Transmission [Abstract] Transmission [Abstract] Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Project Status [Axis] Project Status [Axis] Project Status: in progress or complete [Axis] Project Status [Domain] Project Status [Domain] [Domain] for Project Status: in progress or complete [Axis] Project in Progress [Member] Project in Progress [Member] Project in Progress [Member] Completed Project [Member] [Member] Completed Project [Member] [Member] Completed Project [Member] [Member] Transmission [Axis] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Transmission [Domain] Name of transmission investment. CapX2020 [Member] CapX2020 [Member] CapX2020 (all projects) [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Transmission [Line Items] Transmission [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Number of Projects Number of Projects Number of projects. Estimated Capital Expenditures Estimated Capital Expenditures, Including Past Expenditures The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. Transmission Line Length (Miles) Transmission Line Length (Miles) Length of a transmission line in miles. Transmission Line Capacity (kV) Transmission Line Capacity (kV) Capacity of a transmission line in kilovolts. Capital Cost Spent to Date Capital Cost Spent to Date Capital cost spent to date for project. Pension and Other Postretirement Benefit Plans [Abstract] Pension and Other Postretirement Benefit Plans [Abstract] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Pension and Other Postretirement Benefit Plans [Axis] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Pension and Other Postretirement Benefit Plans [Domain] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Postretirement Health and Life [Member] Other Postretirement Benefit Plan [Member] Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Effect of One Percent Increase on Total of Postretirement Health and Life Service and Interest Cost Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components Effect of One Percent Increase on Postretirement Health and Life Obligation Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation Effect of One Percent Decrease on Total of Postretirement Health and Life Service and Interest Cost Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components Effect of One Percent Decrease on Postretirement Health and Life Obligation Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation Operations and Significant Accounting Policies [Abstract] Operations and Significant Accounting Policies [Abstract] Financial Statement Preparation [Policy Text Block] Basis of Accounting, Policy [Policy Text Block] Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] Principles of Consolidation [Policy Text Block] Consolidation, Policy [Policy Text Block] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Revenue Recognition [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Land Inventory [Policy Text Block] Real Estate Held for Development and Sale, Policy [Policy Text Block] Cash and Cash Equivalents [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] Accounts Receivable [Policy Text Block] Trade and Other Accounts Receivable, Policy [Policy Text Block] Long-Term Finance Receivables [Policy Text Block] Finance, Loan and Lease Receivables, Held-for-investment, Policy [Policy Text Block] Available-for-Sale Securities [Policy Text Block] Marketable Securities, Available-for-sale Securities, Policy [Policy Text Block] Inventories [Policy Text Block] Inventory, Policy [Policy Text Block] Property, Plant and Equipment [Policy Text Block] Property, Plant and Equipment, Policy [Policy Text Block] Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Derivatives [Policy Text Block] Derivatives, Policy [Policy Text Block] Stock-Based Compensation [Policy Text Block] Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Environmental Liabilities [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Unamortized Discount and Premium on Debt [Policy Text Block] Debt, Policy [Policy Text Block] Income Taxes [Policy Text Block] Income Tax, Policy [Policy Text Block] Excise Taxes [Policy Text Block] Excise Tax Policy [Policy Text Block] Policy regarding the collection and recording of excise taxes. Business Combinations Policy [Policy Text Block] Business Combinations Policy [Policy Text Block] Asset Retirement Obligations [Policy Text Block] Asset Retirement Obligations, Policy [Policy Text Block] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Equity Method Investments, Policy [Policy Text Block] Equity Method Investments, Policy [Policy Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Fair Value Transfers [Policy Text Block] Fair Value Transfer, Policy [Policy Text Block] Pension and Other Postretirement Benefit Plans [Policy Text Block] Pension and Other Postretirement Plans, Policy [Policy Text Block] Employee Stock Ownership Plan (ESOP) [Policy Text Block] Employee Stock Ownership Plan (ESOP), Policy [Policy Text Block] Income Tax Expense [Abstract] Income Tax Expense [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Total Current Tax Expense Current Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Total Deferred Tax Expense Deferred Income Tax Expense (Benefit) Total Income Tax Expense Income Tax Expense (Benefit) Investment in ATC [Abstract] Investment in ATC [Abstract] ALLETE's Interest in ATC [Table Text Block] Equity Method Investments [Table Text Block] ATC Summarized Financial Data [Table Text Block] Equity Method Investment, Schedule of Summarized Financial Data [Table Text Block] Tabular disclosure of summarized financial data reported by an equity method investment of the entity. Schedule of Stock by Class [Table] Class of Stock [Axis] Class of Stock [Domain] Common Stock [Member] Class of Stock [Line Items] Common Stock Outstanding, Shares Common Stock, Shares, Outstanding Common Stock Outstanding, Value Common Stock, Value, Outstanding Employee Stock Purchase Program, Shares Stock Issued During Period, Shares, Employee Stock Purchase Plans Employee Stock Purchase Program, Value Stock Issued During Period, Value, Employee Stock Purchase Plan Invest Direct, Shares Stock Issued During Period, Shares, Dividend Reinvestment Plan Invest Direct, Value Stock Issued During Period, Value, Dividend Reinvestment Plan 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. Options and Stock Awards, Value 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. Equity Issuance Program, Shares Stock Issued During Period, Shares, Other Equity Issuance Program, Value Stock Issued During Period, Value, Other Forward Sale Agreement and Issuance, Shares Stock Issued During Period, Shares, Forward Sale Number of shares issued during the period from the forward sales agreement, Forward Sales Agreement and Issuance, Value Stock Issued During Period, Value, Forward Sale Value of shares issued during the period from the forward sales agreement. Contributions to Pension, Shares Stock Issued During Period, Shares, Employee Benefit Plan Contributions to Pension, Value Stock Issued During Period, Value, Employee Benefit Plan 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). 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). Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Acquisitions [Abstract] Acquisitions [Abstract] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Pension [Member] Pension Plan [Member] 2015 Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months 2016 Defined Benefit Plan, Expected Future Benefit Payments, Year Two 2017 Defined Benefit Plan, Expected Future Benefit Payments, Year Three 2018 Defined Benefit Plan, Expected Future Benefit Payments, Year Four 2019 Defined Benefit Plan, Expected Future Benefit Payments, Year Five Years 2020 - 2024 Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter Business Segments [Abstract] Business Segments [Abstract] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Property, Plant and Equipment [Abstract] Property, Plant, and Equipment [Table Text Block] Public Utility Property, Plant, and Equipment [Table Text Block] Estimated Useful Lives of Property, Plant and Equipment [Table Text Block] Property, Plant and Equipment [Table Text Block] Asset Retirement Obligation [Table Text Block] Schedule of Change in Asset Retirement Obligation [Table Text Block] Service Cost Defined Benefit Plan, Service Cost Interest Cost Defined Benefit Plan, Interest Cost Expected Return on Plan Assets Defined Benefit Plan, Expected Return on Plan Assets Amortization of Loss Defined Benefit Plan, Amortization of Gains (Losses) Amortization of Prior Service Cost (Credit) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of Transition Obligation Defined Benefit Plan, Amortization of Transition Obligations (Assets) Effect of Plan Settlement Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements Net Expense (Credit) Defined Benefit Plan, Net Periodic Benefit Cost Investments [Abstract] Investments [Text Block] Investment [Text Block] Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Financial Covenants [Abstract] Required Indebtedness to Total Capital Ratio Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Actual Indebtedness to Total Capital Ratio Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Supplemental Statement of Cash Flow Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Accounts Receivable [Table Text Block] Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Inventories [Table Text Block] Schedule of Inventory, Current [Table Text Block] Prepayments and Other Current Assets [Table Text Block] Schedule of Other Current Assets [Table Text Block] Other Current and Non-Current Liabilities [Table Text Block] Schedule of Other Assets and Other Liabilities [Table Text Block] Forward Contracts [Abstract] Forward Contracts [Abstract] Forward Contract Indexed to Issuer's Equity [Table] Forward Contract Indexed to Issuer's Equity [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Forward