Term Return on Plan Assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Cash Paid During the Period for Interest – Net of Amounts Capitalized Interest Paid, Excluding Capitalized Interest, Operating Activities Debt Instrument [Abstract] Long-term Debt, Fiscal Year Maturity [Abstract] Total Equity Balance, Beginning of Period Balance, Ending of Period Members' Equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Point-in-Time [Member] Point-in-Time [Member] Revenue Recognized From Point-in-Time Sales Operating Revenue Revenue Operating Revenue Revenues Class of Stock [Axis] Class of Stock [Axis] Total Fair Value of Liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure 2023-01-01 [Member] 2023-01-01 [Member] 2023-01-01 [Member] Diluted Earnings Per Share of Common Stock Earnings Per Share Earnings Per Share, Diluted CIP Consolidated Filing [Member] CIP Consolidated Filing [Member] CIP Consolidated Filing [Member] AFUDC - Equity [Member] AFUDC - Equity [Member] AFUDC - Equity [Member] Summary of Common Stock [Table] Schedule of Stock by Class [Table] ALLETE South Wind [Member] ALLETE South Wind [Member] ALLETE South Wind First Mortgage Bonds Issued August 3, 2020 [Table Text Block] Schedule of Debt [Table Text Block] Accounts Receivable [Abstract] Accounts Receivable, after Allowance for Credit Loss, Current [Abstract] Consolidation, Eliminations [Member] Consolidation, Eliminations [Member] Calpine Corporation PPA [Member] Calpine Corporation PPA [Member] Calpine Corporation PPA Long-term Debt [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Depreciation Expense Depreciation Entity Address, Address Line Entity Address, Address Line One Contributions to RSOP, Equity Shares Issued, Value, Share-based Payment Arrangement, before Forfeiture Wholesale Electric Service [Member] Wholesale Electric Service [Member] Wholesale Electric Service [Member] Entity Emerging Growth Company Entity Emerging Growth Company Deferred Income Tax Benefit Total Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) First Mortgage Bonds - 4.47% Series Due 2049 [Member] ALLETE Bonds 4.47% Due March 2049 [Member] ALLETE Bonds 4.47% Due March 2049 [Member] Maximum [Member] Maximum [Member] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Non-Controlling Interest in Subsidiaries [Member] Noncontrolling Interest [Member] Approved Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Recurring Fair Value Measures [Line Items] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Industrial Customers [Member] Industrial Customers [Member] Industrial Customers Stock-Based Compensation [Table] Share-Based Compensation Expense [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] 2022-01-01 [Member] 2022-01-01 [Member] 2022-01-01 [Member] Unrecognized Tax Benefits, Penalties Unrecognized Tax Benefits, Income Tax Penalties Expense Equity Issuance Program Shares Authorized Equity Issuance Program Shares Authorized Aggregate number of shares, without par value, authorized to be issued under the distribution agreement with KCCI (equity issuance program/PIE). Postretirement Health and Life [Member] Other Postretirement Benefits Plan [Member] Other Postretirement Benefits Plan [Member] Renewable Generation [Member] Renewable Generation [Member] Renewable Generation Trading Symbol Trading Symbol Disposal Group Classification [Domain] Disposal Group Classification [Domain] Minnesota Power Ownership % Jointly Owned Utility Plant, Proportionate Ownership Share Regulatory Liabilities [Axis] Regulatory Liability [Axis] Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) WPPI Energy [Member] WPPI Energy [Member] WPPI Energy [Member] Jointly-Owned Facilities and Assets [Axis] Jointly Owned Utility Plant [Axis] 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. Entity Shell Company Entity Shell Company Transmission Line Length (Miles) Transmission Line Length (Miles) Length of a transmission line in miles. Business Segments [Line Items] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Recurring Fair Value Measures [Member] Fair Value, Recurring [Member] Power Sales Agreement - Capacity [Domain] Long-term Contract for Sale of Capacity [Domain] [Domain] for Long-term Contract for Sale of Capacity [Axis] 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 Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Entity Public Float Entity Public Float First Mortgage Bonds - 4.07% Series Due 2048 [Member] ALLETE Bonds 4.07% Due April 2048 [Member] ALLETE Bonds 4.07% Due April 2048 [Member] Regulatory Agency [Domain] Regulatory Agency [Domain] Document Type Document Type ICFR Auditor Attestation Flag ICFR Auditor Attestation Flag Other Total Other Income (Expense) - Other Other Nonoperating Income (Expense) 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 Component of Other Income (Expense) - Other [Domain] Component of Other Income (Expense) - Other [Domain] [Domain] for Component of Other Income (Expense) - Other [Axis] Business Segments [Axis] Business Segments [Axis] Segments [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] Pension and Other Postretirement Benefit Plans [Text Block] Retirement Benefits [Text Block] Awarded Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value First Mortgage Bonds - 3.30% Series Due 2050 ALLETE Bonds 3.30% Due August 2050 [Member] ALLETE Bonds 3.30% Due August 2050 Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Wholesale Electric Contracts (expire December 2024) [Member] Wholesale Electric Contracts (Expire from 2024 through 2029) [Member] Wholesale Electric Contracts (Expire from 2024 through 2029) [Member] Great River Energy [Member] Great River Energy [Member] Great River Energy [Member] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Leases [Policy Text Block] Lessee, Leases [Policy Text Block] Benefits Paid Defined Benefit Plan, Plan Assets, Benefits Paid Other [Member] Other [Member] Regulated Operations Other Revenue Recognized From Other Sources Immaterial For Disaggregation Pension and Other Postretirement Benefit Plans [Policy Text Block] Pension and Other Postretirement Plans, Policy [Policy Text Block] Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Deferred Income Taxes Deferred Income Tax Liabilities, Net 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 First Mortgage Bonds - 4.90% Series Due 2025 [Member] 4.90% Series Due 2025 [Member] First Mortgage Bonds, 4.90% Series Due 2025 [Member] Reclassification of Property, Plant and Equipment to Inventory Reclassification of Property, Plant and Equipment to Inventory Reclassification of Property, Plant and Equipment to Inventory Expected Period for Costs of Compliance Expected Period for Costs of Compliance Expected Period for Costs of Compliance Average Customer Demand (MW) Average Customer Demand (MW) Average demand measured for a specified customer, measured in MW. Bonds Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability 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 ESPP Discount Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date Operating Income Operating Income Operating Income (Loss) Contract Term (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. Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Costs, Future Period Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Costs, Future Period Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Costs, Future Period Minnesota Power Output Entitlement (Percent) Expected Output Entitlement (Percent) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Variable Rate [Axis] Variable Rate [Axis] Commitments, Guarantees and Contingencies (Note 8) Commitments and Contingencies Available-for-sale – Corporate and Governmental Debt Securities Debt Securities, Available-for-sale, Noncurrent Vesting Period (Years) Performance Period (Years) Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Vehicles and Equipment [Member] Vehicles and Equipment [Member] Vehicles and Equipment [Member] Accrued Taxes Taxes Payable, Current Entity Address, State Entity Address, State or Province Awarded Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Income Taxes Computed at Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount First Mortgage Bonds - 4.21% Series Due 2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] Accounts Payable Accounts Payable, Current Debt Instrument, Description of Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Other Income (Expense) - Other [Member] Other Income (Expense) - Other [Member] Other Income (Expense) - Other [Member] Retained Earnings Retained Earnings (Accumulated Deficit) Operating Lease, Right-of-Use Asset Operating Lease, Right-of-Use Asset Sale of Wind Energy Facility Sale of Wind Energy Facility Sale of Wind Energy Facility Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [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] Property-Related Deferred Tax Liabilities, Property, Plant and Equipment Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Plant in Service Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service Recurring Fair Value Measures - Activity in Level 3 [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Nobles 2 [Member] Nobles 2 [Member] Nobles 2 [Member] Equity [Abstract] Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Unsecured Senior Notes Due September 2025 [Member] Unsecured Senior Notes Due September 2025 [Member] Unsecured Senior Notes Due September 2025 First Mortgage Bonds - 3.74% Series Due 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] Asset Retirement Obligations [Table Text Block] Schedule of Change in Asset Retirement Obligation [Table Text Block] Customer [Axis] Customer [Axis] Palm Coast Park Community Development District [Member] Palm Coast Park District Special Assessment Bonds [Member] Palm Coast Park Community Development District [Member] Community development district obligations for Palm Coast Park District. 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 Taxes [Policy Text Block] Income Tax, Policy [Policy Text Block] Generating Capacity Under Construction (MW) Generating Capacity Under Construction (MW) Generating Capacity Under Construction (MW) Payments for Equity Investments Cash Investments Payments to Acquire Equity Method Investments Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Other Investing Activities Payments for (Proceeds from) Other Investing Activities Employer Contributions to Defined Benefit Pension Plans - Common Stock Value Stock Issued During Period, Value, Employee Benefit Plan Boswell Unit 4 [Member] Boswell Unit 4 [Member] Boswell Unit 4 [Member] 2021-01-01 [Member] 2021-01-01 [Member] 2021-01-01 Approximate Proceeds from Sale of Assets Approximate Proceeds Expected or Received from Sale of Assets Approximate Proceeds Expected or Received from Sale of Assets Beginning Obligation Ending Obligation Asset Retirement Obligation Construction Work in Progress Construction in Progress, Gross Revenue from Contract with Customer, Excluding Assessed Tax Revenue from Contract with Customer, Excluding Assessed Tax Health Care Trend Rates [Abstract] Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] Long-Term Debt Maturing Thereafter Long-Term Debt, Maturity, after Year Five Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Total Regulatory Liabilities [Domain] Regulatory Liability [Domain] Long-term PSA [Member] Long-term PSA [Member] Revenue Recognized From Long-term PSAs Geographical [Domain] Geographical [Domain] Capitalized Contract Cost, Amortization Capitalized Contract Cost, Amortization Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Financing Renewal Term (Years) Debt Instrument, Renewal Term Renewal term for a debt instrument disclosing for what period of time after maturity the instrument may be extended. Mutual Funds [Member] Fixed Income Funds [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] Contract with Customer, Duration [Domain] Contract with Customer, Duration [Domain] Accrued Interest Interest Payable, Current Entity Voluntary Filers Entity Voluntary Filers Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Common Stock, Par Value Per Share Common Stock, No Par Value Credit Facility [Axis] Credit Facility [Axis] Proposed 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] Assumptions Used to Determine Benefit Obligation [Abstract] Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] Other Total Operating Expenses – Other Operating Expenses – Other Other Operating Income (Expense), Net Other Non-Current Liabilities Equity Method Investment, Summarized Financial Information, Other Non-Current Liabilities Equity Method Investment, Summarized Financial Information, Other Non-Current Liabilities Revenue Recognition [Abstract] Revenue Recognition [Abstract] Consolidated Statement of Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Receivables from