Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Text Block] Jointly-Owned Electric Facility [Text Block] Jointly-Owned Electric Facility [Text Block] Quarterly Financial Information Disclosure [Abstract] Operating Revenue Revenues Operating Income Operating Income (Loss) Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent Basic Earnings Per Share of Common Stock Earnings Per Share, Basic Diluted Earnings Per Share of Common Stock Earnings Per Share, Diluted Organization, Consolidation and Presentation of Financial Statements [Abstract] Accounts Receivables [Abstract] Receivables [Abstract] Trade Accounts Receivable [Abstract] Accounts Receivable, Net, Current [Abstract] Billed Accounts Receivable, Gross, Current Unbilled Unbilled Receivables, Current Less: Allowance for Doubtful Accounts Allowance for Doubtful Accounts Receivable, Current Total Accounts Receivable Accounts Receivable, Net, Current Inventories – Net [Abstract] Inventory, Net [Abstract] Fuel Energy Related Inventory, Coal Materials and Supplies Inventory, Supplies, Net of Reserves Raw Materials Inventory, Raw Materials, Gross Work in Progress Inventory, Work in Process, Gross Finished Goods Inventory, Finished Goods, Gross Reserve for Obsolescence Inventory Valuation Reserves Total Inventories Inventory, Net Prepayments and Other Current Assets [Abstract] Prepaid Expense and Other Assets, Current [Abstract] Deferred Fuel Adjustment Clause Deferred Fuel Cost Restricted Cash Restricted Cash and Cash Equivalents, Current Other Other Assets, Miscellaneous, Current Total Prepayments and Other Current Assets Other Assets, Current Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] 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] Taconite Harbor [Member] Taconite Harbor [Member] Taconite Harbor [Member] Boswell Units 1 and 2 [Member] Boswell Units 1 and 2 [Member] Boswell Units 1 and 2 [Member] Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Business Segments [Axis] Segments [Axis] Business Segments [Domain] Segments [Domain] Regulated Operations [Member] Regulated Operations [Member] [Member] Operation of entities that are regulated by governmental organizations for example, but not limited to, local, state, county, country, or other regulatory organizations. ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] Corporate and Other [Member] Corporate and Other [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Net Book Value Property, Plant and Equipment, Net Impairment of Long-Lived Assets [Abstract] Asset Impairment Charges [Abstract] Impairment of Real Estate Impairment of Real Estate Indicators of Impairment Asset Impairment Charges Goodwill and Intangible Assets [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Goodwill [Table] Schedule of Goodwill [Table] Business Segments [Axis] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Goodwill [Line Items] Goodwill [Line Items] Impairment of Goodwill Goodwill, Impairment Loss Discount Rate Fair Value Inputs, Discount Rate Annual Revenue Growth Rate Fair Value Inputs, Long-term Revenue Growth Rate Terminal Growth Rate Terminal Growth Rate Terminal Growth Rate Reporting Unit Fair Value in Excess of Carrying Value Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount Intangible Assets [Abstract] Intangible Assets, Net (Excluding Goodwill) [Abstract] Definite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Definite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Useful Life (Years) Finite-Lived Intangible Asset, Useful Life Indefinite-Lived Intangible Assets [Abstract] Indefinite-Lived Intangible Assets (Excluding Goodwill) [Abstract] Impairment of Intangible Assets Impairment of Intangible Assets (Excluding Goodwill) Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Contract Payment Contract Payment Non-Current Carrying value of a non-current deferred asset related to a payment made as part of a long-term contract. Financing Receivable Financing Receivable, Gross Restricted Cash Restricted Cash and Cash Equivalents, Noncurrent Other Other Assets, Miscellaneous, Noncurrent Total Other Non-Current Assets Other Assets, Noncurrent Contract Payment Made to Cliffs Contract Payment Made to Counterparty Contract payment made to a counterparty as part of a long-term contract. Real Estate Sale Consideration [Abstract] Real Estate Sale Consideration [Abstract] Real Estate Sale Consideration [Abstract] Total Consideration for Land Inventory Noncash or Part Noncash Divestiture, Amount of Consideration Received Down Payment of ALLETE Common Stock (Shares) Stock Repurchased and Retired During Period, Shares Down Payment of ALLETE Common Stock Stock Repurchased and Retired During Period, Value Finance Receivable Term (Years) Financing Receivable, Term Contractual term of a financing receivable. Other Current Liabilities [Abstract] Other Liabilities, Current [Abstract] Customer Deposits Customer Deposits, Current Power Sales Agreements Power Purchase Agreement - Current Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability. Other Other Accrued Liabilities, Current Total Other Current Liabilities Other Liabilities, Current Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent Power Sales Agreements Power Purchase Agreement - Noncurrent Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent liability. Contingent Consideration Business Combination, Contingent Consideration, Liability, Noncurrent Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Other Liabilities, Noncurrent Revenue Recognition [Abstract] Revenue Recognition [Abstract] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Line Items] Segment Reporting Information [Line Items] Alternative Revenue Program, Required Collection Period (Months) Alternative Revenue Program, Required Collection Period Alternative Revenue Program, Required Collection Period Amortization of Power Sales 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. Minimum Cash Collections Requirement for Real Estate Sales (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). Operating Expenses – Other [Abstract] Other Income and Expenses [Abstract] Change in Fair Value of Contingent Consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Total Operating Expenses – Other Other Operating Income (Expense), Net Income Taxes [Abstract] Income Tax Disclosure [Abstract] More-Likely-Than-Not Percentage MoreLikelyThanNotDefined The percentage guideline to determine more-likely-than-not status Investments [Abstract] ALLETE Properties Real Estate Investments, Net Available-for-sale Securities Available-for-sale Securities, Noncurrent Cash Equivalents Other Restricted Assets, Noncurrent Other Other Long-term Investments Total Other Investments Long-term Investments Available-for-sale Corporate Debt Securities, Maturities [Abstract] Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract] One Year or Less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value 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). 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). 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). Impairment of Land Inventory Regulated Operations [Abstract] Regulatory Matters [Table] Public Utilities General Disclosures [Table] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] MPUC [Member] MPUC [Member] Minnesota Public Utilities Commission [Member] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] CIP Triennial Filing [Member] CIP Triennial Filing [Member] Conservation improvement program triennial filing [Member] CIP Consolidated Filing [Member] CIP Annual Filing [Member] Conservation Improvement Program Annual Filing [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] CIP Spending Requirement CIP Spend Requirement Minnesota's Next Generation Act of 2007 introduced a spend goal of 1.5% of net gross operating revenues from service provided in the state on energy CIPs each year. Investment Goal CIP Investment Goal Conservation improvement program Investment Goal approved by MPUC. Actual Spending CIP Actual Spend Conservation improvement program actual annual spend. Investment Goal in 2017 CIP Investment Goal, Next Twelve Months Conservation improvement program Investment Goal for the next twelve months approved by the MPUC. Investment Goal in 2018 CIP Investment Goal, Year Two Conservation improvement program Investment Goal for year two approved by the MPUC. Investment Goal in 2019 CIP Investment Goal, Year Three Conservation improvement program Investment Goal for year three approved by the MPUC. Annual Energy-Savings Goal CIP Energy-Savings Goal Minnesota's Next Generation Act of 2007 introduced an energy-savings goal of 1.5 percent of net gross annual retail electric energy sales beginning with the 2010 program year. Financial Incentive Regulated Operating Revenue, Other 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] Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment The decrease in accounts payable in the current period related to capital additions recognized in prior periods. Increase in Accounts Payable for Capital Additions to Property, Plant and Equipment Capital Expenditures Incurred but Not yet Paid Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations Camp Ripley Solar Project Financing Liabilities Assumed AFUDC–Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction ALLETE Common Stock Contributed to the Defined Benefit Pension Plan Stock Issued During Period, Value, Employee Benefit Plan Contingent Consideration Noncash or Part Noncash Acquisition, Value of Liabilities Assumed Noncash Consideration for Land Inventory [Table] Noncash or Part Noncash Divestitures [Table] Unique Name [Axis] Unique Name [Axis] Noncash Consideration for Land Inventory [Domain] Noncash or Part Noncash Divestiture, Name [Domain] ALLETE Common Stock Received for Land Inventory [Member] ALLETE Common Stock Received for Land Inventory [Member] ALLETE Common Stock Received for Land Inventory [Member] Long-Term Finance Receivable for Land Inventory [Member] Long-Term Finance Receivable for Land Inventory [Member] Long-Term Finance Receivable for Land Inventory [Member] Noncash Consideration for Land Inventory [Line Items] Noncash or Part Noncash Divestitures [Line Items] Consideration for Land Inventory Debt Disclosure [Abstract] Long-term Debt [Table Text Block] Schedule of Long-term Debt Instruments [Table 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] Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Income Statement [Abstract] Operating Expenses [Abstract] Costs and Expenses [Abstract] Fuel and Purchased Power Cost of Purchased Power Transmission Services Cost of Transmission Cost of Sales Cost of Goods and Services Sold Operating and Maintenance Operating Expenses Depreciation and Amortization Depreciation, Depletion and Amortization Taxes Other Than Income Taxes Taxes, Other Other Total Operating Expenses Costs and Expenses Operating Income Other Income (Expense) [Abstract] 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, Noncontrolling Interest Income Tax Expense Income Tax Expense (Benefit) Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Less: Non-Controlling Interest in Subsidiaries Net Income (Loss) Attributable to Noncontrolling Interest Net Income Attributable to ALLETE Average Shares of Common Stock and Per Share Data [Abstract] Earnings Per Share [Abstract] Basic (Shares) Weighted Average Number of Shares Outstanding, Basic Diluted (Shares) Weighted Average Number of Shares Outstanding, Diluted Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Commitments and Contingencies Disclosure [Abstract] Estimated Minimum Annual Payments for Certain Long-Term Commitments [Text Block] Unrecorded Unconditional Purchase Obligations Disclosure [Table Text Block] Investments [Text Block] Investment [Text Block] Equity [Abstract] Summary of Common Stock [Table] Schedule of Stock by Class [Table] Class of Stock [Axis] Class of Stock [Domain] Common Stock [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Summary of Common Stock [Line Items] Class of Stock [Line Items] Balance, Shares Common Stock, Shares, Outstanding Balance, Equity Common Stock, Value, Outstanding Employee Stock Purchase Program, Shares Stock Issued During Period, Shares, Employee Stock Purchase Plans Employee Stock Purchase Program, Equity Stock Issued During Period, Value, Employee Stock Purchase Plan Invest Direct, Shares Stock Issued During Period, Shares, Dividend Reinvestment Plan Invest Direct, Equity 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, Equity Stock Issued During Period, Value, Share-based Compensation and ESOP, Gross Gross value of stock (or other type of equity) issued during the period as a result of any equity-based compensation plan, including employee stock ownership plans (ESOP). Stock issued could result from the issuance of retricted stock units, the exercise of stock options, stock issued under an ESOP and/or other employee benefit plans. Contributions to RSOP, Shares Stock Issued During Period, Shares, Share-based Compensation, Gross Contributions to RSOP, Equity Stock Issued During Period, Value, Share-based Compensation, Gross Equity Issuance Program, Shares Stock Issued During Period, Shares, Other Equity Issuance Program, Equity Stock Issued During Period, Value, Other Received for Sale of Land Inventory, Shares Received for Sale of Land Inventory, Equity Acquisition of Non-Controlling Interest, Shares Stock Issued During Period, Shares, Acquisitions Acquisition of Non-Controlling Interest, Equity Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests 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, Equity 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, Equity 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 Financial Covenants [Abstract] Debt Instruments [Abstract] Allowed Indebtedness to Total Capitalization Ratio Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Actual Indebtedness to Total Capitalization Ratio Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Compliance with Financial Covenants Debt Instrument, Covenant Compliance Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] First Mortgage Bonds - 7.70% Series Due 2016 [Member] 7.70% Series Due 2016 [Member] First Mortgage Bonds, 7.70% Series Due 2016 [Member] First Mortgage Bonds - 1.83% Series Due 2018 [Member] ALLETE Bonds 1.83% Due April2018 [Member] ALLETE Bonds 1.83% Due April2018 [Member] First Mortgage Bonds - 8.17% Series Due 2019 [Member] 8.17% Series Due 2019 [Member] First Mortgage Bonds, 8.17% Series Due 2019 [Member] First Mortgage Bonds - 5.28% Series Due 2020 [Member] 5.28% Series Due 2020 [Member] First Mortgage Bonds, 5.28% Series Due 2020 [Member] First Mortgage Bonds - 2.80% Series Due 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] First Mortgage Bonds - 4.85% Series Due 2021 [Member] 4.85% Series Due 2021 [Member] First Mortgage Bonds, 4.85% Series Due 2021 [Member] First Mortgage Bonds - 3.02% Series Due 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] First Mortgage Bonds - 3.40% Series Due 2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] First Mortgage Bonds - 6.02% Series Due 2023 [Member] 6.02% Series Due 2023 [Member] First Mortgage Bonds, 6.02% Series Due 2023 [Member] First Mortgage Bonds - 3.69% Series Due 2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] First Mortgage Bonds - 4.90% Series Due 2025 [Member] 4.90% Series Due 2025 [Member] First Mortgage Bonds, 4.90% Series Due 2025 [Member] First Mortgage Bonds - 5.10% Series Due 2025 [Member] 5.10% Series Due 2025 [Member] First Mortgage Bonds, 5.10% Series Due 2025 [Member] First Mortgage Bonds - 3.20% Series Due 2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] First Mortgage Bonds - 5.99% Series Due 2027 [Member] 5.99% Series Due 2027 [Member] First Mortgage Bonds, 5.99% Series Due 2027 [Member] First Mortgage Bonds - 3.30% Series Due 2028 [Member] ALLETE Bonds 3.30% Due October2028 [Member] ALLETE Bonds 3.30% Due October2028 [Member] First Mortgage Bonds - 3.74% Series Due 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] First Mortgage Bonds - 3.86% Series Due 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] First Mortgage Bonds - 5.69% Series Due 2036 [Member] 5.69% Series Due 2036 [Member] First Mortgage Bonds, 5.69% Series Due 2036 [Member] First Mortgage Bonds - 6.00% Series Due 2040 [Member] 6.00% Series Due 2040 [Member] First Mortgage Bonds, 6.00% Series Due 2040 [Member] First Mortgage Bonds - 5.82% Series Due 2040 [Member] 5.82% Series Due 2040 [Member] First Mortgage Bonds, 5.82% Series Due 2040 [Member] First Mortgage Bonds - 4.08% Series Due 2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] First Mortgage Bonds - 4.21% Series Due 2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] First Mortgage Bonds - 4.95% Series Due 2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] First Mortgage Bonds - 5.05% Series Due 2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] First Mortgage Bonds - 4.39% Series Due 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] Unsecured Term Loan Variable Rate Due 2017 [Member] ALLETE Term Loan Variable Rate Due 2017 [Member] ALLETE Term Loan Variable Rate Due 2017 [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 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] Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006 Due 2025 [Member] Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] Armenia Mountain Senior Secured Notes 3.26% Due 2024 [Member] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] A415SeriesDue2028 [Member] A415SeriesDue2028 [Member] Other Long-Term Debt, 3.11% – 6.20% Due 2017 – 2037 [Member] Other Long-Term Debt [Member] Other long-term debt instruments not specifically identified elsewhere Debt Instrument [Line Items] Long-term Debt Long-term Debt, Gross Interest Rate Debt Instrument, Interest Rate, Stated Percentage Unamortized Debt Issuance Costs Debt Issuance Costs, Net Total Long-Term Debt Long-term Debt Less: Due Within One Year Long-term Debt, Current Maturities Net Long-Term Debt Long-term Debt, Excluding Current Maturities Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Share-Based Compensation Expense [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 Costs 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 (Years/Months) Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Equity Method Investments and Joint Ventures [Abstract] Investment in ATC [Table] Schedule of Equity Method Investments [Table] Equity Method Investment [Axis] Investment, Name [Axis] Equity Method Investment [Domain] Investment, Name [Domain] ATC [Member] ATC [Member] ATC [Member] FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Return on Equity Complaint 2 [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] 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. Expense Presentation [Axis] Expense Presentation [Axis] Expense Presentation [Axis] Expense Presentation [Domain] Expense Presentation [Domain] [Domain] for Expense Presentation [Axis] After-tax [Member] After-tax [Member] After-tax [Member] Investment in ATC [Line Items] Schedule of Equity Method Investments [Line Items] Ownership Percentage Equity Method Investment, Ownership Percentage Expected Additional Investment in 2017 Equity Method Investment, Expected Additional Investment Next Fiscal Year Additional investments expected to be made in the next fiscal year in an equity method investment. ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Beginning Balance Equity Method Investments Cash Investments Payments to Acquire Equity Method Investments Distributed ATC Earnings Proceeds from Equity Method Investment, Dividends or Distributions Equity Investment Ending Balance Equity Method Investment, Summarized Financial Information [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 Authorized Return on Equity Equity Method Investment, Approved Return on Common Equity The approved percentage return on common equity dedicated to the utility plant in the investee accounted for under the equity method of accounting. Authorized Return on Equity, Including Incentive Adder Equity Method Investment, Approved Return on Common Equity, Including Incentive Adder The approved percentage return on common equity, including an incentive adder for participation in a regional transmission organization dedicated to the utility plant in the investee accounted for under the equity method of accounting. Proposed Return on 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. Proposed Return on Equity, Including Incentive Adder Equity Method Investment, Proposed Return on Common Equity, Including Incentive Adder The proposed return on common equity, including an incentive adder for participation in a regional transmission organization, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting. 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. Annual Effect on Future Equity Earnings in ATC Effect on Future Earnings, Amount Compensation and Retirement Disclosure [Abstract] Pension and Other Postretirement Benefit Plan Disclosures [Table] Schedule of Defined 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] Pension [Member] Pension Plan [Member] Postretirement Health and Life [Member] Other Postretirement Benefit Plan [Member] 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 Percent 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 Actual and Target Allocations [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] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] VEBA [Member] VEBA [Member] VEBA (Voluntary Employee Benefits Association trusts) [Member] Irrevocable Grantor Trust [Member] Trust for Benefit of Employees [Member] Pension Plan [Member] Employer Contributions Pension and Other Postretirement Benefit Contributions Employer Contributions Expected Employer Contributions in 2017 Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year Defined Contribution RSOP, 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). Defined Benefit Plan, Plan Amendments Defined Benefit Plan, Description of Plan Amendment Integrated Resource Plan [Member] Integrated Resource Plan [Member] Integrated Resource Plan which contains a utility's analysis of existing and future energy resource alternatives and a projection of customer cost impact by class [Member] Natural Gas-Fired [Member] Natural Gas Processing Plant [Member] Generating Capacity (MW) Capacity of a generating facility, unit, or project in megawatts. Common Stock and Earnings Per Share [Text Block] Stockholders' Equity Note Disclosure [Text Block] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Net Loss Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax Prior Service Credit Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax Total Unrecognized Cost (Credit) Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax Stock-Based Compensation [Table] 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] Non-Qualified Stock Options [Member] Employee Stock Option [Member] Employee Stock Purchase Plan (ESPP) [Member] Employee Stock [Member] Vesting [Axis] Vesting [Axis] Vesting [Domain] Vesting [Domain] Qualified Retirement, Death or Disability [Member] Qualified Retirement, Death or Disability [Member] Qualified Retirement, Death or Disability [Member] Voluntary Termination or Involuntary Termination Without Cause [Member] Voluntary Termination or Involuntary Termination Without Cause [Member] Voluntary Termination or Involuntary Termination Without Cause [Member] Stock-based Compensation [Line Items] Common Stock Reserved (Shares) 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 Requisite Service Period (Years) Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period Annual Vesting Percentage Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Vesting Period (Years) Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Exercise Period (Years/Months) Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period ESPP Discount Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date Business Combinations [Abstract] Schedule of Business Acquisitions [Table] Schedule of Business Acquisitions, by Acquisition [Table] 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 Acquisition [Axis] Business Acquisition [Axis] Business Acquisition [Domain] Business Acquisition, Acquiree [Domain] Lake Benton [Member] Lake Benton [Member] Lake Benton wind facility acquired from The AES Corporation[Member] Storm Lake II [Member] Storm Lake II [Member] Storm Lake II [Member] Condon [Member] Condon [Member] Condon wind facility acquired from The AES Corporation [Member] ACE Wind [Member] ACE Wind [Member] ACE Wind [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Name of Acquired Entity Business Acquisition, Name of Acquired Entity Payments to Acquire Business Payments to Acquire Businesses, Gross Number of Wind Energy Facilities Acquired Number of Wind Energy Facilities Acquired Number of Wind Energy Facilities Acquired Assets Acquired [Abstract] 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 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 Assumed [Abstract] Liabilities [Abstract] Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Long-Term Debt Due Within One Year Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Long-Term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Power Sales Agreements - Non-Current Liability Other Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Non-Controlling Interest Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Total Liabilities and Non-Controlling Interest Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities and Non-Controlling Interest Assumed Amount of liabilities and non-controlling interest assumed at the acquisition date. Net Identifiable Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Purchase option Purchase option Purchase option to acquire entity. Goodwill Goodwill Tax Deductible Goodwill Business Acquisition, Goodwill, Expected Tax Deductible Amount Power Sales Agreements - Current Liability Acquisition-Related Costs Business Combination, Acquisition Related Costs Purchase of Non-Controlling Interest Gain or Loss Recognized in Net Income or Other Comprehensive Income Other Nonoperating Gains (Losses) Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Domain] Guarantee Entity and Type [Domain] [Domain] for Guarantee Entity and Type [Axis] BNI Energy Reclamation Liability [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. ALLETE Properties Development and Maintenance Obligations [Member] ALLETE Properties Performance and Maintenance Obligations [Member] Performance and maintenance obligations to governmental entities to construct improvements in various projects. Town Center District Capital Improvement Bonds [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. Palm Coast Park District Special Assessment Bonds [Member] Palm Coast Park Community Development District [Member] Community development district obligations for Palm Coast Park District. Guarantor Obligations by Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Letters of Credit [Member] Financial Standby Letter of Credit [Member] Surety Bonds [Member] Surety Bond [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Collateral Guarantor Obligations, Collateral Held Directly or by Third Parties, Amount Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Bonds Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability 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. Bond Term (Years) Community Development District Obligation, Bond Term, in Years Term (in years) of capital improvement revenue and special assessment bonds used to fund real estate development projects. Ownership Percentage of Benefited Property Community Development District Obligation, Ownership of Property Benefited Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. Annual Assessment Community Development District Obligation, Annual Assessment Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. Water & Energy Systems Technology [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] Chanarambie/Viking [Member] Chanarambie/Viking [Member] Chanarambie/Viking [Member] A and W Technologies [Member] A and W Technologies, Inc. [Member] A and W Technologies, Inc. [Member] Storm Lake I [Member] Storm Lake I [Member] Storm Lake I [Member] Schedule of Business Acquisitions [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Transmission [Table] Public Utility, Property, Plant and Equipment [Table] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Power Purchase Agreements [Axis] Long-term Contract for Purchase of Electric Power [Axis] Long-term Contract for Purchase of Electric Power [Domain] Long-term Contract for Purchase of Electric Power [Domain] Manitoba Hydro PPA (expires May 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Transmission [Axis] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Transmission [Domain] Name of transmission investment. Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Transmission [Line Items] Public Utility, Property, Plant and Equipment [Line Items] Output Being Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased Transmission Line Length (Miles) Transmission Line Length (Miles) Length of a transmission line in miles. Transmission Line Capacity (kV) Transmission Line Capacity (kV) Capacity of a transmission line in kilovolts. Total Project Cost in the U.S. 