Contract Indexed to Issuer's Equity, Type [Axis] Forward Contract Indexed to Issuer's Equity, Type [Axis] Forward Contract Indexed to Issuer's Equity, Type [Domain] Forward Contract Indexed to Issuer's Equity, Type [Domain] Forward Contracts [Member] Forward Contracts [Member] Forward Contract Indexed to Issuer's Equity [Line Items] Forward Contract Indexed to Issuer's Equity [Line Items] Forward Contract Indexed to Issuer's Equity, Indexed Shares Forward Contract Indexed to Issuer's Equity, Indexed Shares Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share Common Stock, Shares, Issued Common Stock, Shares, Issued Proceeds from Issuance or Sale of Equity Proceeds from Issuance or Sale of Equity Forward Contract Indexed to Issuer's Equity, Dilutive effect Forward Contract Indexed to Issuer's Equity, Dilutive effect Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of equity forward arrangements using the treasury stock method. Net (Gain) Loss Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax Prior Service Cost (Credit) Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax Total Unrecognized Costs (Credit) Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax Employee Stock and Incentive Plans [Abstract] Employee Stock and Incentive Plans [Abstract] Employee Stock and Incentive Plans [Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Table] Jointly Owned Utility Plant Interests [Table] Jointly-Owned Facilities Projects [Axis] Jointly Owned Utility Plant [Axis] Jointly-Owned Facilities and Projects [Domain] Jointly Owned Utility Plant [Domain] Boswell Unit 4 [Member] Boswell Unit 4 [Member] CapX2020 Projects [Member] Jointly-Owned Facilities and Projects [Line Items] Jointly Owned Utility Plant Interests [Line Items] % Ownership Jointly Owned Utility Plant, Proportionate Ownership Share Generating Capacity (MW) Capacity of a generating facility, unit, or project in megawatts. % Not Owned Jointly-Owned Electric Facility Percentage Not Owned Jointly-Owned Electric Facility Percentage Not Owned Plant in Service Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service Accumulated Depreciation Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation Construction Work in Progress Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Consolidated Statement of Comprehensive Income Parentheticals [Abstract] Consolidated Statement of Comprehensive Income Parentheticals [Abstract] Unrealized Gain (Loss) on Securities, Taxes Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Unrealized Loss on Derivatives, Taxes Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Taxes Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax Fair Value [Abstract] Fair Value Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value Measures [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Recurring Fair Value Measures [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Investments [Abstract] Available-for-sale - Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Available-for-sale - Corporate Debt Securities Available-for-sale Securities, Debt Securities, Noncurrent Cash Equivalents Investments, Cash and Cash Equivalents, Fair Value Disclosure This element represents the portion of the balance sheet Investments line item's cash and cash equivalents, valued at fair value by the entity. Total Fair Value of Assets Assets, Fair Value Disclosure, Recurring Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Deferred Compensation Deferred Compensation, Fair Value Disclosure This element represents the portion of the balance sheet assertion, Deferred Compensation, valued at fair value by the entity. Derivatives - Interest Rate Swap Interest Rate Cash Flow Hedge Liability at Fair Value Total Fair Value of Liabilities Liabilities, Fair Value Disclosure, Recurring Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Fair Value Activity in Level 3 Fair Value Activity in Level 3 Fair Value Activity in Level 3 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. Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Deferred Tax Assets and Liabilities [Table Text Block] Schedule of Deferred Tax Assets and Liabilities [Table Text Block] NOL and Tax Credit Carryforwards [Table Text Block] Summary of Operating Loss and Tax Credit Carryforwards [Table Text Block] Summary of Operating Loss and Tax Credit Carryforwards [Table Text Block] [Table Text Block] Gross Unrecognized Income Tax Benefits [Table Text Block] Summary of Income Tax Contingencies [Table Text Block] Short-Term and Long-Term Debt [Abstract] SHORT-TERM AND LONG-TERM DEBT [Abstract] Long-term Debt [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Pension and Other Postretirement Benefit Plan Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Obligation and Funded Status [Table Text Block] Schedule of Net Funded Status [Table Text Block] Unrecognized Costs [Table Text Block] Schedule of Net Periodic Benefit Cost Not yet Recognized [Table Text Block] Components of Net Periodic Expense [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] 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] Information for Pension Plans with an Accumulated Benefit Obligation in Excess of Plan Assets [Table Text Block] Schedule of Accumulated Benefit Obligations in Excess of Fair Value of Plan Assets [Table Text Block] Estimated Future Benefit Payments [Table Text Block] Schedule of Expected Benefit Payments [Table Text Block] Defined Benefit Costs Recorded in Regulatory Long-Term Assets or Liabilities and Accumulated Other Comprehensive Income Expected to be Recognized over Next Fiscal Year [Table Text Block] Schedule of Amounts in Accumulated Other Comprehensive Income (Loss) to be Recognized over Next Fiscal Year [Table Text Block] Assumptions Used to Determine Benefit Obligation and Net Periodic Benefit Costs [Table Text Block] Schedule of Assumptions Used [Table Text Block] Sensitivity of a One-Percentage-Point Change in Health Care Trend Rates [Table Text Block] Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates [Table Text Block] Plan Asset Allocations, Actual and Target [Table Text Block] Schedule of Allocation of Plan Assets [Table Text Block] Recurring Fair Value Measures [Table Text Block] Fair Value, Assets Measured on Recurring Basis [Table Text Block] Recurring Fair Value Measures - Activity in Level 3 [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Supplemental Cash Flow Information [Abstract] Cash Paid During the Period for Interest - Net of Amounts Capitalized Interest Paid, Net Cash Paid During the Period for Income Taxes Income Taxes Paid, Net Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment Capital Expenditures Incurred but Not yet Paid Capitalized Asset Retirement Costs Asset Retirement Obligation, Period Increase (Decrease) AFUDC - Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction ALLETE Common Stock Contributed to the Defined Benefit Pension Plan Consolidated Statement of Comprehensive Income [Abstract] Consolidated Statement of Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Gain (Loss) on Securities - Net of Income Taxes of $(0.2), $– and $0.8 Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Unrealized Gain (Loss) on Derivatives - Net of Income Taxes of $0.1, $– and $(0.1) Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Taxes of $(2.8), $3.3 and $3.9 Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Other Comprehensive Income (Loss) Other Comprehensive Income (Loss), Net of Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Less: Non-Controlling Interest in Subsidiaries Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Available-for-sale Securities [Abstract] Cost Available-for-sale Securities, Amortized Cost Basis Gross Unrealized Gain Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Loss Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Available-for-sale Securities, Noncurrent Net Proceeds Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds Gross Realized Gain Available-for-sale Securities, Gross Realized Gains Gross Realized Loss Available-for-sale Securities, Gross Realized Losses Acquisitions [Text Block] Business Combination Disclosure [Text Block] Pension Plan [Member] VEBA [Member] VEBA (Voluntary Employee Benefits Association trusts) [Member] Irrevocable Grantor Trust [Member] Grantor Trust [Member] Irrevocable Grantor Trusts [Member] Defined Benefit Plans, Employer Stock Contributions Defined Benefit Plans, Employer Cash Contributions Pension and Other Postretirement Benefit Contributions Defined Benefit Plans, Estimated Employer Contributions Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year Defined Contribution Plan, Employer Contributions Defined Contribution Plan, Employer Matching Contribution, Amount Amount employer contributed during the period, to match employee contributions, to a defined contribution plan. Includes, but not limited to, contributions made through Employee Stock Ownership Plans (ESOP). Commitments, Guarantees and Contingencies [Abstract] Commitments, Guarantees and Contingencies [Abstract] Commitments, Guarantees and Contingencies [Text Block] Commitments and Contingencies Disclosure [Text Block] Regulatory Matters [Abstract] Regulatory Matters [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Deferred Fuel Adjustment Clause [Member] Deferred Fuel Costs [Member] Defined Benefit Pension and Other Postretirement Plans [Member] Pension and Other Postretirement Plans Costs [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligation [Member] Asset Retirement Obligation Costs [Member] Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] PPACA Income Tax Deferral [Member] Medicare Part D Regulatory Asset [Member] Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). Other [Member] Other Regulatory Assets (Liabilities) [Member] Regulatory Liability [Axis] Regulatory Liability [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Plant Removal Obligations [Member] Removal Costs [Member] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Current Regulatory Asset Regulatory Assets, Current Non-Current Regulatory Asset Regulatory Assets, Noncurrent Total Regulatory Assets Regulatory Assets Non-Current Regulatory Liability Regulatory Liability, Noncurrent Types of Net Regulatory Assets Earning Returns Types of Net Regulatory Assets Earning Returns Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument, Name [Domain] Lines of Credit [Member] Consolidated Lines of Credit [Member] [Member] Consolidated Lines of Credit [Member] includes the lines of credit of the reporting entity and all subsidiaries. ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] Debt Instrument [Line Items] Short-Term Debt Debt, Current Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Lines of Credit, Current Line of Credit, Current Letters of Credit Drawn Letters of Credit Drawn Outstanding draws under lines of credit Long-Term Debt Long-term Debt, Excluding Current Maturities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt, Including current portion Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Long-Term Debt, 2016 Maturities Long-term Debt, Maturities, Repayments of Principal in Year Two Long-Term Debt, 2017 Maturities Long-term Debt, Maturities, Repayments of Principal in Year Three Long-Term Debt, 2018 Maturities Long-term Debt, Maturities, Repayments of Principal in Year Four Long-Term Debt, 2019 Maturities Long-term Debt, Maturities, Repayments of Principal in Year Five Long-Term Debt, Thereafter Maturities Long-term Debt, Maturities, Repayments of Principal after Year Five Proceeds from Issuance of First Mortgage Bond Proceeds from Issuance of First Mortgage Bond First Mortgage Bonds, Interest Rate Debt Instrument, Interest Rate, Stated Percentage Consolidated Statement of Income [Abstract] Consolidated Statement of Income [Abstract] Operating Revenue Revenues Operating Expenses Cost of Revenue [Abstract] Fuel and Purchased Power Cost of Purchased Power Operating and Maintenance Operating Expenses Depreciation Depreciation, Depletion and Amortization Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Other Income (Expense) Nonoperating Income (Expense) [Abstract] Interest Expense Interest Expense Equity Earnings in ATC Income (Loss) from Equity Method Investments Other Other Nonoperating Income (Expense) Total Other Expense Nonoperating Income (Expense) Income Before Non-Controlling Interest and Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest Income Tax Expense Net Income Less: Non-Controlling Interest in Subsidiaries Net Income (Loss) Attributable to Noncontrolling Interest Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent Average Shares of Common Stock and Per Share Data Earnings Per Share [Abstract] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Basic Earnings Per Share of Common Stock Earnings Per Share, Basic Diluted Earnings Per Share of Common Stock Earnings Per Share, Diluted Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared FERC Complaint [Abstract] FERC Complaint [Abstract] FERC Complaint [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] ATC [Member] ATC [Member] ATC [Member] Rainy River Energy [Member] Rainy River Energy [Member] Rainy River Energy, a wholly-owned subsidiary of ALLET, Inc. [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Equity Method Investment, Proposed Return on Common Equity Equity Method Investment, Proposed Return on Common Equity The proposed return on common equity, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting. Equity Method Investment, Approved Return on Common 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. Loss Contingency, Estimate of Possible Loss Loss Contingency, Estimate of Possible Loss Equity Method Investment, Ownership Percentage Equity Method Investment, Ownership Percentage Sensitivity Analysis [Table] Sensitivity Analysis [Table] Sensitivity Analysis [Table] Sensitivity Analysis [Axis] Sensitivity Analysis [Axis] The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Domain] Sensitivity Analysis [Domain] [Domain] for The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Member] Sensitivity Analysis [Member] [Member] for The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Line Items] Sensitivity Analysis [Line Items] [Line Items] for Sensitivity Analysis [Table] Basis Point Reduction on Approved Rate of Return on Common Equity Basis Point Reduction on Approved Rate of Return on Common Equity A sensitivity analysis using basis point reduction. Effect on Future Earnings, Amount Effect on Future Earnings, Amount Consolidated Balance Sheet [Abstract] Consolidated Balance Sheet [Abstract] Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Assets Assets [Abstract] Current Assets Assets, Current [Abstract] Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Accounts Receivable (Less Allowance of $1.1 and $1.1) Accounts Receivable, Net, Current Inventories Inventory, Net Prepayments and Other Other Assets, Current Deferred Income Taxes Deferred Tax Assets, Net, Current Total Current Assets Assets, Current Property, Plant and Equipment - Net Property, Plant and Equipment, Net Regulatory Assets Investment in ATC Equity Method Investments Other Investments Long-term Investments Other Non-Current Assets Other Assets, Noncurrent Total Assets Assets Liabilities and Equity Liabilities and Equity [Abstract] Liabilities Liabilities [Abstract] Current Liabilities Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest Payable, Current Notes Payable Notes Payable, Current Other Other Liabilities, Current Total Current Liabilities Liabilities, Current Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Defined Benefit Pension and Other Postretirement Benefit Plans Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent Other Non-Current Liabilities Other Liabilities, Noncurrent Total Liabilities Liabilities Commitments and Contingencies (Note 12) Commitments and Contingencies Equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 45.9 and 41.4 Shares Outstanding Common Stock, Value, Issued Unearned ESOP Shares Unearned ESOP Shares Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained Earnings Retained Earnings (Accumulated Deficit) Total ALLETE Equity Stockholders' Equity Attributable to Parent Non-Controlling Interest in Subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Total Equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Total Liabilities and Equity Liabilities and Equity Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] ACE Agreement [Member] ACE Agreement [Member] Purchase agreement signed on December 31, 2014. Expected to close in the first quarter of 2015. Business Acquisition [Line Items] Business Acquisition [Line Items] Member's Interest Purchase Agreement Member's Interest Purchase Agreement Member's Interest Purchase Agreement Generating Capacity (MW) Common Stock and Earnings Per Share [Abstract] Common Stock and Earnings Per Share [Abstract] Summary of Common Stock [Table Text Block] Schedule of Common Stock Outstanding Roll Forward [Table Text Block] Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Other Income (Expense) [Abstract] Other Income (Expense) [Abstract] Other Income (Expense) [Text Block] Other Income and Other Expense Disclosure [Text Block] Employee Stock Ownership Plan [Table Text Block] Employee Stock Ownership Plan (ESOP) Disclosures [Table Text Block] Share-Based Compensation Expense [Table Text Block] Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] Non-Qualified Stock Options [Table Text Block] Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block] Stock Options by Exercise Price Range [Table Text Block] Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block] Performance Shares [Table Text Block] Schedule of Nonvested Performance-based Units Activity [Table Text Block] Restricted Stock Units [Table Text Block] Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] Gain on Sale of Available-for-sale Securities Investment and Other Income Other Income Total Other Income Net Loss Defined Benefit Plan, Future Amortization of Gain (Loss) Prior Service Cost (Credit) Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit) Total Pension and Postretirement Health and Life Cost (Credit) Defined Benefit Plan, Amount to be Amortized from Accumulated Other Comprehensive Income (Loss) Next Fiscal Year ALLETE Properties Real Estate Investments, Net Available-for-sale Securities Other Other Long-term Investments Total Other Investments Available-for-sale Corporate Debt Securities, Maturities, One Year or Less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Available-for-sale Corporate Debt Securities, Maturities, One Year to Less Than Three Years Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the first fiscal year through the third fiscal year following the latest fiscal year (maturing in more than one year to less than three years). Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less Than Five Years Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the third fiscal year through the fifth fiscal year following the latest fiscal year (maturing in three years to less than five years). Available-for-sale Corporate Debt Securities, Maturities, Five or More Years 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). ALLETE Properties [Roll Forward] SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward] Land Inventory Beginning Balance SEC Schedule III, Real Estate, Gross Cost of Sales SEC Schedule III, Real Estate, Cost of Real Estate Sold Other ALE Real Estate Other Additions Amount of other additions including deeds to collateralized property and capitalized improvements to real estate investments for entities with a substantial portion of business acquiring and holding investment real estate. Land Inventory Ending Balance Long-Term Finance Receivables (net of allowances of $0.6 and $0.6) Notes, Loans and Financing Receivable, Net, Noncurrent Other Real Estate Investments, Other Total Real Estate Assets Land Impairment Impairment of Real Estate Long-Term Finance Receivables, Allowance Allowance for Notes, Loans and Financing Receivable, Noncurrent Assumptions Used to Determine Benefit Obligation [Abstract] Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] Discount Rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Rate of Compensation Increase Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Trend Rate Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year Ultimate Trend Rate Defined Benefit Plan, Ultimate Health Care Cost Trend Rate Year Ultimate Trend Rate Effective Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate Assumptions Used to Determine Net Periodic Benefit Costs [Abstract] Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Expected Long-Term Return on Plan Assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets Rate of Compensation Increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase Change in Obligation, Discount Rate Defined Benefit Plans, Impact on Benefit Obligation, Discount Rate Amount of change in the defined benefit obligation due to a change in the discount rate used to calculate the defined benefit obligation. Change in Obligation, Mortality Assumption Defined Benefit Plans, Impact on Benefit Obligation, Mortality Assumption Amount of change in the defined benefit obligation due to a change in the mortality assumption used to calculate the defined benefit obligation. Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Financial Instruments [Abstract] Financial Instruments [Abstract] Long-Term Debt, Including Current Portion - Carrying Amount Long-term Debt Long-Term Debt, Including Current Portion - Fair Value Long-term Debt, Fair Value Jointly-Owned Facilities and Projects [Text Block] Jointly-Owned Electric Facility [Text Block] Jointly-Owned Electric Facility [Text Block] Other Income (Expense) [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Deferred Tax Assets [Abstract] Deferred Tax Assets, Gross [Abstract] Employee Benefits and Compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation Property Related Deferred Tax Assets, Property, Plant and Equipment NOL Carryforwards Deferred Tax Assets, Operating Loss Carryforwards Tax Credit Carryforwards Deferred Tax Assets, Tax Credit Carryforwards Power Purchase Agreements Power Purchase Agreements, Deferred Tax Asset Power Purchase Agreements, Deferred Tax Asset Other Deferred Tax Assets, Other Gross Deferred Tax Assets Deferred Tax Assets, Gross Deferred Tax Asset Valuation Allowance Deferred Tax Assets, Valuation Allowance Total Deferred Tax Assets Deferred Tax Assets, Net of Valuation Allowance Deferred Tax Liabilities [Abstract] Deferred Tax Liabilities, Gross [Abstract] Property Related Deferred Tax Liabilities, Property, Plant and Equipment Regulatory Asset for Benefit Obligations Deferred Tax Liabilities, Regulatory Assets Unamortized Investment Tax Credits Accumulated Deferred Investment Tax Credit Partnership Basis Differences Deferred Tax Liabilty, Partnership Basis Differences Deferred Tax Liabilty, Partnership Basis Differences Other Deferred Tax Liabilities, Other Total Deferred Tax Liabilities Deferred Tax Liabilities, Net Net Deferred Income Taxes Deferred Tax Assets, Net Net Current Deferred Tax Assets Deferred Tax Assets, Net of Valuation Allowance, Current Net Long-Term Deferred Tax Liabilities Public Utility, Property, Plant and Equipment [Table] Property, Plant and Equipment by Regulated or Unregulated Status [Axis] Regulation Status [Axis] Property, Plant and Equipment Regulated and Unregulated Operation [Domain] Regulated and Unregulated Operation [Domain] Regulated Utility [Member] Regulated Operation [Member] Investments and Other Operations [Member] Unregulated Operation [Member] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Generation [Member] Electricity Generation Plant, Non-Nuclear [Member] Transmission [Member] Electric Transmission [Member] Distribution [Member] Electric Distribution [Member] Investments and Other Operations [Member] Property, Plant and Equipment, Other Types [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment Public Utilities, Property, Plant and Equipment, Plant in Service Construction Work in Progress Public Utilities, Property, Plant and Equipment, Construction Work in Progress Accumulated Depreciation Public Utilities, Property, Plant and Equipment, Accumulated Depreciation Property, Plant and Equipment - Net Public Utilities, Property, Plant and Equipment, Net Total Property, Plant and Equipment - Net Estimated Useful Lives Property, Plant and Equipment, Useful Life Asset Retirement Obligation [Roll Forward] Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] Beginning Obligation Asset Retirement Obligation Accretion Asset Retirement Obligation, Accretion Expense Liabilities recognized Asset Retirement Obligation, Liabilities Incurred Liabilities settled Asset Retirement Obligation, Liabilities Settled Revisions in estimated cash flows Asset Retirement Obligation, Revision of Estimate Ending Obligation Income Before Non-Controlling Interest and Income Taxes Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Taxes Computed at 35 Percent Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes - Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Regulatory Differences for Utility Plant Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Amount Other Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Effective Income Tax Rate Effective Income Tax Rate Reconciliation, Percent Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Axis] Segments [Axis] Consolidated [Domain] Segments [Domain] Regulated Operations [Member] Regulated Operations [Member] Regulated Operations [Member] Investments and Other [Member] Other Segments [Member] Business Segments [Line Items] Segment Reporting Information [Line Items] Land in Minnesota (in acres) Area of Land Fuel and Purchased Power Expense Operating and Maintenance Expense Depreciation Expense Operating Income (Loss) Other Income Income (Loss) Before Non-Controlling Interest and Income Taxes Income Tax Expense (Benefit) Net Income (Loss) Net Income (Loss) Attributable to ALLETE Total Assets Capital Additions Property, Plant and Equipment, Additions Storm Lake I [Member] Storm Lake I [Member] Storm Lake I [Member] Business Acquisition, Effective Date of Acquisition Business Acquisition, Effective Date of Acquisition Business Acquisition, Name of Acquired Entity Business Acquisition, Name of Acquired Entity Payments to Acquire Businesses, Gross Payments to Acquire Businesses, Gross Assets [Abstract] Cash and Cash Equivalents Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Other Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Property, Plant and Equipment - Net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Other Non-Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Total Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets Liabilities [Abstract] Other Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Power Purchase Agreements Power Purchase Agreement - Noncurrent 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. Other Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Total Liabilities and Non-Controlling Interest Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Net Identifiable Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Restricted Cash and Investments, Noncurrent Restricted Cash and Investments, Noncurrent Goodwill Description Goodwill Description Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Power Purchase Agreements - Current Power Purchase Agreement - Current 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. Business Combination, Acquisition Related Cost Business Combination, Acquisition Related Cost Business Combination, Acquisition Related Costs Business Acquisition, Pro Forma Information, Description Business Acquisition, Pro Forma Information, Description Accumulated Benefit Obligation Defined Benefit Plan, Accumulated Benefit Obligation Change in Benefit Obligation [Roll Forward] Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Obligation, Beginning of Year Defined Benefit Plan, Benefit Obligation Actuarial (Gain) Loss Defined Benefit Plan, Actuarial Gain (Loss) Benefits Paid Defined Benefit Plan, Benefits Paid Participant Contributions Defined Benefit Plan, Contributions by Plan Participants Plan Amendments Defined Benefit Plan, Plan Amendments Plan Curtailments Defined Benefit Plan, Curtailments Settlements Defined Benefit Plan, Settlements, Benefit Obligation Obligation, End of Year Change in Plan Assets [Roll Forward] Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Fair Value, Beginning of Year Defined Benefit Plan, Fair Value of Plan Assets Actual Return on Plan Assets Defined Benefit Plan, Actual Return on Plan Assets Employer Contribution Defined Benefit Plan, Contributions by Employer Participant Contributions Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Benefits Paid Defined Benefit Plan, Benefits Paid, Excluding Key Employees Defined Benefit Plan, Benefits Paid, Excluding Key Employees Fair Value, End of Year Funded Status, End of Year Defined Benefit Plan, Funded Status of Plan Non-Current Assets Defined Benefit Plan, Assets for Plan Benefits, Noncurrent Current Liabilities Pension and Other Postretirement Defined Benefit Plans, Current Liabilities Non-Current Liabilities Other Investments, Irrevocable Grantor Trusts Consolidated Statement of Shareholders' Equity [Abstract] Consolidated Statement of Shareholders' Equity [Abstract] Statement [Table] Statement [Table] Shareholders' Equity Components [Axis] Equity Components [Axis] Total Shareholders' Equity [Domain] Equity Component [Domain] Retained Earnings Retained Earnings [Member] Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) [Member] Unearned ESOP Shares Deferred Compensation, Share-based Payments [Member] Common Stock Statement [Line Items] Statement [Line Items] Balance Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Other