Large Power Customers Accounts Receivable, before Allowance for Credit Loss Proceeds from Issuance of Common Stock Proceeds from Issuance of Common Stock 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] Activity in Level 3 [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] NOL and Tax Credit Carryforwards [Line Items] NOL and Tax Credit Carryforwards [Line Items] NOL and Tax Credit Carryforwards [Line Items] Recurring Fair Value Measures [Table Text Block] Fair Value, Assets Measured on Recurring Basis [Table Text Block] Real Estate [Member] Real Estate [Member] Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense (Benefit) Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] Long-term Contract with Customer [Member] Long-term Contract with Customer [Member] Plan Asset Categories [Axis] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis] Defined Benefit Plan, Plan Assets, Category [Axis] Short-term Leases [Policy Text Block] Short-term Leases [Policy Text Block] Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Entity Central Index Key Entity Central Index Key Related Party [Domain] Related Party [Domain] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges Equity Issuance Program Shares Available for Issuance Equity Issuance Program Shares Available for Issuance The number of shares of common stock that remain available for issuance under the equity issuance program (KCCI distribution agreement). Employer Contributions to Pension - Shares Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares Stock Issued During Period, Shares, Employee Benefit Plan Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Other Noncurrent Liabilities Other Noncurrent Liabilities [Member] 2025-01-01 [Member] 2025-01-01 [Member] 2025-01-01 Geographical [Axis] Geographical [Axis] Additions, Other Charges SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account Granted, Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Total Grant Date Fair Value The total grant date fair value of equity-based awards other than options, granted during the period. Business Segments [Domain] Segments [Domain] First Mortgage Bonds - 4.85% Series Due 2021 [Member] 4.85% Series Due 2021 [Member] First Mortgage Bonds, 4.85% Series Due 2021 [Member] Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Lines of Credit [Abstract] Line of Credit Facility [Abstract] Defined Benefit Plan - Amendments Defined Benefit Plan, Changes in Benefit Obligation and Plan Assets, Description Long-Term Debt Maturing in 2023 Long-Term Debt, Maturity, Year Four U.S. Small-cap [Member] U.S. Small Cap [Member] Equity Securities, U.S. Small Cap [Member] Power Purchase Agreement - Capacity [Domain] Long-term Contract for Purchase of Electric Capacity [Domain] [Domain] for Long-term Contract for Purchase of Electric Capacity [Axis] Income Statement Location [Axis] Income Statement Location [Axis] Capitalized Contract Cost [Line Items] Capitalized Contract Cost [Line Items] Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Long-Term Service Agreements [Member] Long-Term Service Agreements [Member] Long-Term Service Agreements [Member] Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax Federal Current Federal Tax Expense (Benefit) Taxes Other than Income Taxes Taxes, Other Lines of Credit [Table] Line of Credit Facility [Table] Prior Service Credit Arising During the Period Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax Capitalized Contract Cost [Domain] Capitalized Contract Cost [Domain] Equity Investments [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Segment Reporting Disclosure [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. 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. Asset Retirement Obligation [Roll Forward] Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] Unsecured Term Loan Variable Rate Due 2021 Unsecured Term Loan Due 2021 [Member] Unsecured Term Loan Due 2021 Title of 12(b) Security Title of 12(b) Security Disposal Group Classification [Axis] Disposal Group Classification [Axis] ALLETE Clean Energy Unsecured Term Loan Variable Rate Due 2021 ALLETE Clean Energy Unsecured Term Loan Due 2021 [Member] ALLETE Clean Energy Unsecured Term Loan Due 2021 U.S. Mid-cap Growth [Member] U.S. Mid-Cap Growth [Member] Equity Securities, U.S. Mid-Cap Growth [Member] Public Utilities, Interim Rate Increase (Decrease), Amount Public Utilities, Interim Rate Increase (Decrease), Amount Non-Current Assets Assets for Plan Benefits, Defined Benefit Plan Unrealized Gain (Loss) on Securities - Net of Income Tax Expense (Benefit) of $0.1, $0.1 and $- Unrealized Gain (Loss) on Securities OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax Provision (Payments) for Interim Rate Refund Provision (Payments) for Interim Rate Refund Provision (Payments) for Interim Rate Refund Unrealized Gain (Loss) on Securities, Income Tax Expense (Benefit) OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax ESOP Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense Interest Expense Interest Expense Interest Expense Gross Unrecognized Income Tax Benefits [Roll Forward] Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Depreciation and Amortization Depreciation, Depletion and Amortization Depreciation, Depletion and Amortization Partnership Basis Differences Deferred Tax Liabilty, Partnership Basis Differences Deferred Tax Liability, Partnership Basis Differences Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] PPA Annual Debt Service Obligation (Percentage) Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required, Obligation (Percentage) Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required, Obligation (Percentage) Largest Customer [Member] Largest Customer [Member] Largest Customer [Member] Chanarambie and Viking Wind Energy Facilities [Member] Chanarambie and Viking Wind Energy Facilities [Member] Chanarambie and Viking Wind Energy Facilities Oconto Electric Cooperative Power Sales Agreement [Member] Oconto Electric Cooperative Power Sales Agreement [Member] Oconto Electric Cooperative Power Sales Agreement Actual Indebtedness to Total Capitalization Ratio Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset) Accumulated Net Periodic Benefit Cost in Excess of Contributions Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset) Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset) Product and Service [Axis] Product and Service [Axis] Proceeds