Retirement Benefits [Abstract] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [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] Postretirement Health and Life [Member] Other Postretirement Benefits Plan [Member] Plan Asset Categories [Axis] Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Domain] Equity Securities [Member] Equity Securities [Member] Fixed Income Securities [Member] Private Equity [Member] Private Equity Funds [Member] Real Estate [Member] Real Estate [Member] Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Actual Plan Asset Allocations Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage ALLETE Common Stock Included in Pension Plan Equity Securities (Shares) Defined Benefit Plan, Plan Assets, Employer, Related Party, Number of Shares Plan Asset Target Allocations Defined Benefit Plan, Plan Assets, Target Allocation, Percentage Organization, Consolidation and Presentation of Financial Statements [Abstract] Financial Statement Preparation [Policy Text Block] Basis of Accounting, Policy [Policy Text Block] Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] Principles of Consolidation [Policy Text Block] Consolidation, Policy [Policy Text Block] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] 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, 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 Obligation [Policy Text Block] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Equity Method Investments [Policy Text Block] Equity Method Investments [Policy Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] 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] 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] Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Table] Jointly Owned Utility Plant Interests [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] 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] Jointly-Owned Electricity Generation Plant [Member] Jointly Owned Electricity Generation Plant [Member] Transmission [Member] Jointly Owned Electricity Transmission and Distribution System [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] WPPI Energy [Member] WPPI Energy [Member] WPPI Energy [Member] Transmission [Axis] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Transmission [Domain] Name of transmission investment. CapX2020 Projects [Member] CapX2020 [Member] CapX2020 (all projects) [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Jointly-Owned Facilities and Projects [Line Items] Jointly Owned Utility Plant Interests [Line Items] Minnesota Power Ownership % Jointly Owned Utility Plant, Proportionate Ownership Share Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) WPPI Energy Ownership % Jointly-Owned Electric Facility Percentage Not Owned Jointly-Owned Electric Facility Percentage Not Owned Plant in Service Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service Accumulated Depreciation Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation Construction Work in Progress Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress Net Loss Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax Prior Service Credit Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax Accumulated Net Periodic Benefit Cost in Excess of Contributions Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset) Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset) Total Net Postretirement Health and Life Amounts Recognized in Consolidated Balance Sheet Defined Benefit Plan, Funded (Unfunded) Status of Plan Commitments and Contingencies Disclosure [Abstract] Estimated Minimum Annual Payments for Certain Long-Term Commitments [Table Text Block] Unrecorded Unconditional Purchase Obligations Disclosure [Table Text Block] Other Postretirement Benefits Plan [Member] 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 Loss Defined Benefit Plan, Amortization of Gain (Loss) Amortization of Prior Service Cost Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Net Pension Cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Statement of Comprehensive Income [Abstract] Unrealized Gain (Loss) on Securities, Income Tax Expense (Benefit) Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense (Benefit) Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Fair Value Disclosures [Abstract] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] Fair Value, Measurements, Nonrecurring [Member] Equity Method Investment [Axis] Investment, Name [Axis] Equity Method Investment [Domain] Investment, Name [Domain] ATC [Member] ATC [Member] ATC [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. U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] Corporate and Other [Member] Corporate and Other [Member] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Recurring Fair Value Measure, Unobservable Inputs, Period Increase (Decrease) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Equity Investments Equity Method Investments Equity Method Investment, Indicators of Impairment Equity Method Investment, Other than Temporary Impairment Goodwill, Carrying Amount Goodwill Goodwill, Impairment Charge Goodwill, Impairment Loss Intangible Assets, Carrying Amount Intangible Assets, Net (Excluding Goodwill) Intangible Assets, Indicators of Impairment Impairment of Intangible Assets (Excluding Goodwill) Property, Plant and Equipment, Indicators of Impairment Impairment of Long-Lived Assets Held-for-use Pension and Other Postretirement Benefit Plan Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Obligation and Funded Status [Table Text Block] Schedule of Net Funded Status [Table Text Block] Amounts Recognized in Consolidated Balance Sheet [Table Text Block] Schedule of Amounts Recognized in Balance Sheet [Table Text Block] Components of Net Periodic Cost [Table Text Block] Schedule of Net Benefit Costs [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] 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] Unrecognized Postretirement Health and Life Costs [Table Text Block] Schedule of Net Periodic Benefit Cost Not yet Recognized [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] Revenue from Contract with Customer [Abstract] Capitalized Contract Cost [Table] Capitalized Contract Cost [Table] Capitalized Contract Cost [Axis] Capitalized Contract Cost [Axis] Capitalized Contract Cost [Domain] Capitalized Contract Cost [Domain] Capitalized Contract Cost [Line Items] Capitalized Contract Cost [Line Items] Contract Asset - Noncurrent Contract with Customer, Asset, Net, Noncurrent Capitalized Contract Cost, Amortization Capitalized Contract Cost, Amortization Equity [Abstract] Summary of Common Stock [Table Text Block] Schedule of Common Stock Outstanding Roll Forward [Table Text Block] Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Equity Method Investments and Joint Ventures [Abstract] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] Selected Quarterly Financial Information [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 Income Tax Disclosure [Abstract] Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Accounting Changes and Error Corrections [Abstract] Schedule of Quantifying Prior Year Misstatement Corrected in Current Year Financial Statements [Table] Schedule of Quantifying Prior Year Misstatement Corrected in Current Year Financial Statements [Table] Nature of Error [Axis] Nature of Error [Axis] Nature of Error [Domain] Nature of Error [Domain] Out-of-Period Adjustment for MISO Rates [Domain] Out-of-Period Adjustment for MISO Rates [Domain] Out-of-Period Adjustment for MISO Rates [Domain] Quantifying Misstatement in Current Year Financial Statements [Line Items] Quantifying Misstatement in Current Year Financial Statements [Line Items] Immaterial Error Correction Immaterial Error Correction 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] 