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. Forward Sale Agreement and Issuance of Common Stock [Table] Forward Contract Indexed to Issuer's Equity [Table] 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 Sale Agreement [Member] Forward Contracts [Member] Option to Purchase Additional Shares [Member] Options Held [Member] Forward Contract Indexed to Issuer's Equity [Line Items] Forward Contract Indexed to Issuer's Equity [Line Items] Indexed Shares Forward Contract Indexed to Issuer's Equity, Indexed Shares Forward Rate Per Share Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share Shares Issued Common Stock, Shares, Issued Proceeds from Issuance of Common Stock Proceeds from Issuance or Sale of Equity Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] 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 Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount 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, Other, Amount Change in Fair Value of Contingent Consideration Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Total Income Tax Expense Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Reason for Business Acquisitions Business Combination, Reason for Business Combination Pro Forma Impact of Business Acquisitions Business Acquisition, Pro Forma Information, Description Fair Value Disclosures [Abstract] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Recurring Fair Value Measures - Activity in Level 3 [Table Text Block] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Number of Shares [Rollforward] Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward] 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 Performance Period (Years) Granted, Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Total Grant Date Fair Value The total grant date fair value of equity-based awards other than options, granted during the period. Awarded, Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Weighted-Average Grant Date Fair Value [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] 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 Acquisitions [Text Block] Business Combination Disclosure [Text Block] Recurring Fair Value Measurements [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] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Domain] Contingent Consideration [Domain] [Domain] for Contingent Consideration [Axis] Recognition of U.S. Water Services Contingent Consideration [Member] Acquired in Business Combination [Member] Acquired in Business Combination [Member] Accretion [Member] Accretion [Member] Accretion expense [Member] Payments [Member] Payments [Member] Payments made that decrease contingent consideration liability [Member] Changes in Cash Flow Projections [Member] Changes in Cash Flow Projections [Member] Changes in Cash Flow Projections [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 and Governmental Debt Securities Available-for-sale Securities, Debt Securities, Noncurrent Total Fair Value of Assets Assets, Fair Value Disclosure, Recurring Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Deferred Compensation Deferred Compensation Liability, Classified, Noncurrent U.S. Water Services Contingent Consideration Total Fair Value of Liabilities Liabilities, Fair Value Disclosure, Recurring Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Activity in Level 3 [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning Balance Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value Activity in Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Ending Balance 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. ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] Concentration Risks, Types, No Concentration Percentage [Abstract] Concentration Risk [Table] Concentration Risk [Table] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Customer Concentration Risk [Member] Customer Concentration Risk [Member] Customer Type [Axis] Customer Type [Axis] Customer Type [Domain] Customer Type [Domain] Large Power Customers [Member] Large Power Customer [Member] Large Power Customer [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Largest Customer [Member] Largest Customer [Member] Largest Customer [Member] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Consolidated Operating Revenue [Member] Sales Revenue, Net [Member] Concentration Risk [Line Items] Concentration Risk [Line Items] Number of Customers Number of Customers Number of Customers Accounts Receivable Accounts Receivable, Gross Percent of Consolidated Operating Revenue Concentration Risk, Percentage Property, Plant and Equipment [Text Block] Property, Plant and Equipment Disclosure [Text Block] Statement of Stockholders' Equity [Abstract] Consolidated Statement of Equity [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Total Equity [Domain] Equity Component [Domain] Retained Earnings [Member] Retained Earnings [Member] Accumulated Other Comprehensive Loss [Member] AOCI Attributable to Parent [Member] Unearned ESOP Shares [Member] Deferred Compensation, Share-based Payments [Member] Common Stock [Member] Non-Controlling Interest in Subsidiaries [Member] Noncontrolling Interest [Member] Consolidated Statement of Equity [Line Items] Statement [Line Items] Consolidated Statement of Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Recognition of Non-Controlling Interest Noncontrolling Interest, Increase from Business Combination Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Net Income Other Comprehensive Income – Net of Tax Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Loss on Securities Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Unrealized Gain on Derivatives Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Defined Benefit Pension and Other Postretirement Plans Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Common Stock Issued Stock Issued During Period, Value, New Issues Common Stock Retired Dividends Declared Dividends, Common Stock ESOP Shares Earned Stock Issued During Period, Value, Employee Stock Ownership Plan Acquisition of Non-Controlling Interest Ending Balance Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis] Defined Benefit Plan, Asset Categories [Axis] Plan Asset Categories [Domain] Plan Asset Categories [Domain] Private Equity Funds [Member] Real Estate [Member] Real Estate [Member] 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 Carryforwards Operating Loss Carryforwards Tax Credit Carryforwards Tax Credit Carryforward, Amount Tax Credit Carryforwards, Valuation Allowance Tax Credit Carryforward, Valuation Allowance NOL Carryforwards, Valuation Allowance Operating Loss Carryforwards, Valuation Allowance 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 Health Care Trend Rates [Abstract] Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] 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 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 Net Loss Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) 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 Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax New Accounting Standards [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Consolidated Statement of Income [Axis] Income Statement Location [Axis] Consolidated Statement of Income [Domain] Income Statement Location [Domain] Income Tax Expense (Benefit) [Member] Income Tax Expense [Member] Income Tax Expense [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Presentation of Debt Issuance Costs [Member] Accounting Standards Update 2015-03 [Member] Improvements to Employee Share-Based Payment Accounting [Member] Accounting Standards Update 2016-09 [Member] Accounting Standards Update 2016-09 [Member] Consolidated Balance Sheet [Axis] Balance Sheet Location [Axis] Consolidated Balance Sheet [Domain] Balance Sheet Location [Domain] Total Assets [Member] Assets, Total [Member] Total Liabilities [Member] Liabilities, Total [Member] New Accounting Standards [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Effect of the Adoption New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification 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 Gain on Sales of Investments and Property, Plant and Equipment Gain (Loss) on Disposition of Assets Depreciation Expense Depreciation Amortization of Power Sales Agreements Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Expense Deferred Income Tax Expense (Benefit) Share-Based Compensation Expense Share-based Compensation ESOP Compensation Expense Other Noncash 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 Plans Pension Contributions 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 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 Cash in Escrow for Acquisition Proceeds from Previous Acquisition Proceeds from Sale of Property, Plant and Equipment Proceeds from Sale of Productive Assets Changes in Restricted Cash Increase (Decrease) in Restricted Cash 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 Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition of Non-Controlling Interest Payments to Acquire Additional Interest in Subsidiaries Construction Deposits Received for Development Project Increase (Decrease) in Other Deposits Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Other Financing Activities Proceeds from (Payments for) Other Financing Activities Cash from (for) 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 Carrying Value Cash and Cash Equivalents at End of Period Power Purchase Agreements [Domain] Chanarambie/Viking PPA (expires 2018) [Member] Chanarambie/Viking PPA (expires 2018) [Member] Chanarambie/Viking PPA (expires 2018) [Member] Chanarambie/Viking PPA (expires 2023) [Member] Chanarambie/Viking PPA (expires 2023) [Member] Chanarambie/Viking PPA (expires 2023) [Member] Percentage of Voting Interests Acquired Business Acquisition, Percentage of Voting Interests Acquired Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets Total Assets Acquired Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Non-Current Liabilities Total Liabilities Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Deferred Tax Assets and Liabilities [Abstract] Components of Deferred Tax Assets and Liabilities [Abstract] Deferred Tax Assets [Abstract] Deferred