Comprehensive Income - Net of Tax Unrealized Gain (Loss) on Securities - Net Unrealized Gain (Loss) on Derivatives - Net Defined Benefit Pension and Other Postretirement Plans - Net Non-Controlling Interest in Subsidiaries Common Stock Issued - Net Stock Issued During Period, Value, New Issues Dividends Declared Dividends, Common Stock ESOP Shares Earned Stock Issued During Period, Value, Employee Stock Ownership Plan Balance Accounts Receivable [Abstract] Accounts Receivable, Net, Current [Abstract] Accounts Receivable, Allowance Allowance for Doubtful Accounts Receivable, Current Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value Common Stock, No Par Value Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Outstanding Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Table Text Block] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument, Name [Domain] 6.94% Series Due 2014 [Member] First Mortgage Bonds, 6.94% Series Due 2014 [Member] 7.70% Series Due 2016 [Member] First Mortgage Bonds, 7.70% Series Due 2016 [Member] 1.83% Series Due 2018 [Member] ALLETE Bonds 1.83% Due April2018 [Member] ALLETE Bonds 1.83% Due April2018 [Member] 8.17% Series Due 2019 [Member] First Mortgage Bonds, 8.17% Series Due 2019 [Member] 5.28% Series Due 2020 [Member] First Mortgage Bonds, 5.28% Series Due 2020 [Member] 4.85% Series Due 2021 [Member] First Mortgage Bonds, 4.85% Series Due 2021 [Member] 3.02% Series Due 2021 [Member] 3.40% Series Due 2022 [Member] 4.95% Pollution Control Series F Due 2022 [Member] First Mortgage Bonds, 4.95% Pollution Control Series F Due 2022 [Member] 6.02% Series Due 2023 [Member] First Mortgage Bonds, 6.02% Series Due 2023 [Member] 3.69% Series Due 2024 [Member] 4.90% Series Due 2025 [Member] First Mortgage Bonds, 4.90% Series Due 2025 [Member] 5.10% Series Due 2025 [Member] First Mortgage Bonds, 5.10% Series Due 2025 [Member] 3.20% Series Due 2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] 5.99% Series Due 2027 [Member] First Mortgage Bonds, 5.99% Series Due 2027 [Member] 3.30% Series Due 2028 [Member] ALLETE Bonds 3.30% Due October2028 [Member] ALLETE Bonds 3.30% Due October2028 [Member] 3.74% Series Due 2029 [Member] 5.69% Series Due 2036 [Member] First Mortgage Bonds, 5.69% Series Due 2036 [Member] 6.00% Series Due 2040 [Member] First Mortgage Bonds, 6.00% Series Due 2040 [Member] 5.82% Series Due 2040 [Member] First Mortgage Bonds, 5.82% Series Due 2040 [Member] 4.08% Series Due 2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] 4.21% Series Due 2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] 4.95% Series Due 2044 [Member] 5.05% Series Due 2044 [Member] 4.39% Series Due 2044 [Member] Unsecured Term Loan Variable Rate Due 2015 [Member] ALLETE Amended Term Loan Variable Rate Due August 2015 [Member] ALLETE Amended Term Loan Variable Rate Due August 2015 [Member] Senior Unsecured Notes 5.99% Due 2017 [Member] Senior Unsecured Notes 5.99% Due 2017 Variable Demand Revenue Refunding Bonds Series 1997 A Due 2015-2020 [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] 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] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] A415SeriesDue2028 [Member] A415SeriesDue2028 [Member] Other Long-Term Debt, 0.08%-7.50% Due 2015-2037 [Member] Other Long-Term Debt, 1.0%-8.0% Due 2012-2037 [Member] Other long-term debt instruments not specifically identified elsewhere Total Long-term Debt Less: Due Within One Year Net Long-Term Debt Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Public Utilities General Disclosures [Table] Public Utilities General Disclosures [Table] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] MPUC 2010 MN Rate Case [Member] 2010 Rate Case [Member] Retail rate increase request filed with the Minnesota Public Utilities Commission (MPUC). FERC-Approved Wholesale Rates [Member] Wholesale rates approved by the Federal Energy Regulatory Commission (FERC). PSCW 2012 Wisconsin Rate Case [Member] 2012 Wisconsin Rate Case [Member] 2012 retail rate increase requested filed with the Public Service Commission of Wisconsin (PSCW). Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Rate Information by Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Electric Rates [Member] Electric Rates [Member] Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Customer Informtaion by Customer Class [Domain] Customer Informtaion by Customer Class [Domain] Customer Informtaion by Customer Class [Domain] Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. North Dakota Wind Development [Axis] North Dakota Wind Development [Axis] Information about each wind project/transmission line in North Dakota. North Dakota Wind Development [Domain] North Dakota Wind Development [Domain] Name of wind project/transmission line in North Dakota. Bison Wind Energy Center [Member] Bison Wind Energy Center [Member] Bison Wind Energy Center [Member] includes all Bison Wind Facilities Bison 4 Wind Project [Member] Bison 4 [Member] Bison 4 [Member] Hydro Operations [Axis] Hydro Operations [Axis] Hydro Operations [Axis] Hydro Operations [Domain] Hydro Operations [Domain] [Domain] for Hydro Operations [Axis] Hydro Project [Member] Hydro Project [Member] Hydro Project [Member] Public Utilities, General Disclosures [Line Items] Public Utilities, General Disclosures [Line Items] Approved Rate of Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Approved Percentage of Capital Structure Related to Equity Public Utilities, Approved Equity Capital Structure, Percentage Number of Customers Number of Customers Number of Customers Length of Notice Required to Terminate (Years) Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract (in years). Common Stock and Earnings Per Share [Text Block] Earnings Per Share [Text Block] Quarterly Financial Data (Unaudited) [Abstract] Quarterly Financial Data (Unaudited) [Abstract] Quarterly Financial Data (Unaudited) [Table Text Block] Schedule of Quarterly Financial Information [Table Text Block] Debt Instrument [Abstract] Debt Instrument [Abstract] Debt Instrument [Table] Term Loan [Member] Loans Payable [Member] Debt Instrument [Line Items] Term Loan Principal Amount Debt Instrument, Face Amount Percentage of Debt Hedged by Interest Rate Derivatives Percentage of Debt Hedged by Interest Rate Derivatives Concentration Risks, Types, No Concentration Percentage [Abstract] Schedule of Customer Information [Table] Schedule of Customer Information [Table] Customer Type [Axis] Customer Type [Axis] Customer Type [Domain] Customer Type [Domain] Large Power Customer [Member] Large Power Customer [Member] Customer Information [Line Items] Customer Information [Line Items] Large Power Customers, Accounts Receivable Accounts Receivable, Gross Major Customer, Percent of Consolidated Revenue Concentration Risk, Percentage Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Award Type [Domain] Equity Award [Domain] Performance Shares [Member] Performance Shares [Member] Restricted Stock Units [Member] Restricted Stock Units (RSUs) [Member] Share-Based Compensation Expense [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-Based Compensation Expense Allocated Share-based Compensation Expense Income Tax Benefit Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Capitalized Share-Based Compensation Expense Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Unrecognized Compensation Cost Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted Average Period for Recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition North Dakota Wind Development [Abstract] North Dakota Wind Development [Abstract] North Dakota Wind Development [Table] North Dakota Wind Development [Table] North Dakota Wind Development [Table] DC Transmission Line from Square Butte [Member] DC Transmission Line from Square Butte [Member] DC Transmission Line from Square Butte [Member] North Dakota Wind Development [Line Items] North Dakota Wind Development [Line Items] [Line Items] for North Dakota Wind Development [Table] Generating Unit Capacity (MW) Wind Development Project [Member] Wind Development Project [Member] Wind Development Project [Member] Contractual Sales Price Contractual Sales Price Contractual Sales Price Construction in Progress, Gross Construction in Progress, Gross Goodwill Goodwill Total Liabilities and Non-Controlling Interest Assumed Deposits Deposits Equity Method Investment, Total Expected Investment for 2015 Equity Method Investment, Total Expected Investment for Current Year The investments expected to be made during the current fiscal year, including any investments made already in the current period/year-to-date or as a subsequent event. Subsequent Events [Abstract] Subsequent Events [Abstract] Subsequent Event [Table] Subsequent Event [Table] Subsequent Event [Line Items] Subsequent Event [Line Items] Cash Investments Payments to Acquire Equity Method Investments ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Beginning Balance Distributed ATC Earnings Proceeds from Equity Method Investment, Dividends or Distributions Equity Investment Ending Balance ATC Summarized Financial Data [Abstract] ATC Summarized Financial Data [Abstract] Balance Sheet Data [Abstract] Equity Method Investment Summarized Financial Information Balance Sheet [Abstract] Equity Method Investment Summarized Financial Information Balance Sheet [Abstract] Current Assets Equity Method Investment, Summarized Financial Information, Current Assets Non-Current Assets Equity Method Investment, Summarized Financial Information, Noncurrent Assets Total Assets Equity Method Investment, Summarized Financial Information, Assets Current Liabilities Equity Method Investment, Summarized Financial Information, Current Liabilities Long-Term Debt Equity Method Investment, Summarized Financial Information, Long-Term Debt Equity Method Investment, Summarized Financial Information, Long-Term Debt Other Non-Current Liabilities Equity Method Investment, Summarized Financial