Revenue from Contract with Customer [Abstract] Contract with Customer, Asset, Net, Noncurrent Contract with Customer, Asset, Net, Noncurrent Contract Asset Non-Cash Amortization Capitalized Contract Cost, Amortization Goodwill and Intangible Assets Disclosure [Abstract] Goodwill Goodwill Amortization of Intangible Assets Amortization of Intangible Assets Fair Value Disclosures [Abstract] Recurring Fair Value Measures [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Recurring Fair Value Measures [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Hierarchy [Axis] 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] Accretion [Member] Accretion [Member] Accretion expense [Member] Recurring Fair Value Measures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Investments [Abstract] Investments [Abstract] Available-for-sale – Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Available-for-sale – Corporate and Governmental Debt Securities Available-for-sale Securities, Debt Securities, Noncurrent Cash Equivalents Other Restricted Assets, Noncurrent Total Fair Value of Assets Assets, Fair Value Disclosure, Recurring Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Deferred Compensation Deferred Compensation, Fair Value Disclosure This element represents the portion of the balance sheet assertion, Deferred Compensation, valued at fair value by the entity. U.S. Water Services Contingent Consideration Business Combination, Contingent Consideration, Liability 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, Assets 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. Organization, Consolidation and Presentation of Financial Statements [Abstract] Cash, Cash Equivalents and Restricted Cash [Abstract] Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract] Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Restricted Cash included in Prepayments and Other Restricted Cash, Current Restricted Cash included in Other Non-Current Assets Restricted Cash, Noncurrent Total Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Inventories – Net [Abstract] Inventory, Net [Abstract] Fuel Energy Related Inventory, Coal Materials and Supplies Inventory, Supplies, Net of Reserves Construction of Wind Energy Facility Other Inventory, Gross 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 – Net Inventory, Net Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Contract Assets Contract Payment Non-Current Carrying value of a non-current deferred asset related to a payment made as part of a long-term contract. Finance Receivable Financing Receivable, Gross Other Other Assets, Miscellaneous, Noncurrent Total Other Non-Current Assets Other Assets, Noncurrent Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] Provision for Interim Rate Refund Provision for Interim Rate Refunds Provision for Interim Rate Refund PSAs Power Purchase Agreement - Current Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability. Contract Liabilities Contract with Customer, Liability, Current Provision for Tax Reform Refunds Provision for Tax Reform Refunds Provision for Tax Reform Refunds Contingent Consideration - Current Business Combination, Contingent Consideration, Liability, Current Other Other Accrued Liabilities, Current Total Other Current Liabilities Other Liabilities, Current Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent PSAs Power Purchase Agreement - Noncurrent Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent liability. Contingent Consideration - Non-current Business Combination, Contingent Consideration, Liability, Noncurrent Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Other Liabilities, Noncurrent 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] 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] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Minnesota Solar Energy Standard - Overall Mandate Percentage Solar Energy Standard Mandate, Overall Mandate Percentage Minnesota's solar energy mandate for total retail electric sales, excluding sales to certain industrial customers, by the end of 2020. Minnesota Solar Energy Standard - Small Scale Solar Mandate Percentage Solar Energy Standard Mandate, Small Scale Solar Mandate Percentage Minnesota's solar energy mandate for small scale solar with photovoltaic devices with a nameplate capacity of 20 kilowatts or less by the end of 2020. Expressed as a percentage of the overall mandate. Minnesota Solar Energy Standard - Qualifying Capacity for Small Scale Solar Mandate (MW) Solar Energy Standard Mandate, Qualifying Capacity for Small Scale Solar Mandate The maximum amount of nameplate capacity of a solar device to qualify for counting towards the small scale solar mandate of Minnesota's solar energy standard. Generating Capacity Owned (MW) Capacity of a generating facility, unit, or project in megawatts. Minnesota Solar Energy Standard - 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. Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Statement of Cash Flows [Abstract] Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest AFUDC – Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Change in Fair Value of Contingent Consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Loss (Gain) on Sales of Investments and Property, Plant and Equipment Gain (Loss) on Disposition of Assets Depreciation Expense Depreciation Amortization of PSAs Amortization of Power Purchase Agreements The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method. Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Expense Deferred Income Tax Expense (Benefit) Share-Based and ESOP Compensation Expense Share-based Compensation Defined Benefit Pension and Postretirement Benefit Expense Pension and Other Postretirement Benefits Cost (Reversal of Cost) Provision for Interim Rate Refund Provision for Interim Rate Refund Provision for Interim Rate Refund Provision for Tax Reform Refund Provision for Tax Reform Refund Provision for Tax Reform Refund Bad Debt Expense (Recoveries) Provision for Doubtful Accounts Changes in Operating Assets and Liabilities [Abstract] Increase (Decrease) in Operating Capital [Abstract] Accounts Receivable Increase (Decrease) in Accounts and Other Receivables Inventories Increase (Decrease) in Inventories Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Accounts Payable Increase (Decrease) in Accounts Payable Other Current Liabilities Increase (Decrease) in Other Current Liabilities Cash Contributions to Defined Benefit Pension Plans Payment for Pension Benefits Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Investing Activities [Abstract] Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Investment in ATC Payments to Acquire Equity Method Investments Changes to Other Investments Payments for (Proceeds from) Investments Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Other Investing Activities Proceeds from Sale of Other Assets, Investing Activities Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities Financing Activities [Abstract] Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from Issuance of Common Stock Changes in Notes Payable Proceeds from (Repayments of) Notes Payable Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition-Related Contingent Consideration Payments Payment for Contingent Consideration Liability, Financing Activities Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Other Financing Activities Proceeds from (Payments for) Other Financing Activities Cash from (for) Financing Activities Net Cash Provided by (Used in) Financing Activities Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, Cash Equivalents and Restricted Cash at End of Period 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] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] 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. Total Project Costs Incurred to Date Capital Cost Spent to Date Capital cost spent to date for project. Project Costs Recovered from Counterparty Capital Costs Spent To Date, Recovered Capital Costs Spent To Date, Recovered Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [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] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Definite-Lived Intangible Assets [Roll Forward] Finite-lived Intangible Assets [Roll Forward] Beginning Balance Finite-Lived Intangible Assets, Net Amortization Ending Balance Remaining Useful Life (Years) Finite-Lived Intangible Asset, Useful Life Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Estimated Annual Amortization Expense for Definite-Lived Intangible Assets [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] Remainder of 2018 Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five 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) Ending Balance Intangible Assets [Abstract] Intangible Assets, Net (Excluding Goodwill) [Abstract] Total Intangible Assets Intangible Assets, Net (Excluding Goodwill) Total Intangible Assets, Amortization Equity Method Investments and Joint Ventures [Abstract] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] Retirement Benefits [Abstract] Pension and Other Postretirement Benefit Plans [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Pension [Member] Pension Plan [Member] Other Postretirement [Member] Other Postretirement Benefits Plan [Member] Pension and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Components of Net Periodic Benefit Cost (Income) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Service Cost Defined Benefit Plan, Service Cost Interest Cost Defined Benefit Plan, Interest Cost Expected Return on Plan Assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Amortization of Prior Service Credits Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of Net Loss Defined Benefit Plan, Amortization of Gain (Loss) Net Periodic Benefit Cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Employer Contributions to Defined Benefit Pension Plans - Common Stock Value Stock Issued During Period, Value, Employee Benefit Plan Employer Contributions to Defined Benefit Plans Defined Benefit Plan, Plan Assets, Contributions by Employer Expected Future Employer Contributions in 2018 Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Segment Reporting [Abstract] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] 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] Revenue Streams [Axis] Contract with Customer, Sales Channel [Axis] Revenue Streams [Domain] Contract with Customer, Sales Channel [Domain] Residential [Member] Residential [Member] Revenue Recognized From Residential Customers Commercial [Member] Commercial [Member] Revenue Recognized From Commercial Customers Municipal [Member] Municipal [Member] Municipal Sales Channel Industrial [Member] Industrial [Member] Industrial Sales Channel [Member] Other Power Suppliers [Member] Other Power Suppliers [Member] Other Power Suppliers Sales Channel [Member] Other [Member] Other [Member] Regulated Operations Other Revenue Recognized From Other Sources Immaterial For Disaggregation Long-term PSA [Member] Long-term PSA [Member] Revenue Recognized From Long-term PSAs Other [Member] ALLETE Clean Energy Other [Member] ALLETE Clean Energy Other Revenue Recognized From Other Sources Immaterial For Disaggregation [Member] Point-in-Time [Member] Point-in-Time [Member] Revenue Recognized From Point-in-Time Sales Contract [Member] Contract [Member] Revenue Recognized by U.S. Water From Contract Sales Capital Project [Member] Capital Project [Member] Revenue Recognized From Capital Projects Long-term Contract [Member] Long-term Contract [Member] Long-term Contract Revenue Stream [Member] Other [Member] Corporate & Other - Other [Member] Revenue Recognized by Corporate & Other From Other Sources Immaterial for Disaggregation Business Segments [Line Items] Segment Reporting Information [Line Items] Number of Reportable Segments Number of Reportable Segments Number of Operating Segments Number of Operating Segments Land in Minnesota (Acres) Area of Land Operating Revenue Revenue from Contract with Customer, Excluding Assessed Tax Operating Revenue, Other Than Customer Revenue Revenue, Other Than Customer Revenue Non-cash Revenue Recognized From Amortization of PSAs Assumed Through Acquisitions Operating Revenue Revenues Net Income (Loss) Net Income (Loss) Attributable to Parent Assets Assets Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Asset [Axis] Total Regulatory Asset [Domain] Regulatory Asset [Domain] Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension and Other Postretirement Plans Costs [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [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). Conservation Improvement Program [Member] Conservation Improvement Program [Member] Conservation Improvement Program [Member] Other Other Regulatory Assets (Liabilities) [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Provision for Interim Rate Refund [Domain] Provision for Interim Rate Refund [Domain] Provision for Interim Rate Refund [Domain] Provision for Tax Reform Refund [Domain] Provision for Tax Reform Refund [Domain] Provision for Tax Reform Refund [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Plant Removal Obligations [Member] Removal Costs [Member] North Dakota Investment Tax Credits [Member] North Dakota Investment Tax Credits Subject to Refund [Member] North Dakota Investment Tax Credits Subject to Refund [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] Provision for Interim Rate Refund [Member] Revenue Subject to Refund [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. Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Rate Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Energy-Intensive Trade-Exposed Customer Rates [Member] Energy-Intensive Trade-Exposed Customer Rates [Member] Energy-Intensive Trade-Exposed Customer Rates [Member] 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 Non-Current Regulatory Assets Regulatory Assets, Noncurrent Current Regulatory Liabilities Regulatory Liability, Current Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Total Regulatory Liabilities Regulatory Liabilities Provided Discounts to EITE Customers that will be Offset Against Interim Rate Refunds Provided Discounts to EITE Customers to be Offset Against Interim Rate Refunds Provided Discounts to EITE Customers to be Offset Against Interim Rate Refunds Commitments and Contingencies Disclosure [Abstract] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Square Butte [Member] Square Butte [Member] Square Butte [Member] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Oconto Electric Cooperative [Member] Oconto Electric Cooperative [Member] Oconto Electric Cooperative Power Purchase Agreements [Axis] Long-term Contract for Purchase of Electric Power [Axis] Power Purchase Agreements [Domain] Long-term Contract for Purchase of Electric Power [Domain] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Power Sales Agreements [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Power Sales Agreements [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Minnkota Power PSA [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Oconto Electric Cooperative PSA [Member] Oconto Electric Cooperative Power Sales Agreement [Member] Oconto Electric Cooperative Power Sales Agreement Generating Facility [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Expected 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 Cost of Power Purchased 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 Energy and Capacity Expected to be Provided (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. Other