Tax Assets, Net of Valuation Allowance [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 Sales 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 Liability, Partnership Basis Differences Other Deferred Tax Liabilities, Other Total Deferred Tax Liabilities Deferred Tax Liabilities, Gross Net Deferred Income Taxes Deferred Tax Liabilities, Net Leasing Agreements [Table] Schedule of Operating Leased Assets [Table] Leasing Agreements [Axis] Property, Plant and Equipment, Type [Axis] Leasing Agreements [Domain] Property, Plant and Equipment, Type [Domain] BNI Energy Dragline [Member] Exploration and Production Equipment [Member] Leasing Agreements [Line Items] Operating Leased Assets [Line Items] Minimum Lease Payments in 2017 Operating Leases, Future Minimum Payments Due, Next Twelve Months Minimum Lease Payments in 2018 Operating Leases, Future Minimum Payments, Due in Two Years Minimum Lease Payments in 2019 Operating Leases, Future Minimum Payments, Due in Three Years Minimum Lease Payments in 2020 Operating Leases, Future Minimum Payments, Due in Four Years Minimum Lease Payments in 2021 Operating Leases, Future Minimum Payments, Due in Five Years Minimum Lease Payments Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Total Lease Expense Operating Leases, Rent Expense Jointly-Owned Facilities and Projects [Table Text Block] Schedule of Jointly Owned Utility Plants [Table Text Block] 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 – Net [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 Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Operating Expenses – Other [Table Text Block] Schedule of Other Operating Cost and Expense, by Component [Table Text Block] Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value Schedule of Indefinite-Lived Intangible Assets [Table] Schedule of Indefinite-Lived Intangible Assets [Table] Indefinite-Lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-Lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] Trademarks and Trade Names [Member] Trademarks and Trade Names [Member] Indefinite-Lived Intangible Assets [Line Items] Indefinite-lived Intangible Assets [Line Items] Indefinite-Lived Intangible Assets [Roll Forward] Indefinite-lived Intangible Assets [Roll Forward] Beginning Balance Indefinite-Lived Intangible Assets (Excluding Goodwill) Additions Indefinite-lived Intangible Assets Acquired Ending Balance Schedule of Definite-Lived Intangible Assets [Table] Definite-Lived Intangible Assets [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Definite-Lived Intangible Assets [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Customer Relationships [Member] Customer Relationships [Member] Developed Technology and Other [Member] Other Intangible Assets [Member] Weighted Average [Member] Weighted Average [Member] Definite-Lived Intangible Assets [Roll Forward] Finite-lived Intangible Assets [Roll Forward] Beginning Balance Finite-Lived Intangible Assets, Net Additions Finite-lived Intangible Assets Acquired Amortization Amortization of Intangible Assets Ending Balance Definite-Lived Intangible Assets, Estimated Annual Amortization Expense [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] 2017 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 2018 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Total Intangible Assets Intangible Assets, Net (Excluding Goodwill) Total Intangible Assets, Additions Intangible Assets Acquired Amount of increase in assets, excluding financial assets and goodwill, lacking physical substance, resulting from a business combination. Total Intangible Assets, Amortization Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Minnesota Solar Energy Standard [Member] Minnesota Solar Energy Standard [Member] Minnesota Solar Energy Standard [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Solar Energy Standard Mandate - Overall Mandate Percentage Solar Energy Standard Mandate, Overall Mandate Percentage Minnesota's solar energy mandate for total retail electric sales, excluding sales to certain industrial customers, by the end of 2020. Solar Energy Standard Mandate - Small Scale Solar Mandate Percentage Solar Energy Standard Mandate, Small Scale Solar Mandate Percentage Minnesota's solar energy mandate for small scale solar with photovoltaic devices with a nameplate capacity of 20 kilowatts or less by the end of 2020. Expressed as a percentage of the overall mandate. Solar Energy Standard Mandate - Qualifying Capacity for Small Scale Solar Mandate (MW) Solar Energy Standard Mandate, Qualifying Capacity for Small Scale Solar Mandate The maximum amount of nameplate capacity of a solar device to qualify for counting towards the small scale solar mandate of Minnesota's solar energy standard. Number of Completed Projects Number of Projects Number of projects. Solar Energy Standard Mandate - Percentage of Overall Mandate Expected to be Met with Current Filings or Projects Solar Energy Standard Mandate, Percentage of Overall Mandate Expected to be Met with Current Filings or Projects The percentage of the solar energy standard overall mandate expected to be met with current filings and projects. Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Total Liabilities Assumed Restricted Cash - Non-Current Segment Reporting [Abstract] Total [Domain] Eliminations [Member] Consolidation, Eliminations [Member] Wind Construction Project [Member] Wind Construction Project [Member] Wind Construction Project [Member] Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments Number of Reportable Segments Number of Reportable Segments Number of Operating Segments Number of Operating Segments Land in Minnesota (Acres) Area of Land Net Income (Loss) Attributable to ALLETE Operating Expenses – Other Other Cost and Expense, Operating Interest Expense Income Tax Expense (Benefit) Assets Assets Capital Expenditures Property, Plant and Equipment, Additions Certificate of Need and Route Permit [Member] Great Northern Transmission Line Certificate of Need and Route Permit [Member] Great Northern Transmission Line Certificate of Need and Route Permit [Member] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Square Butte [Member] Square Butte [Member] Square Butte [Member] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Great River Energy [Member] Great River Energy [Member] Great River Energy [Member] Oliver Wind [Member] Oliver Wind [Member] Oliver Wind [Member] Shell Energy [Member] Shell Energy [Member] Shell Energy [Member] TransAlta [Member] TransAlta [Member] TransAlta [Member] Basin [Member] Basin [Member] Basin [Member] Silver Bay Power [Member] Silver Bay Power [Member] Silver Bay Power [Member] Square Butte PPA (expires December 2026) [Member] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Great River Energy Capacity and Energy PPA (expires May 2020) [Member] Great River Energy PPA (expires 2020) [Member] Great River Energy Capacity and Energy PPA (expires 2020) [Member] Great River Energy Capacity PPA Beginning June 2016 (expires May 2020) [Member] Great River Energy Capacity Only PPA Beginning 2016 (expires 2020) [Member] Great River Energy Capacity Only PPA (begins 2016/expires 2020) [Member] Great River Energy Capacity PPA Beginning June 2017 (expires May 2020) [Member] Great River Energy Capacity Only PPA Beginning 2017 (expires 2020) [Member] Great River Energy Capacity Only PPA Beginning 2017 (expires 2020) [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 Beginning June 2015 (expires May 2020) [Member] Manitoba Hydro PPA (expires May2020) [Member] Manitoba Hydro PPA (expires May2020) [Member] Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member] Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member] Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Minnkota Power PPA (expires May 2020) [Member] Minnkota PPA [Member] Minnkota PPA [Member] Oliver Wind I PPA (expires December 2031) [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 (expires December 2032) [Member] Oliver Wind II PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. Shell Energy PPA (expires December 2019) [Member] Shell Energy PPA [Member] Contract with Shell Energy North America, for the purchase of energy [Member] TransAlta Off-Peak Hours PPA (expires December 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta On-Peak Hours PPA (expire December 2019) [Member] TransAlta On-Peak Hours PPA (expire 2019) [Member] TransAlta On-Peak Hours PPA (expires 2019) Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Minnkota Power PSA (expires December 2026) [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Basin PSA (expires April 2020) [Member] Basin Power Sales Agreement (expires April 2020) [Member] Basin Power Sales Agreement (expires April 2020) [Member] Basin PSA (expires June 2018) [Member] Basin Power Sales Agreement (expires 2018) [Member] Basin Power Sales Agreement (expires 2018) [Member] Silver Bay Power PSA through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Silver Bay Power Self-Generation [Member] Silver Bay Power Coal, Liquid and Gaseous Fuels-fired Units [Member] Silver Bay Power Coal, Liquid and Gaseous Fuels-fired Units [Member] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Generating Unit Capacity (MW) Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased PPA Counterparty Total Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding PPA Counterparty Annual Debt Service Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required Cost of Purchased Power Utilities Operating Expense, Purchased Power under Long-term Contracts Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges 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. Output Being Sold (MW) Long-term Contract for Sale of Electric Power, Share of Plant Output Being Sold (MW) Share of plant output being purchased in megawatts under a long-term contract to sell electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Contract