from Sale of U.S. Water Services - Net of Transaction Costs and Cash Retained Proceeds from Sale of Productive Assets Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] Nemadji Trail Energy Center [Member] Bond Interest Rate Community Development District Obligation, Bond Interest Rate Interest rate of capital improvement revenue and special assessment bonds used to fund real estate development projects. Amortization of PSAs 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. Proceeds from Issuance of Unsecured Debt Proceeds from Issuance of Unsecured Debt Weighted-Average Period for Recognition (Years/Months) Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Long-Term Debt Maturing in 2021 Long-Term Debt, Maturity, Year Two Asset Retirement Obligation [Member] Asset Retirement Obligation Costs [Member] Liabilities Settled Asset Retirement Obligation, Liabilities Settled Environmental Matters [Table] Site Contingency [Table] Generating Capacity (MW) Anticipated Additional Generating Capacity (MW) Anticipated Additional Generating Capacity (MW) Real Estate Investments, Net Real Estate Investments, Net Recurring Fair Value Measurements [Table] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Table] Fair Value, Recurring and Nonrecurring [Table] Accounts Payable Increase (Decrease) in Accounts Payable Debt Instrument [Axis] Debt Instrument [Axis] Quarterly Financial Data (Unaudited) [Table Text Block] Quarterly Financial Information [Table Text Block] Great River Energy Power Sales Agreement (expires May 2025) Great River Energy Power Sales Agreement (expires May 2025) [Member] Great River Energy Power Sales Agreement (expires May 2025) Capitalized Contract Cost [Axis] Capitalized Contract Cost [Axis] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Summary of Common Stock [Line Items] Class of Stock [Line Items] Taconite Harbor [Member] Taconite Harbor [Member] Taconite Harbor Energy Center Other Power Suppliers [Member] Other Power Suppliers [Member] Other Power Suppliers Sales Channel [Member] Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] Tax Credit Carryforwards, Valuation Allowance Tax Credit Carryforward, Valuation Allowance Minimum Annual Payments for Certain Long-Term Commitments [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Billed Accounts Receivable, before Allowance for Credit Loss, Current Revenue from Contract with Customer [Abstract] Revenue from Contract with Customer [Abstract] Bond Term (Years) Community Development District Obligation, Bond Term, in Years Term (in years) of capital improvement revenue and special assessment bonds used to fund real estate development projects. NOL Carryforwards, Valuation Allowance Operating Loss Carryforwards, Valuation Allowance 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 First Mortgage Bonds - 3.20% Series Due 2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Projected Benefit Obligation Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation ALLETE, Inc. [Member] ALLETE, Inc. [Member] -- None. No documentation exists for this element. -- Subsequent Event [Member] Subsequent Event [Member] Distribution [Member] Electric Distribution [Member] Letters of Credit Outstanding, Amount Letters of Credit Outstanding, Amount Power Purchase Agreement - Energy [Domain] Long-term Contract for Purchase of Energy [Domain] [Domain] for Long-term Contract for Purchase of Energy [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Land and Other [Member] Land and Other [Member] Land and Other [Member] Revenue Collection Period Following the Annual Period it is Recognized Revenue Collection Period Following the Annual Period it is Recognized Revenue Collection Period Following the Annual Period it is Recognized Energy Generation Owned (MW) Additional Natural Gas-fired Generation (MW) Generating Capacity Owned (MW) Generating Capacity Owned (MW) Revenue Recognition [Policy Text Block] Revenue [Policy Text Block] Impairment of Long-Lived Assets Held-for-use Impairment of Long-Lived Assets Held-for-use First Mortgage Bonds - 2.50% Due August 2030 [Member] ALLETE Bonds 4.08% Due March 2029 [Member] ALLETE Bonds 4.08% Due March 2029 [Member] Boswell Unit 3 [Member] Boswell Unit 3 [Member] Boswell Unit 3 Unrecognized Tax Benefits [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Capital Cost Spent to Date Total Project Costs Incurred to Date Capital Cost Spent to Date Capital cost spent to date for project. Operating Lease, Liability, Noncurrent Long-term Lease Obligations Operating Lease, Liability, Noncurrent Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense (Benefit) of $(3.1), $1.4 and $0.3 Defined Benefit Pension and Other Postretirement Plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Oliver Wind I PPA [Member] Oliver Wind I PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. CapX2020 Projects [Member] CapX2020 [Member] CapX2020 (all projects) [Member] Depreciation [Policy Text Block] Depreciation, Depletion, and Amortization [Policy Text Block] Current Assets [Abstract] Assets, Current [Abstract] Provision for Interim Rate Refund [Domain] Provision for Interim Rate Refund [Domain] Provision for Interim Rate Refund [Domain] 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] Nobles 2 [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] Long-Term Debt Maturing in 2020 Long-Term Debt, Maturity, Year One Rate of Compensation Increase Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Rate Information by Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Deductions from Reserves SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction Legal Entity [Axis] Legal Entity [Axis] City Area Code City Area Code Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] Income Tax Authority [Axis] Income Tax Authority [Axis] Document Period End Date Document Period End Date 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). Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Minimum Payments in 2023 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Three Net Income Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Impairment of Long-Lived Assets [Policy Text Block] Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Income Taxes [Member] Deferred Income Tax Charge [Member] Liabilities Recognized Asset Retirement Obligation, Liabilities Incurred Capitalized Share-Based Compensation Costs Share-based Payment Arrangement, Amount Capitalized Share-Based Compensation Expense [Table Text Block] Share-based Payment Arrangement, Cost by Plan [Table Text Block] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Share-Based and ESOP Compensation Expense Share-based Payment Arrangement, Noncash Expense Property Related Deferred Tax Assets, Property, Plant and Equipment Other Financing Activities Proceeds from (Payments for) Other Financing Activities Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Total Regulatory Assets [Domain] Regulatory Asset [Domain] Distributed ATC Earnings Proceeds from Equity Method Investment, Distribution Fixed Income [Member] Fixed Income Securities [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] Impairment of Long-Lived Assets [Abstract] Asset Impairment Charges [Abstract] 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. Total Long-Term Debt Long-term Debt Contract with Customer, Sales Channel [Axis] Contract with Customer, Sales Channel [Axis] Guarantee Entity and Type [Domain] Guarantee Entity and Type [Domain] [Domain] for Guarantee Entity and Type [Axis] Conservation Improvement Program Goal, In 3 Years Conservation Improvement Program Goal, In 3 Years Conservation Improvement Program Goal, In 3 Years Participant Contributions Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant 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. Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Cost of Sales - Non-utility Cost of Goods and Services Sold Total Operating Expenses Costs and Expenses Equity Components [Axis] Equity Components [Axis] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Asset Retirement Obligations [Policy Text Block] Asset Retirement Obligation [Policy Text Block] 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] Gross Deferred Income Tax Assets Deferred Tax Assets, Gross Minimum [Member] Minimum [Member] Accounts Receivables [Abstract] Receivables [Abstract] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Equity Method Investment, Indicators of Impairment Equity Method Investment, Other than Temporary Impairment Options and Stock Awards, Equity Stock Issued During Period, Value, Share-based Compensation and ESOP, Gross Gross value of stock (or other type of equity) issued during the period as a result of any equity-based compensation plan, including employee stock ownership plans (ESOP). Stock issued could result from the issuance of retricted stock units, the exercise of stock options, stock issued under an ESOP and/or other employee benefit plans. Contracts with Customers – Non-utility Unregulated Operating Revenue Accumulated Benefit Obligation Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation Supplemental Statement of Cash Flow Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Defined Benefit Plan, Plan Assets, Category [Domain] Defined Benefit Plan, Plan Assets, Category [Domain] 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] Concentration Risks, Types, No Concentration Percentage [Abstract] Concentration Risks, Types, No Concentration Percentage [Abstract] Amortization of Loss Defined Benefit Plan, Amortization of Gain (Loss) Amounts Recognized in Consolidated Balance Sheet [Table Text Block] Schedule of Amounts Recognized in Balance Sheet [Table Text Block] Contract Asset - Noncurrent 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 Entity Interactive Data Current Entity Interactive Data Current Net Pension Cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Plan Asset Actual and Target Allocations [Table Text Block] Schedule of Allocation of Plan Assets [Table Text Block] Employee Benefits and Compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation Interest and Investment Earnings [Member] Interest and Investment Earnings [Member] Interest and Investment Earnings [Member] Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] Common Stock and Earnings Per Share [Text Block] Stockholders' Equity Note Disclosure [Text Block] Deferred Income Tax Assets and Liabilities [Abstract] Components of Deferred Tax Assets and Liabilities [Abstract] Accounting for Stock-Based Compensation [Policy Text Block] Share-based Payment Arrangement [Policy Text Block] Valuation and Qualifying Accounts and Reserves [Roll Forward] SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward] Conservation Improvement Program Goal, Next Fiscal Year Conservation Improvement Program Goal, Next Fiscal Year Conservation Improvement Program Goal, Next Fiscal Year Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Income Tax Expense (Benefit), Due to Sale of Subsidiary Income Tax Expense (Benefit), Due to Sale of Subsidiary Income Tax Expense (Benefit), Due to Sale of Subsidiary Long-term Contract for Purchase of Electric Power [Domain] Long-term Contract for Purchase of Electric Power [Domain] Long-term Contract for Purchase of Electric Power [Domain] Power Sales Agreements Power Purchase Agreements, Deferred Tax Asset Power Purchase Agreements, Deferred Tax Asset 2020 Defined Benefit Plan, Expected Future Benefit Payment, Year One Total Deferred Income Tax Liabilities Deferred Tax Liabilities, Gross SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] Number of Shares [Rollforward] Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward] BNI Energy [Domain] BNI Energy [Domain] BNI Energy [Domain] Number of Reportable Segments Number of Reportable Segments Regulatory Assets and Liabilities Disclosure [Abstract] Regulatory Assets and Liabilities Disclosure [Abstract] Change in Fair Value of Contingent Consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Retirement Plan Type [Domain] Retirement Plan Type [Domain] Defined Benefit Pension and Other Postretirement Benefit Expense Pension and Other Postretirement Benefits Cost (Reversal of Cost) Additions, Charged to Income SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Cost and Expense Equity Investments [Table] Schedule of Equity Method Investments [Table] Consolidation Items [Axis] Consolidation Items [Axis] Net Book Value Public Utilities, Property, Plant and Equipment, Net Net (Gain) Loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax Other Expense Equity Method Investment, Summarized Financial Information, Other Expense The amount of other expense reported by an equity method investment of the entity. Accounts Receivable (Less Allowance of $2.5 and $0.9) Total Accounts Receivable Accounts Receivable, after Allowance for