Information, Other Non-Current Liabilities Equity Method Investment, Summarized Financial Information, Other Non-Current Liabilities Members' Equity Equity Method Investment Summarized Financial Information, Equity Total Liabilities and Members' Equity Equity Method Investment, Summarized Financial Information, Liabilities and Equity Income Statement Data [Abstract] Equity Method Investment, Summarized Financial Information, Income Statement [Abstract] Revenue Equity Method Investment, Summarized Financial Information, Revenue 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. Other Expense Equity Method Investment, Summarized Financial Information, Other Expense The amount of other expense reported by an equity method investment of the entity. Net Income Equity Method Investment, Summarized Financial Information, Net Income (Loss) ALLETE's Equity in Net Income Jointly-Owned Facilities and Projects [Table Text Block] Schedule of Jointly Owned Utility Plants [Table Text Block] Consolidated Statement of Cash Flows [Abstract] Consolidated Statement of Cash Flows [Abstract] Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] Allowance for Funds Used During Construction - Equity Income from Equity Investments, Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Loss (Gain) on Sale of Assets / Investments Gains Loss on Sale of Assets / Investments Gain or loss on sale or disposal of other assets. Also included are investments. This element represents the undistributed income (or loss) of equity method investments, net of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporations; such investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities. Depreciation Expense Depreciation Other Amortization Amortization of Financing Costs Amortization of Power Purchase Agreements 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. Deferred Income Tax Expense Share-Based Compensation Expense Share-based Compensation ESOP Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense Defined Benefit Pension and Other Postretirement Benefit Expense Pension and Other Postretirement Benefit Expense Bad Debt Expense Provision for Doubtful Accounts Changes in Operating Assets and Liabilities Increase (Decrease) in Operating Capital [Abstract] Accounts Receivable Increase (Decrease) in Accounts and Other Receivables Inventories Increase (Decrease) in Inventories Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Accounts Payable Increase (Decrease) in Accounts Payable Other Current Liabilities Increase (Decrease) in Other Current Liabilities Cash Contributions to Defined Benefit Pension and Other Postretirement Plans Increase (Decrease) in Pension and Postretirement Obligations Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Investing Activities Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Acquisitions of Subsidiaries – Net of Cash Acquired Payments to Acquire Businesses, Net of Cash Acquired Investment in ATC Payments to Acquire Interest in Subsidiaries and Affiliates Changes to Other Investments Payments for (Proceeds from) Other Investing Activities Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Construction Costs for Development Project Payments for Construction in Process Changes in Escrow for Acquisition Increase (Decrease) in Restricted Cash Proceeds from Sale of Assets Proceeds from Sale of Productive Assets Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from Issuance of Common Stock Proceeds from Issuance of Common Stock Proceeds from Issuance of Long-Term Debt Proceeds from Issuance of Long-term Debt Changes in Restricted Cash Proceeds from (Repayments of) Restricted Cash, Financing Activities Changes in Notes Payable Proceeds from (Repayments of) Notes Payable Reductions of Long-Term Debt Repayments of Long-term Debt Acquisition of Non-Controlling Interest Payments for Repurchase of Redeemable Noncontrolling Interest Construction Deposits Received for Development Project Proceeds from (Payments for) Other Financing Activities Debt Issuance Costs Payments of Debt Issuance Costs Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Cash from Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and Cash Equivalents at Beginning of Period Cash and Cash Equivalents at End of Period Schedule II [Abstract] Schedule II [Abstract] Schedule II [Text Block] Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Text Block] Disclosure of Reclassification Amount [Text Block] Coal Rail and Shipping Contracts [Abstract] Coal Rail and Shipping Contracts [Abstract] Coal Rail and Shipping Contracts [Table] Recorded Unconditional Purchase Obligation [Table] Coal Rail and Shipping Contracts [Axis] Recorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Coal Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Coal and Transportation [Member] Coal and Transportation [Member] Coal and Transportation [Member] Coal Rail and Shipping Contracts [Line Items] Recorded Unconditional Purchase Obligation [Line Items] Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation in 2015 Recorded Unconditional Purchase Obligation Due in Next Twelve Months Boswell Mercury Emission Reduction Plan [Abstract] Boswell Mercury Emission Reduction Plan [Abstract] Schedule of Utility Generating Facilities [Table] Schedule of Utility Generating Facilities [Table] Schedule of Generating Facilities owned by the Utility Generating Facility Axis [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Boswell Unit 4 [Member] Schedule of Utility Generating Facilities [Line Items] Schedule of Utility Generating Facilities [Line Items] [Line Items] for Schedule of Generating Facilities owned by the Utility ACE Wind [Member] ACE Wind [Member] ACE Wind [Member] Storm Lake II [Member] Storm Lake II [Member] Storm Lake II [Member] Lake Benton [Member] Lake Benton [Member] Lake Benton wind facility acquired from The AES Corporation[Member] Condon [Member] Condon [Member] Condon wind facility acquired from The AES Corporation [Member] Purchase Option [Axis] Purchase Option [Axis] Purchase Option [Axis] Purchase Option [Domain] Purchase Option [Domain] [Domain] for Purchase Option [Axis] Armenia Mountain [Member] Armenia Mountain [Member] Armenia Mountain [Member] Business Combination, Reason for Business Combination Business Combination, Reason for Business Combination Business Acquisition, Transaction Costs Long-Term Debt Due Within one Year Long-Term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Non-Controlling Interest Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Purchase option Purchase option Purchase option to acquire entity. Business Combination, Acquisition Related Costs Business Combination, Acquisition Related Costs Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Net (Gain) Loss Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax Prior Service Credit Arising During the Period Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Net Prior Service Cost (Credit) Arising During Period, before Tax Amortization of Prior Service (Cost) Credit Other Comprehensive (Income) Loss, Amortization Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Prior Service Cost (Credit), before Tax Amortization of Loss Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax Amount Recognized due to Plan Settlement Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax Number of Options [Abstract] Number of Options [Abstract] Outstanding as of January 1, Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Outstanding as of December 31, Exercisable as of December 31, Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Weighted Average Exercise Price [Abstract] Weighted Average Exercise Price [Abstract] Outstanding as of January 1, Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Granted Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Exercised Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Forfeited Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Outstanding as of December 31, Exercisable as of December 31, Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Weighted Average Grant-Date Intrinsic Value of Options Granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Grant Date Intrinsic Value Proceeds from Stock Options Exercised Total Intrinsic Value of Options Exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Executive Long-Term Incentive Compensation Plan [Member] Executive Plan [Member] Stock based, long-term incentive compensation, Executive Plan [Member] Employee Stock Purchase Plan (ESPP) [Member] Employee Stock [Member] Stock-based Compensation [Line Items] Common Stock Reserved Common Stock, Capital Shares Reserved for Future Issuance Shares Available for Issuance Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Vesting Period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period ESPP Discount Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date Other Investments and ALLETE Properties [Table Text Block] Investment [Table Text Block] Available-for-sale Securities [Table Text Block] Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Available-for-sale Securities [Table Text Block] Schedule of Realized Gain (Loss) [Table Text Block] Leasing Agreements [Abstract] Leasing Agreements [Abstract] Leasing Agreements [Table] Schedule of Operating Leased Assets [Table] BNI Dragline [Member] Exploration and Production Equipment [Member] Leasing Agreements [Line Items] Operating Leased Assets [Line Items] Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Operating Leases, Future Minimum Payments, Current Operating Leases, Future Minimum Payments Due, Next Twelve Months Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Future Minimum Payments, Due Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Operating Leases, Rent Expense Operating Leases, Rent Expense Plan Asset Categories [Axis] Defined Benefit Plan, Asset Categories [Axis] Plan Asset Categories [Domain] Equity Securities [Member] Equity Securities [Member] Debt Securities [Member] Private Equity [Member] Private Equity