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] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] 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] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Nobles 2 [Member] Nobles 2 [Member] Nobles 2 [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] 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] 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 [Domain] Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Additional Natural Gas-fired Generation (MW) Generating Capacity Owned (MW) Capacity of a generating facility, unit, or project in megawatts. Generating Capacity to be Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity 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 Jointly Owned (MW) Expected Output Entitlement (Percent) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Goodwill [Table Text Block] Schedule of Goodwill [Table Text Block] Schedule of Intangible Assets [Table Text Block] Schedule of Intangible Assets and Goodwill [Table Text Block] Business Combinations [Abstract] Acquisitions [Text Block] Business Combination Disclosure [Text Block] Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] 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] 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] 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] Accounts Receivables [Abstract] Receivables [Abstract] 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 Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Business Segments [Axis] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Impairment of Long-Lived Assets [Abstract] Asset Impairment Charges [Abstract] Impairment of Real Estate Impairment of Real Estate Goodwill and Intangible Assets [Abstract] Schedule of Goodwill [Table] Schedule of Goodwill [Table] Measurement Input Type [Axis] Measurement Input Type [Axis] Measurement Input Type [Domain] Measurement Input Type [Domain] Discount Rate [Member] Measurement Input, Discount Rate [Member] Annual Revenue Growth Rate [Member] Measurement Input, Long-term Revenue Growth Rate [Member] Measurement Input, Terminal Growth Rate [Member] Measurement Input, Terminal Growth Rate [Member] Measurement Input, Terminal Growth Rate [Member] Goodwill [Line Items] Goodwill [Line Items] Impairment of Goodwill Discounted Cash Flow Model Significant Assumptions Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] 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 [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Definite-Lived Intangible Assets [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Developed Technology and Other [Member] Other Intangible Assets [Member] Customer Relationships [Member] Customer Relationships [Member] Definite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Provision for Interim Rate Refunds Provision for Interim Rate Refunds Provision for Interim Rate Refund Provision for Tax Reform Refunds Provision for Tax Reform Refunds Provision for Tax Reform Refunds 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 Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Financing Receivable Financing Receivable, Gross Other Other Assets, Miscellaneous, Noncurrent Total Other Non-Current Assets Other Assets, Noncurrent 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 Other Current Liabilities [Abstract] Other Liabilities, Current [Abstract] PSAs Power Purchase Agreement - Current Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability. Contract with Customer, Liability, Current Contract with Customer, Liability, Current Business Combination, Contingent Consideration, Liability, 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 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] 2016 Minnesota General Rate Case [Member] 2016 Minnesota Rate Case [Member] [Member] 2016 Minnesota Rate Case [Member] [Member] Business Segments [Line Items] Segment Reporting Information [Line Items] Other – Non-utility Amortization of Power Purchase Agreements The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method. 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] More-Likely-Than-Not Percentage MoreLikelyThanNotDefined The percentage guideline to determine more-likely-than-not status Remeasurement of Deferred Income Taxes as a Result of the TCJA [Table] Remeasurement of Deferred Income Taxes as a Result of the TCJA [Table] Remeausrement of Deferred Income Taxes as a Result of the TCJA [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Income Tax Expense [Member] Income Tax Expense [Member] Income Tax Expense [Member] Remeasurement of Deferred Income Taxes as a Result of the TCJA [Line Items] Remeasurement of Deferred Income Taxes as a Result of the TCJA [Line Items] [Line Items] for Remeausrement of Deferred Income Taxes as a Result of the TCJA [Table] Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Decrease in Investment in ATC Decrease in Investment of Equity Method Investee Decrease in Investment of Equity Method Investee Increase to Regulatory Assets Increase to Regulatory Assets Increase to Regulatory Assets Increase to Regulatory Liabilities Increase to Regulatory Liabilities Increase to Regulatory Liabilities 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 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 – Excess Deferred Tax Benefit Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. Change in Fair Value of Contingent Consideration Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Remeasurement of Deferred Income Taxes Effective Income Tax Rate Reconciliation, Remeasurement of Deferred Income Taxes Effective Income Tax Rate Reconciliation, Remeasurement of Deferred Income Taxes Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Total Income Tax Expense (Benefit) Income Tax Expense (Benefit) Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent 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, Rail and Shipping Contracts [Member] Coal and Transportation [Member] Coal and Transportation [Member] Leasing Agreements [Member] Lease Agreements [Member] Easements [Member] Easements [Member] Easements [Member] Long-Term Service Agreements [Member] Long-Term Service Agreements [Member] Long-Term Service Agreements [Member] PPAs [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 Payments in 2019 Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months Minimum Payments in 2020 Unrecorded Unconditional Purchase Obligation, Due within Two Years Minimum Payments in 2021 Unrecorded Unconditional Purchase Obligation, Due within Three Years Minimum Payments in 2022 Unrecorded Unconditional Purchase Obligation, Due within Four Years Minimum Payments in 2023 Unrecorded Unconditional Purchase Obligation, Due within Five Years Minimum Payments Thereafter Unrecorded Unconditional Purchase Obligation, Due after Five Years Minimum Payments in 2019 Operating Leases, Future Minimum Payments Due, Next Twelve Months Minimum Payments in 2020 Operating Leases, Future Minimum Payments, Due in Two Years Minimum Payments in 2021 Operating Leases, Future Minimum Payments, Due in Three Years Minimum Payments in 2022 Operating Leases, Future Minimum Payments, Due in Four Years Minimum Payments in 2023 Operating Leases, Future Minimum Payments, Due in Five Years Minimum Payments Thereafter Operating Leases, Future Minimum Payments, Due Thereafter 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 Asset [Axis] Regulatory Asset [Axis] Total Regulatory Assets [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 Obligation [Member] Asset Retirement Obligation Costs [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [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). CIP Financial Incentive [Member] CIP Financial Incentive [Member] Conservation Improvement Program [Member] Other [Member] Other Regulatory Assets (Liabilities) [Member] Regulatory Clause