Credit Loss, Current Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Total Lease Payments Due Lessee, Operating Lease, Liability, to be Paid Other Deferred Tax Assets, Other Unrecognized Tax Benefits, Accrued Interest Unrecognized Tax Benefits, Interest on Income Taxes Accrued Debt Instrument, Redemption, Period [Domain] Debt Instrument, Redemption, Period [Domain] 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 Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, 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] Document Transition Report Document Transition Report Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Axis] Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held Deferred Income Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Minimum Annual Payments for Certain Long-Term Commitments [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Accumulated Depreciation Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation Other Income (Expense) [Abstract] Nonoperating Income (Expense) [Abstract] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Current Income Tax Expense [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] First Mortgage Bonds - 5.99% Series Due 2027 [Member] 5.99% Series Due 2027 [Member] First Mortgage Bonds, 5.99% Series Due 2027 [Member] Other Liabilities Other Liabilities [Member] Cumulative Effect, Period of Adoption [Axis] Cumulative Effect, Period of Adoption [Axis] Credit Facility [Domain] Credit Facility [Domain] Basic Earnings Per Share of Common Stock Earnings Per Share Earnings Per Share, Basic Unrecognized Tax Benefits that Would Impact Effective Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Entity File Number Entity File Number Other Current Liabilities Other Current Liabilities [Member] Defined Benefit Pension and Other Postretirement Benefit Plans Non-Current Liabilities Liability, Defined Benefit Plan, Noncurrent Rate of Compensation Increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase Operating Lease, Weighted Average Remaining Lease Term Operating Lease, Weighted Average Remaining Lease Term First Mortgage Bonds - 3.40% Series Due 2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] Standby Letters of Credit Outstanding Line of Credit, Amount Pledged for Letters of Credit Amount of line of credit borrowing capacity pledged against letters of credit at the end of the period limiting the line of credit borrowing capacity but not reflected as debt on the balance sheet. Jointly-Owned Facilities and Projects [Domain] Jointly-Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Balance, Equity Common Stock, Value, Outstanding Customer Type [Domain] Customer Type [Domain] Customer Type [Domain] Proceeds from Non-Controlling Interest in Subsidiaries - Net of Issuance Costs Proceeds from Non-Controlling Interest - Net of Issuance Costs Proceeds from Noncontrolling Interests Prior Service Credit Prior Service Credit Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax Share-Based Compensation Expense Share-based Payment Arrangement, Expense Employee Stock Purchase Plan, Equity Stock Issued During Period, Value, Employee Stock Purchase Plan Balance Sheet Location [Axis] Balance Sheet Location [Axis] International [Member] International [Member] Equity Securities, International [Member] Document Annual Report Document Annual Report NOL and Tax Credit Carryforwards [Table] NOL and Tax Credit Carryforwards [Table] NOL and Tax Credit Carryforwards [Table] Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Income Tax Benefit Share-based Payment Arrangement, Expense, Tax Benefit Accounts Receivable [Table Text Block] Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Property, Plant and Equipment, Additions Property, Plant and Equipment, Additions Retirement Benefits [Abstract] Retirement Benefits [Abstract] Selected Quarterly Financial Information [Abstract] Selected Quarterly Financial Information [Abstract] Gross Unrecognized Income Tax Benefits [Table Text Block] Summary of Income Tax Contingencies [Table Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Inventories – Net [Abstract] Inventory, Net [Abstract] Non-Controlling Interest in Subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Document and Entity Information, Document [Axis] Document Information, Document [Axis] Investing Activities Net Cash Provided by (Used in) Investing Activities [Abstract] SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] Unamortized Discount and Premium on Debt [Policy Text Block] Debt, Policy [Policy Text Block] Other Deferred Tax Liabilities, Other Variable Demand Revenue Refunding Bonds Series 1997 A Due 2020 [Member] Variable Demand Revenue Refunding Bonds Series 1997 ABC Due 2013-2020 [Member] Variable Demand Revenue Refunding Bonds Series 1997 A, B, and C Due 2013-2020 [Member] First Mortgage Bonds - 5.05% Series Due 2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Pension and Other Postretirement Benefit Plan Disclosures [Table] Pension and Other Postretirement Benefit Plans Recurring Fair Value Measures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Transmission Formula Rates [Member] Transmission Formula Rates [Member] Transmission Formula Rates Total Regulatory Liabilities Regulatory Liabilities Fuel Energy Related Inventory, Coal Annual Debt Service Period [Axis] Annual Debt Service Period [Axis] Annual Debt Service Period [Axis] Jointly-Owned Facilities and Assets [Text Block] Jointly-Owned Electric Facility [Text Block] Jointly-Owned Electric Facility [Text Block] 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] Concentration Risk Type [Domain] Concentration Risk Type [Domain] 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] Plan Amendments Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment 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 Average Rate Increase Public Utilities, Requested Rate Increase (Decrease), Percentage Lapse of Statute Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations Operating Lease, Cost Operating Lease, Cost Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] Deferred Income Tax Liabilities [Abstract] Deferred Tax Liabilities, Gross [Abstract] South Peak [Member] South Peak [Member] South Peak [Member] Total Current Income Tax Expense Current Income Tax Expense (Benefit) Lines of Credit [Line Items] Line of Credit Facility [Line Items] Requested Equity Ratio Public Utilities, Requested