Investments [Table Text Block] Investment [Table Text Block] Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Business Combinations [Abstract] Schedule of Business Acquisitions [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Tonka Water [Member] Tonka Water [Member] Tonka Equipment Company [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Schedule of Business Acquisitions [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Alternative Programs [Member] Alternative Programs [Member] Alternative Programs Sales Channel [Member] Customers [Axis] Customer [Axis] Customers [Domain] Customer [Domain] Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Industrial Customers [Member] Industrial Customers [Member] Industrial Customers Other Power Supplier Customers [Member] Other Power Supplier Customers [Member] Other Power Supplier Customers Services and Goods [Axis] Products and Services [Axis] Services and Goods [Domain] Products and Services [Domain] Wholesale Electric [Member] Wholesale Electric Service [Member] Wholesale Electric Service [Member] Retail Electric [Member] Retail Electric Service [Member] Retail Electric Service [Member] Sale of Energy under PSA [Member] Sale of Energy under PSA [Member] Sale of Energy under PSA [Member] Chemicals, Consumable Equipment and Services [Member] Chemicals, Consumable Equipment and Services [Member] Chemicals, Consumable Equipment and Services [Member] Contract Duration [Axis] Contract with Customer, Duration [Axis] Contract Duration [Domain] Contract with Customer, Duration [Domain] Long-term Contract with Customer [Member] Long-term Contract with Customer [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Number of Customers Number of Customers Number of Customers Length of Notice Required to Terminate Contract Length of Notice Required to Terminate Contract Length of Notice Required to Terminate Contract Percentage of Total Regulated Utility kWh Sales Percentage of Total Regulated Utility kWh Sales Percentage of Total Regulated Utility kWh Sales Number of Large Power Customer Contracts Large Power Customer Contracts Large Power Customer Contracts Large Power Customer Contract Serving Requirement Contract Serving Requirement Contract Serving Requirement Revenue Collection Period Following the Annual Period it is Recognized Revenue Collection Period Following the Annual Period it is Recognized Revenue Collection Period Following the Annual Period it is Recognized Income Tax Disclosure [Abstract] 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] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Manitoba Hydro PPA [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Transmission [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Output Being Purchased (MW) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Statement of Financial Position [Abstract] Accounts Receivable [Abstract] Accounts Receivable, Net, Current [Abstract] Accounts Receivable, Allowance Allowance for Doubtful Accounts Receivable, Current Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value Common Stock, No Par Value Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Outstanding Common Stock, Shares, Outstanding Common Stock, Shares Issued Common Stock, Shares, Issued 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 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 Debt Disclosure [Abstract] Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Table] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] 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 [Abstract] Indicators of Impairment Asset Impairment Charges Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Acquisitions [Text Block] Business Combination Disclosure [Text Block] Reclassification of Prior Income Statement [Table] Schedule of Prior Period Reclassification Adjustments [Table] Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period. Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Operating Revenue to Operating Revenue – Utility [Member] Operating Revenue – Utility [Member] The amount of regulated operating revenues recognized during the period. Operating Revenue – Non-utility [Member] Operating Revenue – Non-utility [Member] The amount of unregulated operating revenues recognized during the period. Cost of Sales to Fuel, Purchased Power and Gas – Utility [Member] Fuel, Purchased Power and Gas – Utility [Member] Cost of electricity purchased as well as fuel used to generate electricity used in operations and sold during the period. Reclassification of Prior Income Statement [Line Items] Schedule of Prior Period Reclassification Adjustments [Line Items] [Line Items] for Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period. Prior Period Reclassification Amount Prior Period Reclassification Adjustment Statement of Stockholders' Equity [Abstract] Consolidated Statement of Shareholders' Equity [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Total Shareholders' Equity [Domain] Equity Component [Domain] Retained Earnings [Member] Retained Earnings [Member] Accumulated Other Comprehensive Loss [Member] AOCI Attributable to Parent [Member] Common Stock [Member] Common Stock [Member] Consolidated Statement of Shareholders' Equity [Line Items] Statement [Line Items] Consolidated Statement of Shareholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Adjustments to Opening Balances - Net of Income Taxes Cumulative Effect of New Accounting Principle in Period of Adoption Beginning Balance - Adjusted Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Net Income Other Comprehensive Income – Net of Tax [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Loss on Debt Securities Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Defined Benefit Pension and Other Postretirement Plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Common Stock Issued Stock Issued During Period, Value, New Issues Dividends Declared Dividends, Common Stock Ending Balance Fair Value [Text Block] Fair Value Disclosures [Text Block] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [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] Schedule of Short-Term and Long-Term Debt [Table Text Block] Schedule of Debt [Table Text Block] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Income 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 Income Tax Expense Current Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Total Deferred Income Tax Expense (Benefit) Total Income Tax Expense (Benefit) Income Tax Expense (Benefit) Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Income Before Non-Controlling Interest and Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Taxes Computed at Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (Decrease) in Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Regulatory Differences for Utility Plant Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Gross Unrecognized Tax Benefits Unrecognized Tax Benefits Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate 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 Due Annually Operating Leases, Future Minimum Payments Due, Next Twelve Months Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Minimum Lease Payments Due for Remainder of 2018 Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Minimum Lease Payments Due in 2019 Operating Leases, Future Minimum Payments, Due in Two Years Minimum Lease Payments Due in 2020 Operating Leases, Future Minimum Payments, Due in Three Years Minimum Lease Payments Due in 2021 Operating Leases, Future Minimum Payments, Due in Four Years Minimum Lease Payments Due in 2022 Operating Leases, Future Minimum Payments, Due in Five Years Minimum Lease Payments Thereafter Operating Leases, Future Minimum Payments, Due Thereafter 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] ALLETE First Mortgage Bonds 4.07% Due April 2048 [Member] ALLETE Bonds 4.07% Due April 2048 [Member] ALLETE Bonds 4.07% Due April 2048 [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Short-Term Debt - Principal Debt, Current, Gross Amount of short-term debt and current maturities of long-term debt before deduction of unamortized discount or premium and unamortized debt issuance costs. Short-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Debt, Current The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of short-term debt or the current maturities of long-term debt. Short-Term Debt - Total Debt, Current Long-Term Debt - Principal Long-term Debt, Gross Long-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Long-Term Debt Excluding Current Maturities The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of long-term debt excluding current maturities. Long-Term Debt - Total Long-term Debt, Excluding Current Maturities Total Debt - Principal Debt, Long-Term and Short-Term, Combined Amount, Gross Represents the aggregate of total long-term debt, including current maturities and short-term debt, before deduction of unamortized discount or premium and unamortized debt issuance expense. Total Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense Total Debt - Total Debt, Long-term and Short-term, Combined Amount First Mortgage Bonds - Proceeds from Sale Proceeds from Issuance of First Mortgage Bond First Mortgage Bonds - Interest Rate Debt Instrument, Interest Rate, Stated Percentage Equity [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Text Block] Disclosure of Reclassification Amount [Text Block] Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Accounts Receivable (Less Allowance of $2.1 and $2.1) Accounts Receivable, Net, Current Inventories – Net Prepayments and Other Other Assets, Current Total Current Assets Assets, Current Property, Plant and Equipment – Net Property, Plant and Equipment, Net Regulatory Assets Investment in ATC Equity Method Investments Other Investments Long-term Investments Goodwill and Intangible Assets – Net Intangible Assets, Net (Including Goodwill) Other Non-Current Assets Total Assets Liabilities and Shareholders' Equity [Abstract] Liabilities and Equity [Abstract] Liabilities [Abstract] Current Liabilities [Abstract] Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest Payable, Current Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Other Total Current Liabilities Liabilities, Current Long-Term Debt Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Defined Benefit Pension and Other Postretirement Benefit Plans Liability, Defined