Revenues, under-recovered [Member] Regulatory Clause Revenues, under-recovered [Member] Regulatory Liabilities [Axis] Regulatory Liability [Axis] Total Regulatory Liabilities [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] Transmission Formula Rates [Member] Transmission Formula Rates [Member] Transmission Formula Rates Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Revenue Subject to Refund [Member] Revenue Subject to Refund [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] 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 Information by 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 [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 Liability Regulatory Liability, Current Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Total Regulatory Liabilities Regulatory Liabilities Rider Revenue Recovery Collection Period (Years) Alternative Revenue Program, Required Collection Period Alternative Revenue Program, Required Collection Period Provided Discounts to EITE Customers to 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 Reclassification out of Accumulated Other Comprehensive Income (Loss) [Text Block] Disclosure of Reclassification Amount [Text Block] Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument, Name [Domain] Camp Ripley Financing [Member] Camp Ripley Financing [Member] Camp Ripley Financing [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Term Loan Variable Rate Due 2020 [Member] ALLETE Term Loan Variable Rate Due 2020 [Member] ALLETE Term Loan Variable Rate Due 2020 [Member] ALLETE Bonds 4.07% Due April 2048 [Member] ALLETE Bonds 4.07% Due April 2048 [Member] ALLETE Bonds 4.07% Due April 2048 [Member] Unsecured Term Loan Due in 2020 [Member] Unsecured Term Loan Due in 2020 [Member] Unsecured Term Loan Due in 2020 Debt Instrument [Line Items] Long-Term Debt Long-term Debt, Excluding Current Maturities Long-Term Debt - Unamortized Debt Issuance Costs Debt Issuance Costs, Noncurrent, Net Debt Instrument [Abstract] Long-term Debt, Fiscal Year Maturity [Abstract] Long-Term Debt Maturing in 2019 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months Long-Term Debt Maturing in 2020 Long-term Debt, Maturities, Repayments of Principal in Year Two Long-Term Debt Maturing in 2021 Long-term Debt, Maturities, Repayments of Principal in Year Three Long-Term Debt Maturing in 2022 Long-term Debt, Maturities, Repayments of Principal in Year Four Long-Term Debt Maturing in 2023 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 (Years) Debt Instrument, Renewal Term Renewal term for a debt instrument disclosing for what period of time after maturity the instrument may be extended. Purchase Option Purchase Option, Asset Purchase option to acquire an asset. Proceeds from Issuance of First Mortgage Bond Proceeds from Issuance of First Mortgage Bond Interest Rate Debt Instrument, Interest Rate, Stated Percentage Repayments of Debt Repayments of Debt Estimated Future Benefit Payments [Abstract] Defined Benefit Plan, Expected Future Benefit Payment [Abstract] 2019 Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months 2020 Defined Benefit Plan, Expected Future Benefit Payment, Year Two 2021 Defined Benefit Plan, Expected Future Benefit Payment, Year Three 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year Four 2023 Defined Benefit Plan, Expected Future Benefit Payment, Year Five Years 2024 - 2028 Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter 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 Pension and Other Postretirement Benefit Plans Recurring Fair Value Measures [Table] Defined Benefit Plan, Plan Assets, Category [Domain] U.S. Large-cap [Member] U.S. Large-Cap [Member] Equity Securities, U.S. Large-Cap [Member] U.S. Mid-cap Growth [Member] U.S. Mid-Cap Growth [Member] Equity Securities, U.S. Mid-Cap Growth [Member] U.S. Small-cap [Member] U.S. Small Cap [Member] Equity Securities, U.S. Small Cap [Member] International [Member] Equity Securities, International [Member] Mutual Funds [Member] Fixed Income Funds [Member] Fixed Income [Member] Cash and Cash Equivalents [Member] Cash and Cash Equivalents [Member] Defined Benefit Plan, Fair Value of Plan Assets by Measurement [Axis] Fair Value Hierarchy and NAV [Axis] Total [Domain] Fair Value Hierarchy and NAV [Domain] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Recurring Fair Value Measures [Line Items] Fair Value of Assets Defined Benefit Plan, Plan Assets, Amount Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Trust for Benefit of Employees [Member] Trust for Benefit of Employees [Member] Pension Plan [Member] Long-term Investments Long-term Investments Net Loss Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax Prior Service Credit Accumulated Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset) Total Net Pension Amounts Recognized in Consolidated Balance Sheet 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, Period Increase (Decrease) Ending Balance Schedule of Definite-Lived Intangible Assets [Table] 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, Period Increase (Decrease) 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] 2019 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2023 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Total Intangible Assets Total Intangible Assets, Additions Intangible Assets Increase (Decrease) Amount of increase (decrease) in carrying value of assets, excluding financial assets and goodwill, lacking physical substance. Total Intangible Assets, Amortization Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Income Tax Expense [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) Federal - Remeasurement of Deferred Income Taxes Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes State Deferred State and Local Income Tax Expense (Benefit) Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Total Deferred Income Tax Expense (Benefit) Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent CIP Triennial Filing [Member] CIP Triennial Filing [Member] Conservation improvement program triennial filing [Member] CIP Consolidated Filing [Member] CIP Consolidated Filing [Member] CIP Consolidated Filing [Member] CIP Spending Minimum - Percentage CIP Spend Requirement Minnesota Next Generation Act of 2007 introduced a spend goal of 1.5% of net gross operating revenues from service provided in the state on energy CIPs each year. CIP Investment Goal CIP Investment Goal Conservation improvement program Investment Goal approved by MPUC. CIP Actual Spending CIP Actual Spend Conservation improvement program actual annual spend. CIP Investment Goal in 2019 CIP Investment Goal, Year Two Conservation improvement program Investment Goal for year two approved by the MPUC. CIP Financial Incentive Regulated Operating Revenue, Other 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] 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 Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company 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 Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Common Stock [Member] Common Stock [Member] VEBA [Member] VEBA [Member] VEBA (Voluntary Employee Benefits Association trusts) [Member] Irrevocable Grantor Trust [Member] Employer Contributions to Defined Benefit Pension Plans - Cash Defined Benefit Plan, Plan Assets, Cash Contributions by Employer Defined Benefit Plan, Plan Assets, Cash Contributions by Employer Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares Stock Issued During Period, Shares, Employee Benefit Plan Employer Contributions to Defined Benefit Pension Plans - Common Stock Value Stock Issued During Period, Value, Employee Benefit Plan ESOP Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense Defined Benefit Plan - Amendments Defined Benefit Plan, Changes in Benefit Obligation and Plan Assets, Description Employer Contributions