Equity Capital Structure, Percentage Transmission [Domain] Transmission [Domain] Name of transmission investment. Additions for Tax Positions Related to Prior Years Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Additions for Tax Positions Related to the Current Year Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Total Equity [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. Equity Method Investment, Summarized Financial Information [Abstract] Equity Method Investment, Summarized Financial Information [Abstract] Equity Method Investment, Nonconsolidated Investee [Axis] Equity Method Investment, Nonconsolidated Investee [Axis] Estimated Future Benefit Payments [Table Text Block] Schedule of Expected Benefit Payments [Table Text Block] Unsecured Term Loan Variable Rate Due 2020 [Member] ALLETE Term Loan Variable Rate Due 2020 [Member] ALLETE Term Loan Variable Rate Due 2020 [Member] Property, Plant and Equipment – Net Property, Plant and Equipment – Net Property, Plant and Equipment, Net Entity Tax Identification Number Entity Tax Identification Number Executive Long-Term Incentive Compensation Plan [Member] Executive Plan [Member] Stock based, long-term incentive compensation, Executive Plan [Member] Deferred Compensation Deferred Compensation Liability, Classified, Noncurrent Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Employee Stock and Incentive Plans [Text Block] Share-based Payment Arrangement [Text Block] Net Income Attributable to ALLETE Net Income Attributable to ALLETE Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent First Mortgage Bonds - 4.08% Series Due 2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] Changes in Operating Assets and Liabilities Increase (Decrease) in Operating Capital [Abstract] Current Fiscal Year End Date Current Fiscal Year End Date Private Equity [Member] Private Equity Funds [Member] First Mortgage Bonds - 3.02% Series Due 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] Minnesota Solar Energy Standard - Qualifying Capacity for Small Scale Solar Mandate (MW) Solar Energy Standard Mandate, Qualifying Capacity for Small Scale Solar Mandate The maximum amount of nameplate capacity of a solar device to qualify for counting towards the small scale solar mandate of Minnesota's solar energy standard. Minnesota Solar Energy Standard - Overall Mandate Percentage Solar Energy Standard Mandate, Overall Mandate Percentage Minnesota's solar energy mandate for total retail electric sales, excluding sales to certain industrial customers, by the end of 2020. Tax Credit Carryforwards Deferred Tax Assets, Tax Credit Carryforwards Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Calpine Corporation [Member] Calpine Corporation [Member] Calpine Corporation Output Being Purchased - Electric Capacity (MW) Long-term Contract for Purchase of Electric Capacity (MW) Long-term Contract for Purchase of Electric Capacity (MW) Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Materials and Supplies Inventory, Supplies, Net of Reserves Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments 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] Property, Plant and Equipment Impairment [Policy Text Block] Property, Plant and Equipment, Impairment [Policy Text Block] Employee Stock Purchase Plan, Shares Stock Issued During Period, Shares, Employee Stock Purchase Plans Customer Class [Domain] 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] Output Being Purchased (MWh) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MWh) Share of plant output being purchased in megawatthours under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Basin Electric Cooperative PSA June 2022 - May 2023 [Member] Basin Electric Cooperative PSA June 2022 - May 2023 [Member] Basin Electric Cooperative PSA June 2022 - May 2023 [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Property, Plant and Equipment, Type [Domain] Long-Lived Tangible Asset [Domain] Trade Accounts Receivable [Member] SEC Schedule, 12-09, Allowance, Credit Loss [Member] Benefits Paid Defined Benefit Plan, Benefit Obligation, Benefits Paid Regulatory Differences – Excess Deferred Tax Benefit Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. Output Being Sold - Electric Energy (MW) Long-term Contract for Sale of Electric Energy (MW) Long-term Contract for Sale of Electric Energy (MW) Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities [Table Text Block] Schedule of Defined Benefit Plan Amounts Recognized in Other Comprehensive Income (Loss) [Table Text Block] Property, Plant and Equipment and Estimated Useful Lives of Property, Plant and Equipment [Table Text Block] Property, Plant and Equipment [Table Text Block] First Mortgage Bonds - 3.86% Series Due 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] Residential [Member] Residential [Member] Revenue Recognized From Residential Customers Total Unrecognized Postretirement Health and Life Cost Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax 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 Accumulated Depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Financial Covenants [Abstract] Debt Instruments [Abstract] Net Loss Net Loss Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax Oconto Electric Cooperative [Member] Oconto Electric Cooperative [Member] Oconto Electric Cooperative First Mortgage Bonds - 5.69% Series Due 2036 [Member] 5.69% Series Due 2036 [Member] First Mortgage Bonds, 5.69% Series Due 2036 [Member] Lessee, Operating Lease, Liability, Maturity [Table Text Block] Lessee, Operating Lease, Liability, Maturity [Table Text Block] Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Financial Statement Preparation [Policy Text Block] Basis of Accounting, Policy [Policy Text Block] Change in Fair Value of Contingent Consideration Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Employee Stock Ownership Plan [Policy Text Block] Employee Stock Ownership Plan (ESOP), Policy [Policy Text Block] Basic (Shares) Average Common Shares Weighted Average Number of Shares Outstanding, Basic Debt Instrument, 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). Document and Entity Information [Line Items] Document Information [Line Items] Cash and Cash Equivalents [Member] Cash and Cash Equivalents [Member] Large Power Customer Contracts Large Power Customer Contracts Large Power Customer Contracts Variable Interest Entities [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Loss Contingencies [Table] Loss Contingencies [Table] Municipal [Member] Municipal [Member] Municipal Sales Channel Plan Name [Domain] Plan Name [Domain] Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Ownership Percentage Equity Method Investment, Ownership Percentage Equity Method Investment, Ownership Percentage Other Current Liabilities Increase (Decrease) in Other Current Liabilities Equity Issuance Program, Shares Stock Issued During Period, Shares, Other Customer Type [Axis] Customer Type [Axis] Customer Type [Axis] Amortization of Loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Actuarial (Gain) Loss Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) Rider Revenue Recovery Collection Period (Years) Alternative Revenue Program, Required Collection Period Alternative Revenue Program, Required Collection Period Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value Cash and Cash Equivalents [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] ALLETE Common Stock Included in Pension Plan Equity Securities (Shares) Defined Benefit Plan, Plan Assets, Employer, Related Party, Number of Shares Inventories [Policy Text Block] Inventory, Policy [Policy Text Block] Income Tax Benefit Total Income Tax Expense (Benefit) Income Tax Expense (Benefit) Income Tax Expense (Benefit) Year Ultimate Trend Rate Effective Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Property, Plant and Equipment [Policy Text Block] Property, Plant and Equipment, Policy [Policy Text Block] 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 Local Phone Number Local Phone Number Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] MPUC [Member] MPUC [Member] Minnesota Public Utilities Commission [Member] Cumulative Effect, Period of Adoption, Adjustment Cumulative Effect, Period of Adoption [Domain] 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] Jointly-Owned Facilities and Assets [Line Items] Jointly Owned Utility Plant Interests [Line Items] Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount Long-term Debt Long-term Debt, Gross FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Public Utilities, Fuel Adjustment Clause Public Utilities, Fuel Adjustment Clause Public Utilities, fuel adjustment clause regulatory liability Reduction for Tax Positions Related to Prior Years Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Schedule II [Text Block] SEC Schedule, 12-09, Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Award Type [Axis] Award Type [Axis] Assumptions Used to Determine Net Periodic Benefit Costs [Abstract] Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] Federal [Member] Internal Revenue Service (IRS) [Member] Weighted-Average Grant Date Fair Value [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Sale of Energy under PSA [Member] Sale of Energy under PSA [Member] Sale of Energy under PSA [Member] Lease Contractual Term [Domain] Lease Contractual Term [Domain] Awarded, Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity Retained Earnings [Member] Retained Earnings [Member] Revisions in Estimated Cash Flows Asset Retirement Obligation, Revision of Estimate Acquisition-Related Contingent Consideration Payments Payment for Contingent Consideration Liability, Financing Activities Transmission [Line Items] Public Utility, Property, Plant and Equipment [Line Items] Components of Net Periodic Cost [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities PSCW [Member] PSCW [Member] Public Service Corporation of Wisconsin [Member] Operating and Maintenance Other General and Administrative Expense Generating Facility Axis [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Award Type [Domain] Award Type [Domain] Current Liabilities Liability, Defined Benefit Plan, Current Operating Expense Equity Method Investment, Summarized Financial Information, Operating Expense The amount of the operating expense reported by an equity method investment of the entity. Costs Incurred, Capitalized Asset Retirement Obligation Costs Costs Incurred, Capitalized Asset Retirement Obligation Costs Amount of asset retirement obligation costs incurred that were capitalized during the period. This may include liabilities incurred and revisions in cash flow estimates. Output Being Purchased - Electric Energy and Capacity (MW) Long-term Contract for Purchase of Electric Energy and Capacity (MW) Long-term Contract for Purchase of Electric Energy and Capacity (MW) Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Property, Plant and Equipment [Text Block] Property, Plant and Equipment Disclosure [Text Block] Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Unearned Grant Award Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Unearned Grant Award, Weighted Average Grant Date Fair Value Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Unearned Grant Award, Weighted Average Grant Date Fair Value (Per Share) Commitments Contingencies and Guarantees 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] Number of Customers Number of Large Power Customers Number of Customers Number of Customers Pension [Member] Pension Plan [Member] Pension Plan [Member] Discount Rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Other Long-Term Debt, 2.97% – 5.75% Due 2020 – 2037 [Member] Other Long-Term Debt [Member] Other long-term debt instruments not specifically identified elsewhere Power Sales Agreement - 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] Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Jointly-Owned Facilities and Assets [Table Text Block] Schedule of Jointly Owned Utility Plants [Table Text Block] Employer Contributions to Other Postretirement Benefit Plans Payment for Pension and Other Postretirement Benefits Accretion Asset Retirement Obligation, Accretion Expense VEBA [Member] VEBA [Member] VEBA (Voluntary Employee Benefits Association trusts) [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Allowed Indebtedness to Total Capitalization Ratio Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Minnesota Solar Energy Standard - Small Scale Solar Mandate Percentage Solar Energy Standard Mandate, Small Scale Solar Mandate Percentage Minnesota's solar energy mandate for small scale solar with photovoltaic devices with a nameplate capacity of 20 kilowatts or less by the end of 2020. Expressed as a percentage of the overall mandate. 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]