Term (Years) Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Commitments, Guarantees and Contingencies [Text Block] Commitments and Contingencies Disclosure [Text Block] Valuation and Qualifying Accounts [Abstract] Schedule II [Text Block] Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] 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] Fair Value [Text Block] Fair Value Disclosures [Text Block] Range of Exercise Price [Table] Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table] Exercise Price [Axis] Exercise Price Range [Axis] Exercise Price [Domain] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Domain] $39.10 [Member] $39.10 [Member] $39.10 Exercise Price Range, Stock Options [Member] $48.65 [Member] $48.65 Exercise Price Range, Stock Options [Member] Range of Exercise Price [Line Items] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items] Lower Range Limit Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit Upper Range Limit Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit Options Outstanding and Exercisable [Abstract] Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract] Number Outstanding (Shares) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Outstanding Options Number Exercisable (Shares) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options Outstanding - Weighted Average Remaining Contractual Life (Years) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Remaining Contractual Term Exercisable - Weighted Average Remaining Contractual Life (Years) Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Remaining Contractual Term Outstanding - Weighted Average Exercise Price Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Exercise Price Exercisable - Weighted Average Exercise Price Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price Outstanding - Aggregate Intrinsic Value (Millions) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value Exercisable - Aggregate Intrinsic Value (Millions) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Aggregate Intrinsic Value Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Estimated Future Benefit Payments [Abstract] Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract] 2017 Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months 2018 Defined Benefit Plan, Expected Future Benefit Payments, Year Two 2019 Defined Benefit Plan, Expected Future Benefit Payments, Year Three 2020 Defined Benefit Plan, Expected Future Benefit Payments, Year Four 2021 Defined Benefit Plan, Expected Future Benefit Payments, Year Five Years 2022 - 2026 Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter Total Consideration Business Combination, Consideration Transferred Payments Due in April 2018 Business Combination, Consideration Transferred, Liabilities Incurred Assets Acquired [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] Customer Relationships Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Liabilities Assumed [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities Net Identifiable Assets Acquired Regulatory Assets [Table Text Block] Schedule of Regulatory Assets [Table Text Block] Regulatory Liabilities [Table Text Block] Schedule of Regulatory Liabilities [Table Text Block] Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] 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] 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 of Long-Lived Assets [Policy Text Block] Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Derivatives [Policy Text Block] Derivatives, Policy [Policy Text Block] Accounting for Stock-Based Compensation [Policy Text Block] Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Goodwill [Policy Text Block] Goodwill and Intangible Assets, Goodwill, Policy [Policy Text Block] Intangible Assets [Policy Text Block] Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] Environmental Liabilities [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Revenue Recognition [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Revenue from Cost Recovery Riders [Policy Text Block] Revenue Recognition for Alternative Revenue Programs, 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] Income Taxes, Effects of Regulation [Policy Text Block] Regulatory Income Taxes, Policy [Policy Text Block] Unrecognized Tax Benefits [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Excise Taxes [Policy Text Block] Excise Tax Policy [Policy Text Block] Policy regarding the collection and recording of excise taxes. Purchase Accounting [Policy Text Block] Business Combinations Policy [Policy Text Block] New Accounting Standards [Policy Text Block] New Accounting Pronouncements, 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] Land Inventory [Policy Text Block] Real Estate Held for Development and Sale, 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] Property, Plant and Equipment Impairment [Policy Text Block] Property, Plant and Equipment, Impairment [Policy Text Block] Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Earnings Per Share [Policy Text Block] Earnings Per Share, 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 [Policy Text Block] Employee Stock Ownership Plan (ESOP), Policy [Policy Text Block] Gross Unrecognized Income Tax Benefits [Abstract] Income Tax Uncertainties [Abstract] 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 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, Penalties Unrecognized Tax Benefits, Income Tax Penalties Expense Material Changes to Unrecognized Tax Benefits Expected During Next 12 Months Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit Property, Plant and Equipment and Estimated Useful Lives of Property, Plant and Equipment [Table Text Block] Property, Plant and Equipment [Table Text Block] Asset Retirement Obligations [Table Text Block] Schedule of Change in Asset Retirement Obligation [Table Text Block] Number of Options [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] 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] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, 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 Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Grant Date Intrinsic Value Cash Received from Non-Qualified Stock Options Exercised 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 Employee Stock and Incentive Plans [Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Generation [Member] Electricity Generation Plant, Non-Nuclear [Member] Transmission [Member] Electric Transmission [Member] Distribution [Member] Electric Distribution [Member] Property, Plant and Equipment in Service Property, Plant and Equipment, Gross Construction Work in Progress Construction in Progress, Gross Accumulated Depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment 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 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] Valuation and Qualifying Accounts and Reserves [Roll Forward] Movement in Valuation Allowances and Reserves [Roll Forward] Balance at Beginning of Period Valuation Allowances and Reserves, Balance Additions, Charged to Income Valuation Allowances and Reserves, Additions for Charges to Cost and Expense Additions, Other Charges Valuation Allowances and Reserves, Additions for Charges to Other Accounts Deductions from Reserves Valuation Allowances and Reserves, Deductions Balance at End of Period Contingent Consideration Business Combination, Contingent Consideration, Liability Percent of Results of Operations Reflected in Income Statement Business Combination, Percent of Results of Operations Reflected in Income Statement The percent of the acquired business' results of operations reflected in the acquiring entity's income statement. Accounts Receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Inventories Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory Intangible Assets Fair Value Adjustments for Work in Progress and Finished Goods Inventories Business combination, Fair Value Adjustment for Work in Process and Finished Goods Fair Value Adjustment for Work in Process and Finished Goods included in inventories at the acquisition date. Fair Value of Sales Backlog Business Combination, Fair Value of Sales Backlog Business Combination, Fair Value of Sales Backlog included in Other Current Assets at the acquisition date. Restricted Cash - Current 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 Environmental Matters [Table] Site Contingency [Table] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] NOV Consent Decree [Member] NOV Consent Decree [Member] NOV Consent Decree [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] SO2 and NO2 NAAQS [Member] SO2 and NO2 NAAQS [Member] SO2 and NO2 NAAQS [Member] 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] Site Contingency [Line Items] Additional Wind Energy Generating Capacity (MW) Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Estimated Costs of Compliance Site contingency, Loss Exposure Accrued and Best Estimate of Loss Exposure in Excess of Accrual Amount of loss exposure accrued and the best estimate of reasonably possible loss exposure in excess of the accrual. Employee Stock Ownership Plan [Abstract] Share-based Arrangements with Employees and Nonemployees [Abstract] ESOP Employer Loan Employee Stock Ownership Plan (ESOP), Debt Structure, Employer Loan, Description ESOP Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense 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 ESOP Shares, Fair Value of Unallocated Shares Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value Jointly-Owned Facilities and Projects [Table] Jointly Owned Utility Plant Interests [Table] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Jointly-Owned Facilities Projects [Axis] Jointly Owned Utility Plant [Axis] Jointly-Owned Facilities and Projects [Domain] Jointly Owned Utility Plant [Domain] Generation [Member] Jointly Owned Electricity