Business Combinations [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] U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] 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 Contingent Consideration Business Combination, Contingent Consideration, Liability Percent of Results of Operations Reflected in Income Statement Business Combination, Percent of Results of Operations Reflected in Income Statement The percent of the acquired business' results of operations reflected in the acquiring entity's income statement. Percentage of Voting Interests Acquired Business Acquisition, Percentage of Voting Interests Acquired Assets Acquired [Abstract] Assets [Abstract] Cash and Cash Equivalents Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Accounts Receivable Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables Inventories Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory Other Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Property, Plant and Equipment Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Intangible Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill 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] 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 Fair Value Adjustments for Work in Progress and Finished Goods Inventories Business combination, Fair Value Adjustment for Work in Process and Finished Goods Fair Value Adjustment for Work in Process and Finished Goods included in inventories at the acquisition date. Fair Value of Sales Backlog Business Combination, Fair Value of Sales Backlog Business Combination, Fair Value of Sales Backlog included in Other Current Assets at the acquisition date. Restricted Cash - Current Restricted Cash and Cash Equivalents, Current Tax Deductible Goodwill Business Acquisition, Goodwill, Expected Tax Deductible Amount Acquisition-Related Costs Business Combination, Acquisition Related Costs Commitments and Contingencies Disclosure [Abstract] Environmental Matters [Table] Site Contingency [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] NOV Consent Decree [Member] NOV Consent Decree [Member] NOV Consent Decree [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] NO2 NAAQS [Member] NO2 NAAQS [Member] NO2 NAAQS [Member] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Environmental Remediation Costs [Member] Environmental Restoration Costs [Member] Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Environmental Matters [Line Items] Site Contingency [Line Items] Additional Wind Energy Generating Capacity (MW) Generating Capacity Owned (MW) Capacity of a generating facility, unit, or project in megawatts. Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Estimated Costs of Compliance - Accrued Site contingency, Loss Exposure Accrued and Best Estimate of Loss Exposure in Excess of Accrual Amount of loss exposure accrued and the best estimate of reasonably possible loss exposure in excess of the accrual. Chanarambie/Viking [Member] Chanarambie/Viking [Member] Chanarambie/Viking [Member] Power Purchase Agreements [Axis] Long-term Contract for Purchase of Electric Power [Axis] Power Purchase Agreements [Domain] Long-term Contract for Purchase of Electric Power [Domain] Chanarambie/Viking PPA (expires 2018) [Member] Chanarambie/Viking PPA (expires 2018) [Member] Chanarambie/Viking PPA (expires 2018) [Member] Chanarambie/Viking PPA (expires 2023) [Member] Chanarambie/Viking PPA (expires 2023) [Member] Chanarambie/Viking PPA (expires 2023) [Member] Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets Power Sales Agreements - Non-Current Liability 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. Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Total Liabilities Assumed Net Identifiable Assets Acquired Power Sales Agreements - Current Liability 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. Retirement Benefits [Abstract] Pension and Other Postretirement Benefit Plan Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Irrevocable Grantor Trust [Member] Trust for Benefit of Employees [Member] 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] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] 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 Service Cost Defined Benefit Plan, Service Cost Interest Cost Defined Benefit Plan, Interest Cost Actuarial 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 Plan Amendments Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment Obligation, End of Year Change in Plan Assets [Roll Forward] Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Fair Value, Beginning of Year Defined Benefit Plan, Fair Value of Plan Assets Actual Return on Plan Assets Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) Employer Contribution Defined Benefit Plan, Plan Assets, Contributions by Employer Participant Contributions Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees Benefits Paid Defined Benefit Plan, Plan Assets, Benefits Paid Fair Value, End of Year Funded Status, End of Year Defined Benefit Plan, Funded (Unfunded) Status of Plan 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 Other Investments Long-term Investments Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Business Segments [Axis] Segments [Axis] Business Segments [Domain] Segments [Domain] Regulated Operations [Member] Regulated Operations [Member] [Member] Operation of entities that are regulated by governmental organizations for example, but not limited to, local, state, county, country, or other regulatory organizations. ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] Corporate and Other [Member] Corporate and Other [Member] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Generation [Member] Electricity Generation Plant, Non-Nuclear [Member] Transmission [Member] Electric Transmission [Member] Distribution [Member] Electric Distribution [Member] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] 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 Water & Energy Systems Technology [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] 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] Total Assets Acquired Liabilities Assumed [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Segment Reporting [Abstract] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] MPUC [Member] MPUC [Member] Minnesota Public Utilities Commission [Member] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] 2016 Minnesota General Rate Case [Member] 2016 Minnesota Rate Case [Member] [Member] 2016 Minnesota Rate Case [Member] [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] BNI Energy [Domain] BNI Energy [Domain] BNI Energy [Domain] 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 [Member] Retail Customers [Member] Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities 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 [Member] Electric Rates [Member] Electric Rates [Member] Natural Gas [Member] Natural Gas [Member] Natural Gas Customer [Member] Water [Member] Water [Member] Water Customer [Member] 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] 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. Business Segments [Line Items] Segment Reporting Information [Line Items] Alternative Revenue Program, Required Collection Period (Months) Alternative Revenue Program, Required Collection Period Alternative Revenue Program, Required Collection Period 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) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Minnesota Power Output Entitlement (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 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 [Table Text Block] Tabular disclosure or assets, excluding financial assets and goodwill, lacking physical substance, by either major class or business segment. Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Abstract] Accumulated Other Comprehensive Loss [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Components [Axis] Total [Domain] Equity Component [Domain] Unrealized Gain (Loss) on Available-for-sale Securities [Member] Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Defined Benefit Pension, Other Postretirement Items [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Gain (Loss) on Cash Flow Hedge [Member] Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Accumulated Other Comprehensive Loss [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Loss [Roll Forward] AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] Beginning Balance Accumulated Other Comprehensive Income (Loss), Net of Tax Other Comprehensive Income (Loss) Before Reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts Reclassified from Accumulated Other Comprehensive Loss Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Net Other Comprehensive Income (Loss) Other Comprehensive Income (Loss), Net of Tax Ending Balance Supplemental Statement of Cash Flow Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Accounts Receivable [Table Text Block] Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Inventories – Net [Table Text Block] Schedule of Inventory, Current [Table Text Block] Prepayments and Other Current Assets [Table Text Block] Schedule of Other Current Assets [Table Text Block] Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Operating Expenses – Other [Table Text Block] Schedule of Other Operating Cost and Expense, by Component [Table Text Block] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Net Loss 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 Contributions in Excess of Net Periodic Benefit Cost (Prepaid Pension Asset) 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 Equity [Abstract] Forward Sale Agreement and Issuance of Common Stock [Table] Forward Contract Indexed to Issuer's Equity [Table] Forward Contract Indexed to Issuer's Equity, Type [Axis] Forward Contract Indexed to Issuer's Equity, Type [Axis] Forward Contract Indexed to Issuer's Equity, Type [Domain] Forward Contract Indexed to Issuer's Equity, Type [Domain] Forward Sale Agreement [Member] Forward Contracts [Member] Forward Contract Indexed to Issuer's Equity [Line Items] Forward Contract Indexed to Issuer's Equity [Line Items] Indexed Shares Forward Contract Indexed to Issuer's Equity, Indexed Shares Forward Rate Per Share Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share Shares Issued Common Stock, Shares, Issued Proceeds from Issuance of Common Stock Proceeds from Issuance or Sale of Equity Quarterly Financial Information Disclosure [Abstract] Quarterly Financial Data (Unaudited) [Text Block] Quarterly Financial Information [Text Block] Commitments, Guarantees and Contingencies [Text Block] Commitments and Contingencies Disclosure [Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Share-Based Compensation Expense [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Award Type [Domain] Equity Award [Domain] Performance Shares [Member] Performance Shares [Member] Restricted Stock Units [Member] Restricted Stock Units (RSUs) [Member] Share-Based Compensation Expense [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-Based Compensation Expense Allocated Share-based Compensation Expense Income Tax Benefit Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Capitalized Share-Based Compensation