to Other Postretirement Benefit Plans Payment for Pension and Other Postretirement Benefits Expected Employer Contributions in 2019 Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year Net Loss Total Unrecognized Postretirement Health and Life Cost 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 Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets [Table 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 Leasing Agreements [Table] Schedule of Operating Leased Assets [Table] BNI Energy [Member] BNI Energy [Member] BNI Energy 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] Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Total Lease Expense Operating Leases, Rent Expense 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 Plan Amendments Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment Plan Curtailments Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment Actuarial (Gain) Loss Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) Benefits Paid Defined Benefit Plan, Benefit Obligation, Benefits Paid Participant Contributions Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant 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, Plan Assets, Increase (Decrease) for Actual Return (Loss) Employer Contribution Defined Benefit Plan, Plan Assets, Contributions by Employer Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Benefits Paid Defined Benefit Plan, Plan Assets, Benefits Paid Fair Value, End of Year Funded Status, End of Year Non-Current Assets Assets for Plan Benefits, Defined Benefit Plan Current Liabilities Liability, Defined Benefit Plan, Current Non-Current Liabilities Liability, Defined Benefit Plan, Noncurrent Loss Contingencies [Table] Loss Contingencies [Table] FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Return on Equity Complaint [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Equity Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder 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 Equity Investments [Table] Schedule of Equity Method Investments [Table] Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] Long-term Contract for Purchase of Electric Power [Domain] Nobles 2 PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] Return on Equity Complaint 2 [Member] Equity Investments [Line Items] Schedule of Equity Method Investments [Line Items] Ownership Percentage Equity Method Investment, Ownership Percentage 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. Expected Additional Investment in 2019 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 Cash Investments Payments to Acquire Equity Method Investments Equity Earnings in ATC Income (Loss) from Equity Method Investments Distributed ATC Earnings Proceeds from Equity Method Investment, Distribution Remeasurement of Deferred Income Taxes for Equity Method Investments Remeasurement of Deferred Income Taxes for Equity Method Investments Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments 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. Estimated Project Cost Estimated Project Cost Estimated Project Cost Portion of Expected Project Costs Our Portion of Expected Cost Our Portion of Expected Cost Expected Tax Equity Financing Expected Tax Equity Financing Expected Tax Equity Financing Jointly-Owned Facilities and Projects [Text Block] Jointly-Owned Electric Facility [Text Block] Jointly-Owned Electric Facility [Text Block] Projected Benefit Obligation Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation Accumulated Benefit Obligation Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation Fair Value of Plan Assets Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets Investments [Abstract] Other Investments [Table Text Block] Investment [Table Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Employee Stock Ownership Plan [Abstract] Share-based Arrangements with Employees and Nonemployees [Abstract] Schedule of Business Acquisitions [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition [Domain] Business Acquisition, Acquiree [Domain] Water & Energy Systems Technology [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] 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 Payments to Acquire Business Payments to Acquire Businesses, Gross Working Capital Adjustments Goodwill, Purchase Accounting Adjustments 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] 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 Intangible Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill 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 Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent 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 Common Stock and Earnings Per Share [Text Block] Stockholders' Equity Note Disclosure [Text Block] Commitments, Guarantees and Contingencies [Text Block] Commitments and Contingencies Disclosure [Text Block] Net Loss Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year Prior Service Credit Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year Total Pension and Postretirement Health and Life Cost (Credit) Defined Benefit Plan, Expected Amortization, Next Fiscal Year 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 Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Gain (Loss) on Securities - Net of Income Tax Expense (Benefit) of $-, $0.7 and $(0.2) Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense (Benefit) of $0.3, $2.2 and $(2.4) Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Total Other Comprehensive Income (Loss) Other Comprehensive Income (Loss), Net of Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest 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 Total [Domain] Goodwill [Roll Forward] Goodwill [Roll Forward] Beginning Balance Acquired Goodwill Goodwill, Acquired During Period Other Adjustments Ending Balance Jointly-Owned Facilities and Projects [Table Text Block] Schedule of Jointly Owned Utility Plants [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] 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] Statement of Financial Position [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, Outstanding Common Stock, Shares Issued Common Stock, Shares, Issued Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate 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, Next Fiscal Year Ultimate Trend Rate Defined Benefit Plan, Ultimate Health Care Cost Trend Rate Year Ultimate Trend Rate Effective Defined Benefit Plan, Year Health Care Cost Trend 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] Expected Long-Term Return on Plan Assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets Rate of Compensation Increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase Quarterly Financial Data (Unaudited) [Table Text Block] Quarterly Financial Information [Table Text Block] 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 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 Long-term Debt [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] 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 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 Settled Asset Retirement Obligation, Liabilities Settled Revisions in Estimated Cash Flows Asset Retirement Obligation, Revision of Estimate Ending Obligation Income Statement [Abstract] Operating Revenue [Abstract] Regulated Operating Revenue [Abstract] Contracts with Customers – Utility Regulated Operating Revenue Contracts with Customers – Non-utility Unregulated Operating Revenue Total Operating Revenue Operating Expenses [Abstract] Costs and Expenses [Abstract] Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Transmission Services - Utility Transmission Services - Utility Transmission Services - Utility Cost of Sales - Non-utility Cost of Goods and Services Sold Operating and Maintenance Operating Expenses Depreciation and Amortization