Benefit Plan, Noncurrent Other Non-Current Liabilities Total Liabilities Liabilities Commitments, Guarantees and Contingencies (Note 13) Commitments and Contingencies Shareholders’ Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 51.3 and 51.1 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 Shareholders’ Equity Stockholders' Equity Attributable to Parent Total Liabilities and Shareholders' Equity Liabilities and Equity Financial Covenants [Table] Required Ratio of Indebtedness to Total Capitalization Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Actual Ratio of Indebtedness to Total Capitalization Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] Inventories – Net [Policy Text Block] Inventory, Policy [Policy Text Block] Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Revenue Recognition for Alternative Revenue Programs, Policy [Policy Text Block] Revenue Recognition for Alternative Revenue Programs, Policy [Policy Text Block] New Accounting Standards [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Land Inventory [Policy Text Block] Real Estate Held for Development and Sale, Policy [Policy Text Block] Available-for-sale Investments [Policy Text Block] Marketable Securities, Available-for-sale Securities, Policy [Policy Text Block] Acquisitions [Policy Text Block] Business Combinations Policy [Policy Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Equity Method Investments [Policy Text Block] Equity Method Investments [Policy Text Block] Income Tax [Policy Text Block] Income Tax, Policy [Policy Text Block] Uncertain Tax Positions [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Environmental Accruals [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Statement of Comprehensive Income [Abstract] Unrealized Gain (Loss) on Securities, Income Tax Expense Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Schedule of Intangible Assets [Table Text Block] Schedule of Intangible Assets and Goodwill [Table Text Block] Investments [Text Block] Investment [Text Block] Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Other Comprehensive Income (Loss) [Abstract] Unrealized Gain (Loss) on Securities Net of Income Tax Expense of $– and $0.3 Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1 and $0.1 Total Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax Total Comprehensive Income Earnings Per Share [Abstract] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Class of Stock [Line Items] Class of Stock [Line Items] Options to Purchase Shares Excluded from Computation of Diluted Earnings Per Share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Net Income Attributable to ALLETE Average Common Shares Weighted Average Number of Shares Outstanding, Basic Earnings Per Share Earnings Per Share, Basic Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Available to Common Stockholders, Diluted Average Common Shares Weighted Average Number of Shares Outstanding, Diluted Earnings Per Share Earnings Per Share, Diluted Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Goodwill and Intangible Assets [Text Block] Goodwill and Intangible Assets Disclosure [Text Block] FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] PSCW [Member] PSCW [Member] Public Service Corporation of Wisconsin [Member] 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] FERC-Approved Wholesale Rates [Member] Wholesale rates approved by the Federal Energy Regulatory Commission (FERC). 2016 Wisconsin General Rate Case [Member] 2016 Wisconsin Rate Case [Member] 2016 retail rate increase request filed with the PSCW [Member] 2012 Wisconsin Rate Case [Member] 2012 Wisconsin Rate Case [Member] 2012 retail rate increase requested filed with the Public Service Commission of Wisconsin (PSCW). SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Domain] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contracts (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contracts (Expire from 2024 through 2029) [Member] Wholesale Electric Contracts (Expire from 2024 through 2029) [Member] Wholesale Electric Contracts (Expire from 2024 through 2029) [Member] Boswell Energy Center [Member] Boswell Energy Center [Member] Boswell Energy Center [Member] Revenue from Cost Recovery Riders Regulated Operating Revenue, Other Requested Average Rate Increase 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 Annual Additional Revenue Generated from Requested Final Rate Increase, Amended Public Utilities, Requested Rate Increase (Decrease), Amended, Amount Approved Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Approved Equity Ratio Public Utilities, Approved Equity Capital Structure, Percentage Reserve for Interim Rate Refund Reserve for Interim Rate Refund Reserve for Interim Rate Refund Pre-Tax Charge to Fuel, Purchase Power and Gas - Utility Cost of Purchased Power Expected Decrease in Annual Depreciation Expense Increase (Decrease) in Annual Depreciation Expense Based on Regulatory Order Increase (Decrease) in Annual Depreciation Expense Based on Regulatory Order Requested Reconsideration of Certain Decisions Representing Annualized Additional Revenue Public Utilities, Approved Rate Increase (Decrease), Reconsideration Amount Public