Funds [Member] Real Estate [Member] Real Estate [Member] Actual Plan Asset Allocation Defined Benefit Plan, Actual Plan Asset Allocations Employer Stock Included in Equity Securities (in Shares) Defined Benefit Plan, Number of Shares of Equity Securities Issued by Employer and Related Parties Included in Plan Assets Plan Asset Target Allocation Defined Benefit Plan, Target Plan Asset Allocations Gross Unrecognized Income Tax Benefits [Abstract] Gross Unrecognized Income Tax Benefits [Abstract] Reconciliation of Gross Unrecognized Income Tax Benefits [Roll Forward] Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Balance at January 1 Unrecognized Tax Benefits Additions for Tax Positions Related to the Current Year Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Additions for Tax Positions Related to Prior Years Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Reduction for Tax Positions Related to Prior Years Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Reductions for Settlements Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities Lapse of Statute Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations Balance as of December 31 Unrecognized Tax Benefits that Would Impact Effective Tax Rate Unrecognized Tax Benefits, Accrued Interest Unrecognized Tax Benefits, Interest on Income Taxes Accrued Unrecognized Tax Benefits, Interest Expense (Reduction) Unrecognized Tax Benefits, Interest on Income Taxes Expense Unrecognized Tax Benefits, Penalties Unrecognized Tax Benefits, Income Tax Penalties Expense Reasonably Possible Change in Unrecognized Tax Benefits in Next 12 Months Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit Pension and Other Postretirement Benefit Plans Recurring Fair Value Measures [Table] Plan Asset Categories [Domain] U.S. Large-cap [Member] U.S. Large-Cap [Member] Equity Securities, U.S. Large-Cap [Member] U.S. Mid-cap Growth [Member] U.S. Mid-Cap Growth [Member] Equity Securities, U.S. Mid-Cap Growth [Member] U.S. Small-cap [Member] U.S. Small Cap [Member] Equity Securities, U.S. Small Cap [Member] International [Member] Equity Securities, International [Member] Debt Securities [Member] Debt Securities - Mutual Funds [Member] Fixed Income Funds [Member] Debt Securities - Fixed Income [Member] Fixed Income Securities [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash and Cash Equivalents [Member] Cash and Cash Equivalents [Member] Defined Benefit Plan, Fair Value of Plan Assets by Measurement [Axis] Fair Value Plan Asset Measurement [Domain] Fair Value of Assets Derivatives [Abstract] DERIVATIVES [Abstract] Derivatives [Text Block] Derivative Instruments and Hedging Activities Disclosure [Text Block] ESOP Employer Loan, Description Employee Stock Ownership Plan (ESOP), Debt Structure, Employer Loan, Description ESOP Shares, Allocated Employee Stock Ownership Plan (ESOP), Number of Allocated Shares ESOP Shares, Unallocated Employee Stock Ownership Plan (ESOP), Number of Suspense Shares ESOP Shares, Total Employee Stock Ownership Plan (ESOP), Shares in ESOP Fair Value of Unallocated ESOP Shares Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value Range of Exercise Price [Table] Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table] Exercise Price Range [Axis] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] $39.10 to $41.35 [Member] $39.10 to $41.35 [Member] $39.10 to $41.35 Exercise Price Range, Stock Options [Member] $44.15 to $48.65 [Member] $44.15 to $48.65 Exercise Price Range, Stock Options [Member] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] Range of Exercise Price, Lower Range Limit Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit Range of Exercise Price, Upper Range Limit Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit Number Outstanding Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Outstanding Options Weighted Average Remaining Contractual Life (in Years) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Remaining Contractual Term Weighted Average Exercise Price Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Exercise Price Aggregate Intrinsic Value (Millions) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value Number Exercisable Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options Weighted Average Remaining Contractual Life (in Years) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Remaining Contractual Term Weighted Average Exercise Price Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price Aggregate Intrinsic Value (Millions) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value Hydro Operations [Abstract] Hydro Operations [Abstract] Hydro Operations [Table] Hydro Operations [Table] Hydro Operations [Table] Hydro Operations [Line Items] Hydro Operations [Line Items] [Line Items] for Hydro Operations [Table] Quarterly Financial Data (Unaudited) [Text Block] Quarterly Financial Information [Text Block] Transmission [Abstract] Great Northern Transmission Line (GNTL) [Abstract] Great Northern Transmission Line (GNTL) [Abstract] Valuation and Qualifying Accounts and Reserves [Table] Valuation and Qualifying Accounts Disclosure [Table] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves [Domain] Valuation Allowances and Reserves [Domain] Trade Accounts Receivable [Member] Allowance for Trade Receivables [Member] Finance Receivables - Long-Term [Member] Allowance for Losses on Finance Receivables [Member] Deferred Tax Assets [Member] Valuation Allowance of Deferred Tax Assets [Member] Valuation and Qualifying Accounts and Reserves [Line Items] Valuation and Qualifying Accounts Disclosure [Line Items] Balance at Beginning of Period Valuation Allowances and Reserves, Balance Additions, Charged to Income Valuation Allowances and Reserves, Charged to Cost and Expense Additions, Other Charges Valuation Allowances and Reserves, Charged to Other Accounts Deductions from Reserves Valuation Allowances and Reserves, Deductions Balance at End of Period Operating Revenue Regulated and Unregulated Operating Revenue Operating Income Net Income Attributable to ALLETE Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document Information [Table] Document Information [Table] Document Information, Document [Axis] Document Information, Document [Axis] Document [Domain] Document [Domain] Document Information [Line Items] Document Information [Line Items] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Filer Category Entity Filer Category Entity Public Float Entity Public Float Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Changes in Accumulated Other Comprehensive Loss [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Reclassifications from Accumulated Other Comprehensive Loss [Table Text Block] Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments [Rollforward] Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward] Number of Shares [Abstract] Number of Shares [Abstract] As of January 1, Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Awarded Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period 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. Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period As of December 31, Weighted Average Grant Date Fair Value [Abstract] Weighted Average Grant Date Fair Value [Abstract] As of January 1, Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Granted Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Awarded Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value 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) Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value As of December 31, Performance Period Grants, 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. Awards, Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Power Purchase Agreements [Abstract] -- None. No documentation exists for this element. -- Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Power Purchase Agreements [Axis] Long-term Contract for Purchase of Electric Power [Axis] Power Purchase Agreements [Domain] Long-term Contract for Purchase of Electric Power [Domain] Square Butte PPA [Member] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Minnkota PPA [Member] Minnkota PPA [Member] Minnkota PPA [Member] 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. 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. Manitoba Hydro PPA (expires May 2020) [Member] Manitoba Hydro PPA (expires May2020) [Member] Manitoba Hydro PPA (expires May2020) [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. Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Great River Energy PPA (expires 2020) [Member] Great River Energy PPA (expires 2020) [Member] Great River Energy PPA (expires 2020) [Member] Great River Energy Capacity Only PPA (expires 2020) [Member] Great River Energy Capacity Only PPA (expires 2020) [Member] Great River Energy Capacity Only PPA (expires 2020) [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased PPA Counterparty Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Annual Debt Service Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required Power Purchased under Long-term Contracts Utilities Operating Expense, Purchased Power under Long-term Contracts Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges 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. Term of Contract (Years) Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Fixed Capacity Charges Long-Term Contract for Purchase of Electric Power, Fixed Capacity Carges Long-Term Contract for Purchase of Electric Power, Fixed Capacity Carges 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. Accumulated Other Comprehensive Loss [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Component [Domain] Unrealized Gains and Losses on Available-for-sale Securities [Member] Accumulated Net Unrealized Investment Gain (Loss) [Member] Defined Benefit Pension, Other Postretirement Items [Member] Accumulated Defined Benefit Plans Adjustment [Member] Gains and Losses on Cash Flow Hedge [Member] Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Other Income (Expense) - Other [Member] Other Income [Member] Income Tax Expense [Member] Income Tax Expense (Benefit) [Member] Income Tax Expense (Benefit) [Member] Operating Expenses - Operating and Maintenance [Member] Operating Expense [Member] Accumulated Other Comprehensive Loss [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward] Beginning Accumulated Other Comprehensive Loss Other Comprehensive Income (Loss) Before Reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts Reclassified from Accumulated Other Comprehensive Loss Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Net Other Comprehensive Income (Loss) Ending Accumulated Other Comprehensive Loss Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Before Tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Before Tax Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement items, Prior Service Costs, Before Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Actuarial Gains and Losses, Before Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Tax Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax Environmental Matters [Abstract] Environmental Matters [Abstract] Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Environmental Consent Decree [Axis] Environmental Consent Decree [Axis] Environmental Consent Decree [Axis] Environmental Consent Decree [Domain] Environmental Consent Decree [Domain] [Domain] for Environmental Consent Decree [Axis] NOV Consent Decree Obligation [Member] NOV Consent Decree Obligation [Member] NOV Consent Decree Obligation [Member] NOV Consent Decree [Member] NOV Consent Decree [Member] NOV Consent Decree [Member] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Environmental Remediation Expense Environmental Remediation Expense Civil Penalty Civil Penalty Civil Penalty Estimated Capital Expenditures Estimated Capital Expenditures The estimated amount of certain future capital expenditures related to environmental compliance, regulatory matters, or other committments and/or contingencies. Estimated Environmental Compliance Costs Estimated Environmental Compliance Costs Estimated Environmental Compliance Costs Investments Parenthetical [Abstract] Investments Parenthetical [Abstract] Property, Plant and Equipment [Text Block] Property, Plant and Equipment Disclosure [Text Block] Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Average Common Shares Earnings Per Share Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent, Diluted Average Common Shares Earnings Per Share Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Derivative [Table] Derivative [Table] Derivative, by Nature [Axis] Derivative, by Nature [Axis] Derivative, Name [Domain] Derivative, Name [Domain] Interest Rate Swap [Member] Interest Rate Swap [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Other Noncurrent Liabilities [Member] Other Noncurrent Liabilities [Member] Other Current Liabilities [Member] Other Current Liabilities [Member] Derivative [Line Items] Derivative [Line Items] Interest Rate Swap Variable Rate Basis Derivative, Type of Interest Rate Paid on Swap Interest Rate Swap Fixed Interest Rate Derivative, Fixed Interest Rate Interest Rate Cash Flow Hedge Effectiveness Assessment and Measurement Discussion of Interest Rate Cash Flow Hedge Effectiveness Assessment and Measurement Interest Rate Swap Liability Projected Benefit Obligation Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Projected Benefit Obligation Accumulated Benefit Obligation Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Accumulated Benefit Obligation Fair Value of Plan Assets Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets, Aggregate Fair Value of Plan Assets Fair Value [Text Block] Fair Value Disclosures [Text Block] Business Segments [Domain] Subsequent Events [Line Items] Business Acquisition, Acquired Business Number of Customers Business Acquisition, Acquired Business' Number of Customers Business Acquisition, Acquired Business' Number of Customers. The number of customers of an acquired business. Acquired Business, Customer Retention Customer Retention Percentage Customer retention percentage for acquired entity at the acquisition date. Regulated Operations [Abstract] Schedule of Customer Information [Table] Retail Customers [Member] Retail [Member] Retail Customers [Member] Electric [Member] Electric Customer [Member] Natural Gas [Member] Natural Gas Customer [Member] Water [Member] Water Customer [Member] Investments and Other [Abstract] Investments and Other [Abstract] Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement (Percent) Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement The minimum amount of cash collected on the contract price in order to fully recognize profit on the sale of real estate upon closing (company policy). Amortization of Power Purchase Agreements Output Purchased (Percent) Output 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 Accounts Receivable [Abstract] Accounts Receivable Additional Disclosures [Abstract] Trade Accounts Receivable [Abstract] Trade Accounts Receivable [Abstract] Billed Accounts Receivable, Gross, Current Unbilled Unbilled Receivables, Current Less: Allowance for Doubtful Accounts Total Accounts Receivable - Net Inventories [Abstract] Inventory Disclosure [Abstract] Fuel Energy Related Inventory, Coal Materials and Supplies Inventory, Raw Materials and Supplies, Net of Reserves Total Inventories Impairement of Long-Lived Assets [Abstract] Long-Lived Asset Impairment [Abstract] Prepayments and Other Current Assets [Abstract] Prepaid Expense and Other Assets, Current [Abstract] Deferred Fuel Adjustment Clause Deferred Fuel Cost Construction Costs for Development Project Restricted Cash Restricted Cash and Cash Equivalents, Current Other Other Assets, Miscellaneous, Current Total Prepayments and Other Current Assets Prepaid Expense and Other Assets, Current Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Restricted Cash Restricted Cash and Cash Equivalents, Noncurrent Other Current Liabilities [Abstract] Other Liabilities, Current [Abstract] Customer Deposits Customer Deposits, Current Power Purchase Agreements Construction Deposits Received for Development Project Other Other Accrued Liabilities, Current Total Other Current Liabilities Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Income Taxes [Abstract] Income Tax Disclosure [Abstract] More Likely Than Not Definition MoreLikelyThanNotDefined The percentage guideline to determine more-likely-than-not status Other Matters [Abstract] Other Matters [Abstract] Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantor Obligations by Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] BNI Coal Reclamation Liability [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. ALLETE Properties Performance and Maintenance Obligations [Member] ALLETE Properties Performance and Maintenance Obligations [Member] Performance and maintenance obligations to governmental entities to construct improvements in various projects. Town Center Community Development District Obligation [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. Palm Coast Park Community Development District Obligation [Member] Palm Coast Park Community Development District [Member] Community development district obligations for Palm Coast Park District. Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Surety Bond Environmental Loss Contingency, Surety Bond Amount of surety bonds that are available for future payment obligations related to environmental commitments and/or contingencies. Letters of Credit Environmental Loss Contingency, Letters of Credit Amount of letters of credit that are available for the future payment of environmental commitments and/or contingencies. Estimated Reclamation Liability Mine Reclamation and Closing Liability, Noncurrent Environmental Loss Contingency, Surety Bond and Letters of Credit Environmental Loss Contingency, Surety Bond and Letters of Credit Amount of surety bonds outstanding and letters of credit to government entities that are available for future payment obligations related to environmental commitments and contingencies. Performance and Maintenance Obligations Performance and Maintenance Obligations Estimated costs remaining on performance and maintenance obligations. Community Development District Obligation, Bond Special Assessment Bond Community Development Distrcit Obligation, 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. Community Development District Obligation, Bond Term (in 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. Community Development District Obligation, 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. Community Development District Obligation, 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. Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis] Activity in Level 3 [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning Balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Actual Return on Plan Assets Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Purchases, sales, and settlements, net Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, (Sales), Issuances, (Settlements) Ending Balance NOL and Tax Credit Carryforwards [Table] NOL and Tax Credit Carryforwards [Table] NOL and Tax Credit Carryforwards [Table] Income Tax Authority [Axis] Income Tax Authority [Domain] Federal [Member] Internal Revenue Service (IRS) [Member] State [Member] State and Local Jurisdiction [Member] NOL and Tax Credit Carryforwards [Line Items] NOL and Tax Credit Carryforwards [Line Items] NOL Carryforward Operating Loss Carryforwards Tax Credit Carryforwards Tax Credit Carryforward, Amount State Tax Credit Carryforwards, Valuation Allowance Tax Credit Carryforward, Valuation Allowance NOL and Federal Tax Credit Carryforwards, Valuation Allowance Operating Loss and Tax Credit Carryforwards, Valuation Allowance Operating Loss and Tax Credit Carryforwards, Valuation Allowance