Depreciation, Depletion and Amortization Taxes Other Than Income Taxes Taxes, Other Operating Expenses – Other Total Operating Expenses Costs and Expenses Operating Income Other Income (Expense) [Abstract] Nonoperating Income (Expense) [Abstract] Interest Expense Interest Expense Other Other Nonoperating Income (Expense) Total Other Expense Nonoperating Income (Expense) Income Before Non-Controlling Interest and Income Taxes Income Tax Expense Net Income 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 Supplemental Cash Flow Information [Abstract] Cash Paid During the Period for Interest – Net of Amounts Capitalized Interest Paid, Excluding Capitalized Interest, Operating Activities Remeasurement of Deferred Income Taxes Resulting from the TCJA [Abstract] Remeasurement of Deferred Income Taxes Resulting from the TCJA [Abstract] Increase (Decrease) in Deferred Income Taxes Increase (Decrease) in Deferred Income Taxes Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment 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 Camp Ripley Solar 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 Noncash Consideration for Land Inventory [Table] Noncash or Part Noncash Divestitures [Table] Noncash Consideration for Land Inventory [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 Assets [Abstract] Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Accounts Receivable (Less Allowance of $1.7 and $2.1) Inventories – Net Prepayments and Other Other Assets, Current Total Current Assets Assets, Current Property, Plant and Equipment – Net Regulatory Assets Other Investments Goodwill and Intangible Assets – Net Intangible Assets, Net (Including Goodwill) Other Non-Current Assets Total Assets Assets Liabilities and Equity [Abstract] Liabilities and Equity [Abstract] Liabilities [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 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] Shareholders' Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 51.5 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 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 Payments to Acquire Additional Interest in Subsidiaries Gain or Loss Recognized in Net Income or Other Comprehensive Income Other Nonoperating Gains (Losses) Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis] Activity in Level 3 [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning Balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Actual Return on Plan Assets Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss) Purchases, Sales, and Settlements – Net Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, (Sales), Issuances, (Settlements) Ending Balance Subsequent Event [Abstract] Subsequent Event [Abstract] Subsequent Event [Table] Subsequent Event [Table] Subsequent Event [Line Items] Subsequent Event [Line Items] Cash Purchase Price on the Sale of U.S. Water Services Expected Cash Proceeds from Sale of Affiliate Expected Cash Proceeds from Sale of Affiliate Estimated Gain on the Sale of U.S. Water Services Estimated Gain on Sale of Affiliate Estimated Gain on Sale of Affiliate Segment Reporting [Abstract] Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Property, Plant and Equipment [Text Block] Property, Plant and Equipment Disclosure [Text Block] 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] Minnesota Solar Energy Standard - Blanchard Solar Energy Facility [Member] Minnesota Solar Energy Standard - Blanchard Solar Energy Facility [Member] Minnesota Solar Energy Standard - Blanchard Solar Energy Facility 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 Counterparty Owned (MW) Schedule of Prior Period Reclassification Adjustments [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. 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. Other Operating Revenue - Non-utility [Member] Other Operating Revenue - Non-utility [Member] Other Operating Revenue - Non-utility [Member] Schedule of Prior Period Reclassification Adjustments [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 Adjustment Prior Period Reclassification Adjustment Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 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 Cash Flow Statement Location [Axis] Cash Flow Statement Location [Axis] Information by Cash Flow Statement Location [Axis] New Accounting Standards [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Cash Flow Statement Location [Domain] Cash Flow Statement Location [Domain] [Domain] for Information by Cash Flow Statement Location [Axis] Financing Activities [Member] Financing Activities [Member] Financing Activities [Member] Investing Activities [Member] Investing Activities [Member] Investing Activities [Member] 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 2016-01 [Member] Accounting Standards Update 2016-01 [Member] Accounting Standards Update 2016-15 [Member] Accounting Standards Update 2016-15 [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] Accounting Standards Update 2016-02 [Member] Accounting Standards Update 2016-02 [Member] Consolidated Statement of Income [Domain] 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 Standards [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Cumulative Effect of New Accounting Principle in Period of Adoption Cumulative Effect of New Accounting Principle in Period of Adoption Expected Minimum Lease Payment Due in Future Years Operating Leases, Future Minimum Payments Due SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] BNI Energy [Domain] BNI Energy [Domain] BNI Energy [Domain] Customer Class [Domain] Retail Customers [Member] Retail [Member] Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Natural Gas [Member] Natural Gas [Member] Natural Gas Customer [Member] Water [Member] Water [Member] Water Customer [Member] Square Butte [Member] Square Butte [Member] Square Butte [Member] Square Butte [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Number of Reportable Segments Number of Reportable Segments Number of Customers Number of Customers Number of Customers Land in Minnesota (Acres) Area of Land Minnesota Power Output Entitlement (Percent) Minnesota Power Output Entitlement (MW) Statement of Cash Flows [Abstract] Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] AFUDC – Equity Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Deferred Fuel Adjustment Clause Charge Increase (Decrease) in Deferred Fuel Costs 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 Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Expense (Benefit) Share-Based and ESOP Compensation Expense Share-based Compensation Defined Benefit Pension and Other Postretirement Benefit Expense Pension and Other Postretirement Benefits Cost (Reversal of Cost) Bad Debt Expense Provision for Doubtful Accounts 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 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 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 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 and Restricted Cash Acquired Payments to Acquire Businesses, Net of Cash Acquired Equity Investments Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Other Investing Activities Payments for (Proceeds from) Other Investing Activities 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 Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition of Non-Controlling Interest 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 for Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Summary of Common Stock [Table] Schedule of Stock by Class [Table] Summary of Common Stock [Line Items] Class of Stock [Line Items] Balance, Shares Balance, Equity Common Stock, Value, Outstanding Employee Stock Purchase Plan, Shares Stock Issued During Period, Shares, Employee Stock Purchase Plans Employee Stock Purchase Plan, 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. Equity Issuance Program, Shares Stock Issued During Period, Shares, Other Equity Issuance Program, Equity Stock Issued During Period, Value, Other 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 Received for Sale of Land Inventory, Shares Stock Repurchased and Retired During Period, Shares Received for Sale of Land Inventory, Equity Stock Repurchased and Retired During Period, Value Acquisition of Non-Controlling Interest, Shares Stock Issued During Period, Shares, Acquisitions Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Employer Contributions to Pension - Shares 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 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. Transmission [Table] Public Utility, Property, Plant and Equipment [Table] Manitoba Hydro PPA [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [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 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. Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Debt Instrument [Axis] Debt Instrument, Name [Domain] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Domain] Debt Instrument, Redemption, Period [Domain] Debt Instrument [Line Items] 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. Deferred Income Tax Assets and Liabilities [Abstract] Components of Deferred Tax Assets and Liabilities [Abstract] Deferred Income 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 Regulatory Liabilities Deferred Tax Assets, Regulatory Liabilities Deferred Tax Assets, Regulatory Liabilities Other Deferred Tax Assets, Other Gross Deferred Income Tax Assets Deferred Tax Assets, Gross Deferred Income Tax Asset Valuation Allowance Deferred Tax Assets, Valuation Allowance Total Deferred Income Tax Assets Deferred Tax Assets, Net of Valuation Allowance Deferred Income Tax Liabilities [Abstract] Deferred Tax Liabilities, Gross [Abstract] Property-Related Deferred Tax Liabilities, Property, Plant and Equipment Regulatory Assets for Benefit Obligations Deferred Tax Liabilities, Regulatory Assets for Benefit Obligations Deferred Tax Liabilities, Regulatory Assets for Benefit Obligations Unamortized Investment Tax Credits Accumulated Deferred Investment Tax Credit Partnership Basis Differences Deferred Tax Liabilty, Partnership Basis Differences Deferred Tax Liability, Partnership Basis Differences Regulatory Assets Deferred Tax Liabilities, Regulatory Assets Other Deferred Tax Liabilities, Other Total Deferred Income Tax Liabilities Deferred Tax Liabilities, Gross Net Deferred Income Taxes Deferred Tax Liabilities, Net Investments [Text Block] Investment [Text Block] Recurring Fair Value Measurements [Table] Recurring Fair Value Measures [Member] Fair Value, Measurements, Recurring [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] 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] 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] 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 Debt Securities, Available-for-sale, Noncurrent Cash Equivalents Other Restricted Assets, Noncurrent Total Fair Value of Assets Assets, Fair Value Disclosure Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Deferred Compensation Deferred Compensation Liability, Classified, Noncurrent U.S. Water Services Contingent Consideration Business Combination, Contingent Consideration, Liability Total Fair Value of Liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure 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 Reconciliation, Recurring Basis, Liability Value Activity in Level 3 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. Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] 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 Debt Securities, Maturities [Abstract] Debt Securities, Available-for-sale, Fair Value, Fiscal Year Maturity [Abstract] One Year or Less Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, 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). 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 Tax Credit Carryforwards [Table Text Block] Gross Unrecognized Income Tax Benefits [Table Text Block] Summary of Income Tax Contingencies [Table Text Block] Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Tonka Water [Member] Tonka Water [Member] Tonka Equipment Company [Member] Accounts Receivable Business Combination, Acquired Receivable, Fair Value Trade Names Total Liabilities Assumed Working Capital Adjustment Paid in the Fourth Quarter 2017 Fair Value [Text Block] Fair Value Disclosures [Text Block] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Contract with Customer, Duration [Axis] Contract with Customer, Duration [Axis] Contract with Customer, Duration [Domain] Contract with Customer, Duration [Domain] Long-term Contract with Customer [Member] Long-term Contract with Customer [Member] Regulated Operations [Member] Regulated Operations [Member] Regulated Operations [Member] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Wholesale Electric Service [Member] Wholesale Electric Service [Member] Wholesale Electric Service [Member] Retail Electric Service [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 with Customer, Sales Channel [Axis] Contract with Customer, Sales Channel [Axis] Contract with Customer, Sales Channel [Domain] Contract with Customer, Sales Channel [Domain] 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] Alternative Programs [Member] Alternative Programs [Member] Alternative Programs Sales Channel [Member] Contract [Member] Contract [Member] Revenue Recognized by U.S. Water From Contract Sales Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Industrial Customers [Member] Industrial Customers [Member] Industrial Customers Other Power Supplier Customers [Member] Other Power Supplier Customers [Member] Other Power Supplier Customers Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] 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 Large Power Customer Contracts Large Power Customer Contracts Large Power Customer Contracts 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 Revenue, Performance Obligation, Description of Payment Terms Revenue, Performance Obligation, Description of Payment Terms Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Net Income (Loss) Attributable to Parent Average Common Shares Earnings Per Share Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income (Loss) Available to Common Stockholders, Diluted Net Income (Loss) Available to Common Stockholders, Diluted Average Common Shares Earnings Per Share Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Common Stock, Dividends, Per Share, Declared Common Stock, Dividends, Per Share, Declared 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, Gross Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value 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] 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 Adjustments to Opening Balance – Net of Income Taxes (a) Beginning Balance - Adjusted Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Other Comprehensive Income – Net of Taxes Unrealized Gain (Loss) on Securities Defined Benefit Pension and Other Postretirement Plans Total Comprehensive Income Common Stock Issued Stock Issued During Period, Value, New Issues Common Stock Retired Dividends Declared Dividends, Common Stock Acquisition of Non-Controlling Interest Ending Balance 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, Expected Timing of Satisfaction, Period Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period Revenue, Remaining Performance Obligation, Amount Revenue, Remaining Performance Obligation, Amount Reason for Business Acquisitions Business Combination, Reason for Business Combination 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 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] ALLETE Amended and Restated Credit Agreement [Member] ALLETE LOC