Utilities, Approved Rate Increase (Decrease), Reconsideration Amount Provided Discounts to EITE Customers Discount Provided to EITE Customers Discount Provided to EITE Customers Expected Discounts to EITE Customers Annually Estimated Discount to be Provided During the Next Twelve Months Estimated Discount to be Provided During the Next Twelve Months Length of Notice Required to Terminate (Years) Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract (in years). Average Monthly Demand (MW) Average Customer Demand (MW) Average demand measured for a specified customer, measured in MW. Expected Annualized Collection of Additional Revenue Under Retail Rate Order Public Utilities, Approved Rate Increase (Decrease), Amount Reason for Business Acquisitions Business Combination, Reason for Business Combination Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] New Accounting Pronouncements [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2017-07 [Member] Accounting Standards Update 2017-07 [Member] Accounting Standards Update 2017-07 [Member] Accounting Standards Update 2016-01 [Member] Accounting Standards Update 2016-01 [Member] Accounting Standards Update 2016-18 [Member] Accounting Standards Update 2016-18 [Member] Accounting Standards Update 2014-09 [Member] Accounting Standards Update 2014-09 [Member] Operating and Maintenance [Member] Operating and Maintenance [Member] Operating and Maintenance [Member] Cost of Sales - Non-utility [Member] Cost of Sales - Non-utility [Member] Cost of Sales - Non-utility [Member] Other Income/Expense [Member] Other Income/Expense [Member] Other Income/Expense [Member] New Accounting Pronouncements [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Cumulative Effect of New Accounting Principle in Period of Adoption Operating Leases, Future Minimum Payments Due Operating Leases, Future Minimum Payments Due Income Statement [Abstract] Operating Revenue [Abstract] Revenues [Abstract] Contracts with Customers – Utility Regulated Operating Revenue Contracts with Customers – Non-utility Unregulated Operating Revenue Other - Non-utility Total Operating Revenue Operating Expenses [Abstract] Costs and Expenses [Abstract] Fuel, Purchased Power and Gas – Utility Transmission Services – Utility Cost of Transmission Cost of Sales – Non-utility 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 Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) 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 Income Taxes Income Tax Expense (Benefit) Net Income Average Shares of Common Stock [Abstract] Basic (Shares) Diluted (Shares) Basic Earnings Per Share of Common Stock Diluted Earnings Per Share of Common Stock Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Supplemental Cash Flow Information [Abstract] Cash Paid for Interest – Net of Amounts Capitalized Interest 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 Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Reclassification of Property, Plant and Equipment to Inventory Reclassification of Property, Plant and Equipment to Inventory Reclassification of Property, Plant and Equipment to Inventory Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations AFUDC–Equity ALLETE Common Stock Contributed to the Pension Plans Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] Inventories – Net [Table Text Block] Schedule of Inventory, Current [Table Text Block] Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Supplemental Statement of Cash Flows Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [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] 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] Return on Equity Complaint 2 [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] Investment in ATC [Line Items] Schedule of Equity Method Investments [Line Items] Ownership Percentage Equity Method Investment, Ownership Percentage Expected Additional Investment in 2018 Equity Method Investment, Expected Additional Investment Additional investments expected to be made during the current fiscal year but after the filing date, not already separately disclosed as part of current period/year-to-date investments or as a subsequent event. ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Balance as of December 31, 2017 Cash Investments Equity in ATC Earnings Distributed ATC Earnings Proceeds from Equity Method Investment, Distribution Amortization of the Remeasurement of Deferred Income Taxes Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Equity Investment Balance as of March 31, 2018 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. Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Table] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items] Revenue, Remaining Performance Obligation Revenue, Remaining Performance Obligation Expected Period to Recognize Minimum Revenue Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Payment Terms and Conditions Revenue, Performance Obligation, Description of Payment Terms ALLETE Properties Real Estate Investments, Net Available-for-sale Securities Available-for-sale Securities, Noncurrent Other Other Long-term Investments Total Other Investments Available-for-sale Corporate and Governmental 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 Impairment of Real Estate Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount Long-term Debt Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value Components of Net Periodic Benefit Expense (Income) [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] 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, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Letters of Credit [Member] Financial Standby Letter of Credit [Member] Surety Bonds [Member] Surety Bond [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Collateral Guarantor Obligations, Collateral Held Directly or by Third Parties, Amount Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Ownership Percentage of Benefited Property Community Development District Obligation, Ownership of Property Benefited Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. Annual Assessment Community Development District Obligation, Annual Assessment Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] Natural Gas-fired [Member] Natural Gas Processing Plant [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Solar Energy Generation [Member] Solar Energy Generation [Member] Solar Energy Generation [Member] 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] Resource Package [Member] Resource Package [Member] Resource Package [Member] Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Tenaska PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] Solar Energy PPA [Member] Solar Energy PPA [Member] Solar Energy Provider [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Combined-Cycle Natural Gas-fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Jointly Owned Electricity Generation Plant [Member] Jointly Owned Electricity Generation Plant [Member] Additional Natural Gas-fired Generation (MW) Generating Capacity to be Counterparty Owned (MW) Output Being Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased Generating Capacity to be Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Expected Output Entitlement (Percent) Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Percentage of Voting Interests Acquired Business Acquisition, Percentage of Voting Interests Acquired Name of Acquired Entity Business Acquisition, Name of Acquired Entity Total Consideration Business Combination, Consideration Transferred Cash Payment to Acquire Business Payments to Acquire Businesses, Gross Working Capital Adjustment Paid in the Fourth Quarter 2017 Business Combination, Consideration Transferred, Liabilities Incurred Assets Acquired [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] Accounts Receivable Business Combination, Acquired Receivable, Fair Value Other Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Trade Names Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill 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] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Total Liabilities Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Net Identifiable Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Tax Deductible Goodwill Business Acquisition, Goodwill, Expected Tax Deductible Amount Loss Contingencies [Table] Loss Contingencies [Table] Return on Equity Complaint 1 [Member] FERC Complaint 1 [Member] FERC Complaint 1 [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 Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Equity Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder 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 Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] NOV Consent Decree [Member] NOV Consent Decree [Member] NOV Consent Decree [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] NO2 NAAQS [Member] NO2 NAAQS [Member] 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] Regulatory Asset [Axis] Regulatory Asset [Domain] Environmental Restoration Costs [Member] Environmental Restoration Costs [Member] Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Consent Decree Wind Energy Generation Addition (MW) Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Estimated Costs of Compliance (Accrued) 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. Unrecorded Unconditional Purchase Obligation [Table] Coal, Rail and Shipping Contracts [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Coal, Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Coal Supply and Transportation Agreements [Member] Coal and Transportation [Member] Coal and Transportation [Member] Coal, Rail and Shipping Contracts [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Minimum Annual Payment Obligation for Remainder of 2018 Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year Minimum Annual Payment Obligation in 2019 Unrecorded Unconditional Purchase Obligation, Due within Two Years Minimum Annual Payment Obligation in 2020 Unrecorded Unconditional Purchase Obligation, Due within Three Years Minimum Annual Payment Obligation in 2021 Unrecorded Unconditional Purchase Obligation, Due within Four Years Minimum Annual Payment Obligation in 2022 Unrecorded Unconditional Purchase Obligation, Due within Five Years Minimum Annual Payment Obligation Thereafter Unrecorded Unconditional Purchase Obligation, Due after Five Years