Generation Plant [Member] Transmission [Member] Jointly Owned Electricity Transmission and Distribution System [Member] CapX2020 Projects [Member] CapX2020 [Member] CapX2020 (all projects) [Member] Jointly-Owned Facilities and Projects [Line Items] Jointly Owned Utility Plant Interests [Line Items] % Ownership Jointly Owned Utility Plant, Proportionate Ownership Share % Not Owned Jointly-Owned Electric Facility Percentage Not Owned Jointly-Owned Electric Facility Percentage Not Owned Number of Projects 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 Acquisition of Non-Controlling Interest [Table] Noncontrolling Interest [Table] Acquisition of Non-Controlling Interest [Line Items] Noncontrolling Interest [Line Items] Payments to Acquire Additional Interest in Subsidiaries Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Table] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] Fair Value, Measurements, Nonrecurring [Member] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] Equity Method Investment, Carrying Amount Equity Method Investment, Indicators of Impairment Equity Method Investment, Other than Temporary Impairment Goodwill, Carrying Amount Goodwill, Impairment Charge Intangible Assets, Carrying Amount Intangible Assets, Indicators of Impairment Property, Plant and Equipment, Indicators of Impairment Impairment of Long-Lived Assets Held-for-use Quarterly Financial Data (Unaudited) [Table Text Block] Quarterly Financial Information [Table Text Block] 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] Natural Gas Rates [Member] Natural Gas Rates [Member] Natural Gas Rates [Member] Water Rates [Member] Water Rates [Member] Water Rates [Member] PSCW [Member] PSCW [Member] Public Service Corporation of Wisconsin [Member] 2010 Minnesota General Rate Case [Member] 2010 Rate Case [Member] Retail rate increase request filed with the Minnesota Public Utilities Commission (MPUC). Minnesota Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] 2016 Minnesota General Rate Case [Member] 2016 Minnesota Rate Case [Member] [Member] 2016 Minnesota Rate Case [Member] [Member] FERC-Approved Wholesale Rates [Member] Wholesale rates approved by the Federal Energy Regulatory Commission (FERC). Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider [Member] Annual Automatic Adjustment of Charges [Member] AAA Filings [Member] Annual Automatic Adjustment of Charges Filings [Member] 2012 Wisconsin General Rate Case [Member] 2012 Wisconsin Rate Case [Member] 2012 retail rate increase requested filed with the Public Service Commission of Wisconsin (PSCW). 2016 Wisconsin General Rate Case [Member] 2016 Wisconsin Rate Case [Member] 2016 retail rate increase request filed with the PSCW [Member] SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Customer Class [Domain] Customer Class [Domain] [Domain] for Customer Class [Axis] description of customer types or classes Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Wholesale Electric Contracts (expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Boswell Energy Center [Member] Boswell Energy Center [Member] Boswell Energy Center [Member] Approved Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Approved Equity Ratio Public Utilities, Approved Equity Capital Structure, Percentage Revenue from Cost Recovery Riders Requested Rate Increase (Decrease) Public Utilities, Requested Rate Increase (Decrease), Percentage Requested Return on Equity Public Utilities, Requested Return on Equity, Percentage Requested Equity Ratio Public Utilities, Requested Equity Capital Structure, Percentage Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Annual Additional Revenue Generated from Requested Interim Rate Increase Public Utilities, Interim Rate Increase (Decrease), Amount Expected Increase (Decrease) in Annual Depreciation Expense Expected increase (Decrease) in Annual Depreciation Expense Based on a Regulatory Order Expected increase (decrease) in annual depreciation expense based on a regulatory order, if approved. Notice Required to Terminate (Years) Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract (in years). Average Monthly Demand (MW) Average Customer Demand (MW) Average demand measured for a specified customer, measured in MW. Change in Annual Capacity Charge Per Contract, Percentage Change in Annual Capacity Charge Per Contract, Percentage The annual percentage change of the capacity charge specified in a power purchase, power sales, or electric sales agreement. Reduction in Operating Revenue due to Regulatory Outcome Regulated Operating Revenue After-Tax Change for North Dakota Investment Tax Credits Reversed Tax Adjustments, Settlements, and Unusual Provisions Retail Fuel Cost Recovery Collected but Subject to Refund AAA Retail Fuel Cost Recovery Retail fuel cost recovery collected but subject to refund per Annual Automatic Adjustment (AAA) of Charges filings currently outstanding with the MPUC. Pension and Other Postretirement Benefit Plans Recurring Fair Value Measures [Table] Plan Asset Categories [Axis] Equity Securities [Member] Equity Securities [Member] 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] Mutual Funds [Member] Mutual Funds [Member] Mutual Funds [Member] International [Member] Equity Securities, International [Member] Debt Securities [Member] Debt Securities [Member] Debt Securities - Mutual Funds [Member] Fixed Income Funds [Member] Debt Securities - Fixed Income [Member] Fixed Income Securities [Member] Cash and Cash Equivalents [Member] Cash and Cash Equivalents [Member] Defined Benefit Plan, Fair Value of Plan Assets by Measurement [Axis] Total [Domain] Fair Value of Assets Defined Benefit Plan, Fair Value of Plan Assets Income Tax Expense [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Tax Expense [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 Tax Expense [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 Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Table] Document Information [Table] Document and Entity Information, Document [Axis] Document Information, Document [Axis] Document [Domain] Document [Domain] Document and Entity Information [Line Items] Document Information [Line Items] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Entity Tax Identification Number Entity Tax Identification Number 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 BNI Energy [Domain] BNI Energy [Domain] BNI Energy [Domain] Customer Class [Domain] Retail Customers [Member] Retail [Member] Retail Customers [Member] Electric [Member] Natural Gas [Member] Natural Gas [Member] Natural Gas Customer [Member] Water [Member] Water [Member] Water Customer [Member] ALLETE Clean Energy PSA - Future Development Project [Member] ALLETE Clean Energy PSA - Future Development Project [Member] ALLETE Clean Energy PSA - Future Development Project [Member] Square Butte [Member] Generating Capacity Constructed and Sold (MW) Generating Capacity Constructed and Sold (MW) Generating capacity constructed and sold (MW) during the period. Future Generating Capacity Under Development (MW) Future Generating Capacity Under Development Future Generating Capacity Constructed PSA Term (Years) Long-term Contract for Sale of Electric Power, Term of Contract Term in years of long-term contract to sell electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Output Purchased (Percent) Output Being Purchased (MW) 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 Business Segments [Text Block] Segment Reporting 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] Minimum Annual Payments for Certain Long-Term Commitments [Table] Unrecorded Unconditional Purchase Obligation [Table] Minimum Annual Payments for Certain Long-Term Commitments [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Minimum Annual Payments for Certain Long-Term Commitments [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Coal Supply and Transportation Agreements [Member] Coal and Transportation [Member] Coal and Transportation [Member] Power Purchase Agreement [Member] Power Purchase Agreement [Member] Power Purchase Agreement [Member] Minimum Annual Payments for Certain Long-Term Commitments [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Minimum Annual Payment Obligation in 2017 Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months Minimum Annual Payment Obligation in 2018 Unrecorded Unconditional Purchase Obligation, Due within Two Years Minimum Annual Payment Obligation in 2019 Unrecorded Unconditional Purchase Obligation, Due within Three Years Minimum Annual Payment Obligation in 2020 Unrecorded Unconditional Purchase Obligation, Due within Four Years Minimum Annual Payment Obligation in 2021 Unrecorded Unconditional Purchase Obligation, Due within Five Years Minimum Annual Payment Obligation Thereafter Unrecorded Unconditional Purchase Obligation, Due after Five Years Changes in Accumulated Other Comprehensive Loss [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [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) Net Expense (Credit) Defined Benefit Plan, Net Periodic Benefit Cost Statement of Financial Position [Abstract] Accounts Receivable [Abstract] Accounts Receivable, Allowance Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value Per Share Common Stock, No Par Value Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Outstanding Common Stock, Shares Issued Loss Contingencies [Table] Loss Contingencies [Table] Return on Equity Complaint 1 [Member] FERC Complaint 1 [Member] FERC Complaint 1 [Member] Incentive Adder [Member] Incentive Adder [Member] Incentive Adder [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Requested Return on Equity Filed with the FERC by Third Party Requested Return on Common Equity Filed with Regulatory Organization The requested return on common equity, which would be dedicated to the utilities within the regulatory organizations jurisdiction. FERC Authorized Return on Equity FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Equity Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder Proposed Return on Equity by Federal Administrative Law Judge Proposed Return on Common Equity by Regulatory Organization The proposed return on common equity by regulatory organization Proposed Return on Equity by Federal Administrative Law Judge Including Incentive Adder Proposed Return on Equity by Regulatory Organization, Including Incentive Adder Proposed Return on Equity by Regulatory Organization, Including Incentive Adder Incentive Adder for Participation in Regional Transmission Organization (Basis Points) Incentive Adder for Participation in Regional Transmission Organization Incentive Adder for Participation in Regional Transmission Organization Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Assets [Axis] Regulatory Asset [Axis] Total Regulatory Assets [Domain] Regulatory Asset [Domain] Deferred Fuel Adjustment Clause [Member] Deferred Fuel Costs [Member] Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension and Other Postretirement Plans Costs [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Cost Recovery Riders [Member] Regulatory Clause Revenues, under-recovered [Member] Asset Retirement Obligation [Member] Asset Retirement Obligation Costs [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 Liabilities [Axis] Regulatory Liability [Axis] Total Regulatory Liabilities [Domain] Regulatory Liability [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] North Dakota Investment Tax Credits [Member] North Dakota Investment Tax Credits Subject to Refund [Member] North Dakota Investment Tax Credits Subject to Refund [Member] 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] Regulatory Assets and Liabilities Currently Earning a Return Types of Net Regulatory Assets Earning Returns Current Regulatory Assets Regulatory Assets, Current Non-Current Regulatory Assets Regulatory Assets, Noncurrent Total Regulatory Assets Regulatory Assets Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Alternative Revenue Program, Required Collection Period (Years) Private Equity [Member] Actual Plan Asset Allocations Defined Benefit Plan, Actual Plan Asset Allocations Employer Stock Included in Equity Securities (Shares) Defined Benefit Plan, Number of Shares of Equity Securities Issued by Employer and Related Parties Included in Plan Assets Employer Contributions (Shares) Plan Asset Target Allocations Defined Benefit Plan, Target Plan Asset Allocations Goodwill and Intangible Assets [Text Block] Goodwill and Intangible Assets Disclosure [Text Block] 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 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 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 Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent Other Investments Net Loss Defined Benefit Plan, Future Amortization of Gain (Loss) Prior Service Credit Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit) Total Cost (Credit) Defined Benefit Plan, Amount to be Amortized from Accumulated Other Comprehensive Income (Loss) Next Fiscal Year Statement of Comprehensive Income [Abstract] Unrealized Loss on Securities, Income Tax Benefit Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Unrealized Gain on Derivatives, Income Tax Expense Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Benefit Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax Payment Due in April 2017 Schedule of Goodwill [Table Text Block] Schedule of Goodwill [Table Text Block] Schedule of Intangible Assets [Table Text Block] Schedule of Intangible Assets [Table Text Block] Tabular disclosure or assets, excluding financial assets and goodwill, lacking physical substance, by either major class or business segment. Camp Ripley Financing [Member] Camp Ripley Financing [Member] Camp Ripley Financing [Member] Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] Long-Term Debt Long-Term Debt - Unamortized Debt Issuance Costs Debt Issuance Costs, Noncurrent, Net Long-term Debt Maturities [Abstract] Long-term Debt, Fiscal Year Maturity [Abstract] Long-Term Debt Maturing in 2017 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months Long-Term Debt Maturing in 2018 Long-term Debt, Maturities, Repayments of Principal in Year Two Long-Term Debt Maturing in 2019 Long-term Debt, Maturities, Repayments of Principal in Year Three Long-Term Debt Maturing in 2020 Long-term Debt, Maturities, Repayments of Principal in Year Four Long-Term Debt Maturing in 2021 Long-term Debt, Maturities, Repayments of Principal in Year Five Long-Term Debt Maturing Thereafter Long-term Debt, Maturities, Repayments of Principal after Year Five Annual Financing Payment Debt Instrument, Periodic Payment Financing Renewal Term Debt Instrument, Renewal Term Renewal term for a debt instrument disclosing for what period of time after maturity the instrument may be extended, in 'PnYnMnDTnHnMnS' format. Purchase Option Purchase Option, Asset Purchase option to acquire an asset. Expected Proceeds from Future Issuance of Senior Unsecured Notes Expected Proceeds from Future Issuance of Senior Unsecured Notes The expected future cash inflow from amounts received from a future unsecured long-term debt instrument issuance. Expected Future Interest Rate Future Debt Instrument, Interest Rate, Stated Percentage Contractual interest rate for funds expected to be borrowed, under the debt agreement. Assets [Abstract] Current Assets Assets, Current [Abstract] Cash and Cash Equivalents Accounts Receivable (Less Allowance of $3.1 and $1.0) Inventories – Net Prepayments and Other Total Current Assets Assets, Current Property, Plant and Equipment – Net Regulatory Assets Investment in ATC Other Investments Goodwill and Intangible Assets – Net Intangible Assets, Net (Including Goodwill) Other Non-Current Assets Total Assets Liabilities and Equity [Abstract] Liabilities and Equity [Abstract] Liabilities [Abstract] Current Liabilities [Abstract] Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest Payable, Current Long-Term Debt Due Within One Year Notes Payable Notes Payable, Current Other Total Current Liabilities Liabilities, Current Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Defined Benefit Pension and Other Postretirement Benefit Plans Other Non-Current Liabilities Total Liabilities Liabilities Commitments, Guarantees and Contingencies (Note 11) Commitments and Contingencies Equity [Abstract] Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] ALLETE's Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 49.6 and 49.1 Shares Issued and Outstanding Common Stock, Value, Issued Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained Earnings Retained Earnings (Accumulated Deficit) Total ALLETE Equity Stockholders' Equity Attributable to Parent Non-Controlling Interest in Subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Total Equity Total Liabilities and Equity Liabilities and Equity Accumulated Other Comprehensive Loss [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Total [Domain] Unrealized Gain (Loss) on Available-for-sale Securities [Member] Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Defined Benefit Pension, Other Postretirement Items [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Gains (Loss) on Cash Flow Hedge [Member] Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Accumulated Other Comprehensive Loss [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Loss [Roll Forward] AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] Beginning Balance 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) Other Comprehensive Income (Loss), Net of Tax Ending Balance Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Other Comprehensive Income (Loss) [Abstract] Unrealized Loss on Securities - Net of Income Tax Benefit of $(0.2), $(0.3) and $(0.2) Unrealized Gain on Derivatives - Net of Income Tax Expense of $–, $0.1 and $0.1 Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Benefit of $(2.4), $(2.2) and $(2.8) Total Other Comprehensive Loss Total Comprehensive Income Less: Non-Controlling Interest in Subsidiaries Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Total Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Quarterly Financial Data (Unaudited) [Text Block] Quarterly Financial Information [Text Block] Other Investments [Table Text Block] Investment [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 Carryforwards [Table Text Block] NOL and Tax Credit Carryforwards [Table Text Block] Summary of Tax Credit Carryforwards [Table Text Block] Gross Unrecognized Income Tax Benefits [Table Text Block] Summary of Income Tax Contingencies [Table Text Block] Goodwill [Roll Forward] Goodwill [Roll Forward] Beginning Balance Acquired Goodwill Goodwill, Acquired During Period Impairment Charge Ending Balance Power Sales Agreements - Current Asset Power Purchase Agreements - Current Asset The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current asset. Power Sales Agreements - Non-Current Asset Power Purchase Agreements - Noncurrent Asset The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent asset. Short-term Debt Outstanding Debt, Current Short-Term Debt - Unamortized Debt Issuance Costs Debt Issuance Costs, Current, Net Lines of Credit [Abstract] Line of Credit Facility [Abstract] Lines of Credit [Table] Line of Credit Facility [Table] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Bank Lines of Credit [Member] Revolving Credit Facility [Member] Lines of Credit [Line Items] Line of Credit Facility [Line Items] Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Standby Letters of Credit Outstanding Line of Credit, Amount Pledged for Letters of Credit Amount of line of credit borrowing capacity pledged against letters of credit at the end of the period limiting the line of credit borrowing capacity but not reflected as debt on the balance sheet. Draws Outstanding Line of Credit, Current