Nov2018 [Member] ALLETE LOC Nov2018 [Member] Lines of Credit [Line Items] Line of Credit Facility [Line Items] Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility Additional Borrowing Capacity Line of Credit Facility Additional Borrowing Capacity Line of Credit Facility Additional 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 Letters of Credit Maximum Issuances Letters of Credit Maximum Issuances Letters of Credit Maximum Issuances Schedule of Long-term Debt Instruments [Table] 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] 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] 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] A415SeriesDue2028 [Member] A415SeriesDue2028 [Member] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [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] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] SWL&P First Mortgage Bonds 4.14% Series Due 2048 [Member] Other Long-Term Debt, 3.11% – 5.75% Due 2019 – 2037 [Member] Other Long-Term Debt [Member] Other long-term debt instruments not specifically identified elsewhere Long-term Debt Unamortized Debt Issuance Costs Debt Issuance Costs, Net Total Long-Term Debt Long-term Debt Less: Due Within One Year Net Long-Term Debt Debt Instrument, Description of Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Debt Instrument, Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider Electric Rates [Member] Residential [Member] Residential [Member] Revenue Recognized From Residential Customers Commercial [Member] Commercial [Member] Revenue Recognized From Commercial Customers 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 Sale of Wind Energy Facility [Member] Sale of Wind Energy Facility [Member] Sale of Wind Energy Facility [Member] ALLETE Clean Energy 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 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] Corporate & Other - Other [Member] Corporate & Other - Other [Member] Revenue Recognized by Corporate & Other From Other Sources Immaterial for Disaggregation Consolidation, Eliminations [Member] Consolidation, Eliminations [Member] Segment Reporting Information [Line Items] Number of Operating Segments Number of Operating Segments Revenue from Contract with Customer, Excluding Assessed Tax Revenue from Contract with Customer, Excluding Assessed Tax Revenues Sale of Wind Energy Facility Sale of Wind Energy Facility Sale of Wind Energy Facility Revenue, Other Than Customer Revenue Revenue, Other Than Customer Revenue Non-cash Revenue Recognized From Amortization of PSAs Assumed Through Acquisitions Deferred Federal Income Tax Expense (Benefit), Remeasurement of Deferred Income Taxes Tax Adjustments, Settlements, and Unusual Provisions Tax Adjustments, Settlements, and Unusual Provisions Depreciation, Depletion and Amortization Interest Expense Income Tax Expense (Benefit) Assets Property, Plant and Equipment, Additions Property, Plant and Equipment, Additions Net (Gain) Loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax Prior Service Credit Arising During the Period Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax Amortization of Prior Service Cost Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax Prior Service Cost Arising During the Period Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Reclassification Adjustment, before Tax Amortization of Loss Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax Equity Investments [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract] Valuation and Qualifying Accounts and Reserves [Table] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Table] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves Type [Axis] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] SEC Schedule, 12-09, Valuation Allowances and Reserves [Domain] Trade Accounts Receivable [Member] SEC Schedule, 12-09, Allowance, Credit Loss [Member] Finance Receivables – Long-Term [Member] SEC Schedule, 12-09, Allowance, Loss on Finance Receivable [Member] Deferred Tax Assets [Member] SEC Schedule, 12-09, Valuation Allowance, Deferred Tax Asset [Member] Valuation and Qualifying Accounts and Reserves [Line Items] SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] Valuation and Qualifying Accounts and Reserves [Roll Forward] SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward] Balance at Beginning of Period SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount Additions, Charged to Income SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Cost and Expense Additions, Other Charges SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account Deductions from Reserves SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction Balance at End of Period Segment Reporting Disclosure [Text Block] Segment Reporting Disclosure [Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Cypress Creek Renewables [Member] Cypress Creek Renewables [Member] Cypress Creek Renewables Nobles 2 [Member] Oliver Wind [Member] Oliver Wind [Member] Oliver Wind [Member] TransAlta [Member] TransAlta [Member] TransAlta [Member] Basin Electric Power Cooperative [Member] Basin Electric Power Cooperative [Member] Basin Electric Power Cooperative Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Silver Bay Power [Member] Silver Bay Power [Member] Silver Bay Power [Member] Calpine Corporation [Member] Calpine Corporation [Member] Calpine Corporation Great River Energy [Member] Great River Energy [Member] Great River Energy [Member] Shell Energy [Member] Shell Energy [Member] Shell Energy [Member] Basin [Member] Basin [Member] Basin [Member] Oconto Electric Cooperative [Member] Oconto Electric Cooperative [Member] Oconto Electric Cooperative Power Purchase Agreement - Energy and Capacity [Axis] Square Butte PPA (expires 2026) [Member] Cypress Creek Renewables PPA [Member] Cypress Creek Renewables PPA [Member] Cypress Creek Renewables PPA Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] 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] 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 (expires May 2035) [Member] Minnkota Power PPA (expires May 2020) [Member] Minnkota PPA [Member] Minnkota PPA [Member] Power Purchase Agreement - Capacity [Axis] Long-term Contract for Purchase of Electric Capacity [Axis] Long-term Contract for Purchase of Electric Capacity [Axis] Power Purchase Agreement - Capacity [Domain] Long-term Contract for Purchase of Electric Capacity [Domain] [Domain] for Long-term Contract for Purchase of Electric Capacity [Axis] Calpine Corporation PPA [Member] Calpine Corporation PPA [Member] Calpine Corporation PPA Great River Energy Capacity Only PPA Beginning 2016 (expires 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 Only PPA Beginning 2017 (expires 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 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] Power Purchase Agreement - Energy [Axis] Long-term Contract for Purchase of Energy [Axis] Long-term Contract for Purchase of Energy [Axis] Power Purchase Agreement - Energy [Domain] Long-term Contract for Purchase of Energy [Domain] [Domain] for Long-term Contract for Purchase of Energy [Axis] 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. Oliver Wind I PPA [Member] Oliver Wind I PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. Oliver Wind II PPA [Member] Oliver Wind II PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. Shell Energy PPA [Member] Shell Energy PPA [Member] Contract with Shell Energy North America, for the purchase of energy [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta On-Peak Hours PPA (expire 2019) [Member] TransAlta On-Peak Hours PPA (expire 2019) [Member] TransAlta On-Peak Hours PPA (expires 2019) Power Sales Agreement - Energy and Capacity [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Power Sales Agreement - Energy and Capacity [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Basin PSA (expires April 2020) [Member] Basin Power Sales Agreement (expires April 2020) [Member] Basin Power Sales Agreement (expires April 2020) [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Oconto Electric Cooperative Power Sales Agreement [Member] Oconto Electric Cooperative Power Sales Agreement [Member] Oconto Electric Cooperative Power Sales Agreement Power Sales Agreement - Capacity [Axis] Long-term Contract for Sale of Capacity [Axis] Long-term Contract for Sale of Capacity [Axis] Power Sales Agreement - Capacity [Domain] Long-term Contract for Sale of Capacity [Domain] [Domain] for Long-term Contract for Sale of Capacity [Axis] Basin Power Sales Agreement (expires 2019) [Member] Basin Power Sales Agreement (expires 2019) [Member] Basin Power Sales Agreement (expires 2019) [Member] Basin Electric Cooperative PSA June 2022 - May 2023 [Member] Basin Electric Cooperative PSA June 2022 - May 2023 [Member] Basin Electric Cooperative PSA June 2022 - May 2023 [Member] Basin Electric Cooperative PSA June 2023 - May 2025 [Member] Basin Electric Cooperative PSA June 2023 - May 2025 [Member] Basin Electric Cooperative PSA June 2023 - May 2025 [Member] Basin Power Sales Agreement (expires May 2028) [Member] Basin Power Sales Agreement (expires May 2028) [Member] Basin Power Sales Agreement (expires May 2028) [Member] Power Sales Agreement - Energy [Axis] Long-term Contract for Sale of Energy [Axis] Long-term Contract for Sale of Energy [Axis] Power Sales Agreement - Energy [Domain] Long-term Contract for Sale of Energy [Domain] [Domain] for Long-term Contract for Sale of Energy [Axis] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Square Butte Coal-fired Unit [Member] Blanchard Solar Energy Facility [Member] Blanchard Solar Energy Facility [Member] Blanchard Solar Energy Facility 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] 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 through 2023 Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required PPA Annual Debt Service Obligation (Percentage) Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required, Obligation (Percentage) Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required, Obligation (Percentage) 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 - Electric Energy and Capacity (MW) Long-term Contract for Purchase of Electric Energy and Capacity (MW) Long-term Contract for Purchase of Electric Energy and Capacity (MW) Contract Length (Years) Contract Length (Years) Contract Length 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 Purchased - Electric Energy (MW) Long-Term Contract for Purchase of Electric Energy (MW) Long-Term Contract for Purchase of Electric Energy (MW) Output Being Sold Electric - Capacity (MW) Long-term Contract for Sale of Electric Capacity (MW) Long-term Contract for Sale of Electric Capacity (MW) Output Being Purchased - Electric Capacity (MW) Long-term Contract for Purchase of Electric Capacity (MW) Long-term Contract for Purchase of Electric Capacity (MW) Output Being Sold - Electric Energy (MW) Long-term Contract for Sale of Electric Energy (MW) Long-term Contract for Sale of Electric Energy (MW) Output Being Sold Electric - Energy and Capacity (MW) Long-term Contract for Sale of Electric Energy and Capacity (MW) Long-term Contract for Sale of Electric Energy and Capacity (MW) Schedule II [Text Block] SEC Schedule, 12-09, Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Legal Proceedings [Table] Gain Contingencies [Table] U.S. Water Services Patent Defense Litigation [Axis] Litigation Case [Axis] U.S. Water Services Patent Defense Litigation [Domain] Litigation Case [Domain] U.S. Water Services Patent Defense Litigation [Member] U.S. Water Services Patent Defense Litigation [Member] U.S. Water Services Patent Defense Litigation [Member] Legal Proceedings [Line Items] Gain Contingencies [Line Items] Patents Infringed Gain Contingency, Patents Allegedly Infringed upon, Number Addition to Intangible Assets for Patent Defense Costs Finite-Lived Intangible Assets Addition for Patent Defense Costs Finite-Lived Intangible Assets Addition for Patent Defense Costs Restricted Cash included in Prepayments and Other Restricted Cash, Current Restricted Cash included in Other Non-Current Assets Restricted Cash, Noncurrent Cash, Cash Equivalents and Restricted Cash on the Consolidated Statement of Cash Flows 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] 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 Large Power Customers Receivables from Large Power Customers Accounts Receivable, Gross Percent of Consolidated Operating Revenue Concentration Risk, Percentage 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] Minnesota Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] FERC-Approved Wholesale Rates [Member] Wholesale rates approved by the Federal Energy Regulatory Commission (FERC). Tax Cuts and Jobs Act [Member] Tax Cuts and Jobs Act [Member] Tax Cuts and Jobs Act 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). 2018 Wisconsin General Rate Case [Member] 2018 Wisconsin General Rate Case [Member] 2018 retail rate increase request filed with the PSCW [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 December 2024) [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 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 Approved Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Approved Equity Ratio Public Utilities, Approved Equity Capital Structure, Percentage Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Reserve for Interim Rate Refund Reserve for Interim Rate Refund Reserve for Interim Rate Refund Charge to Fuel, Purchased Power and Gas - Utility Decrease in Annual Depreciation Expense Based on Regulatory Order Increase (Decrease) in Annual Depreciation Expense Based on Regulatory Order Increase (Decrease) in Annual Depreciation Expense Based on Regulatory Order Estimated Discount to be Provided During the Next Twelve Months Estimated Discount to be Provided During the Next Twelve Months Estimated Discount to be Provided During the Next Twelve Months Discount Provided to EITE Customers Discount Provided to EITE Customers Discount Provided to EITE Customers 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. Approved Rate Increase - Amount Public Utilities, Approved Rate Increase (Decrease), Amount After-Tax Change for North Dakota Investment Tax Credits Reversed Amortization Period of the Income Tax Benefits from the Remeasurement of Deferred Income Tax Assets and Liabilities Resulting from the TCJA Excluding the Related Property Plant and Equipment Amortization Period of the Income Tax Benefits from the Remeasurement of Deferred Income Tax Assets and Liabilities Resulting from the TCJA Excluding the Related Property Plant and Equipment Amortization Periord of the Income Tax Benefits from the Remeasurement of Deferred Income Tax Assets and Liabilities Resulting from the TCJA Excluding the Related Property Plant and Equipment Goodwill and Intangible Assets [Text Block] Goodwill and Intangible Assets Disclosure [Text Block] 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] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Superior, WI [Member] Superior, WI [Member] Superior, WI Environmental Matters [Line Items] Site Contingency [Line Items] Consent Decree Wind Energy Generation Addition (MW) Expected Period for Costs of Compliance Expected Period for Costs of Compliance Expected Period for Costs of Compliance Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Estimated Costs of Compliance (Accrued) Accrual for Environmental Loss Contingencies Expected Period to Incur Remediation Costs Expected Period to Incur Remediation Costs Expected Period to Incur Remediation Costs Quarterly Financial Data (Unaudited) [Text Block] Quarterly Financial Information [Text Block]