Costs Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Unrecognized Compensation Cost Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted-Average Period for Recognition (Years/Months) Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 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] 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] 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] Resource Package [Member] Resource Package [Member] Resource Package [Member] Jointly-Owned Facilities Projects [Axis] Jointly Owned Utility Plant [Axis] Jointly-Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Jointly-Owned Electricity Generation Plant [Member] Jointly Owned Electricity Generation Plant [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] 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] Long-term Contract for Purchase of Electric Power [Domain] Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Minimum Annual Payments for Certain Long-Term Commitments [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Minimum Payments in 2018 Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months Minimum Payments in 2019 Unrecorded Unconditional Purchase Obligation, Due within Two Years Minimum Payments in 2020 Unrecorded Unconditional Purchase Obligation, Due within Three Years Minimum Payments in 2021 Unrecorded Unconditional Purchase Obligation, Due within Four Years Minimum Payments in 2022 Unrecorded Unconditional Purchase Obligation, Due within Five Years Minimum Payments Thereafter Unrecorded Unconditional Purchase Obligation, Due after Five Years Minimum Payments in 2018 Operating Leases, Future Minimum Payments Due, Next Twelve Months Minimum Payments in 2019 Operating Leases, Future Minimum Payments, Due in Two Years Minimum Payments in 2020 Operating Leases, Future Minimum Payments, Due in Three Years Minimum Payments in 2021 Operating Leases, Future Minimum Payments, Due in Four Years Minimum Payments in 2022 Operating Leases, Future Minimum Payments, Due in Five Years Minimum Payments Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Statement of Financial Position [Abstract] Accounts Receivable [Abstract] Accounts Receivable, Net, Current [Abstract] Accounts Receivable, Allowance Allowance for Doubtful Accounts Receivable, Current Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value 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 Reason for Business Acquisitions Business Combination, Reason for Business Combination Pro Forma Impact of Business Acquisitions Business Acquisition, Pro Forma Information, Description Pension and Other Postretirement Benefit Plans Recurring Fair Value Measures [Table] Plan Asset Categories [Axis] Defined Benefit Plan, Asset Categories [Axis] Plan Asset Categories [Domain] Plan Asset Categories [Domain] Equity Securities [Member] Equity Securities [Member] U.S. Large-cap [Member] U.S. Large-Cap [Member] Equity Securities, U.S. Large-Cap [Member] U.S. Mid-cap Growth [Member] U.S. Mid-Cap Growth [Member] Equity Securities, U.S. Mid-Cap Growth [Member] U.S. Small-cap [Member] U.S. Small Cap [Member] Equity Securities, U.S. Small Cap [Member] International [Member] Equity Securities, International [Member] Debt Securities [Member] Debt Securities [Member] Mutual Funds [Member] Fixed Income Funds [Member] Fixed Income [Member] Fixed Income Securities [Member] Cash and Cash Equivalents [Member] Cash and Cash Equivalents [Member] Private Equity Funds [Member] Real Estate [Member] Defined Benefit Plan, Fair Value of Plan Assets by Measurement [Axis] Fair Value, Hierarchy [Axis] Total [Domain] Fair Value Hierarchy [Domain] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Recurring Fair Value Measures [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value of Assets Debt Disclosure [Abstract] Short-Term Debt - Outstanding Debt, Current Short-Term Debt - Unamortized Debt Issuance Costs Debt Issuance Costs, Current, Net Lines of Credit [Abstract] Line of Credit Facility [Abstract] Lines of Credit [Table] Line of Credit Facility [Table] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Bank Lines of Credit [Member] Revolving Credit Facility [Member] Lines of Credit [Line Items] Line of Credit Facility [Line Items] Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Standby Letters of Credit Outstanding Line of Credit, Amount Pledged for Letters of Credit Amount of line of credit borrowing capacity pledged against letters of credit at the end of the period limiting the line of credit borrowing capacity but not reflected as debt on the balance sheet. Draws Outstanding Line of Credit, Current Private Equity [Member] Real Estate [Member] Actual Plan Asset Allocations Defined Benefit Plan, Actual Plan Asset Allocations ALLETE Common Stock Included in Pension Plan Equity Securities (Shares) Defined Benefit Plan, Number of Shares of Equity Securities Issued by Employer and Related Parties Included in Plan Assets Plan Asset Target Allocations Defined Benefit Plan, Plan Assets, Target Allocation, Percentage Financial Covenants [Abstract] Debt Instruments [Abstract] Allowed Indebtedness to Total Capitalization Ratio Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Actual Indebtedness to Total Capitalization Ratio Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Compliance with Financial Covenants Debt Instrument, Covenant Compliance A and W Technologies [Member] A and W Technologies, Inc. [Member] A and W Technologies, Inc. [Member] Payment Due in April 2017 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] Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities [Table Text Block] Schedule of Defined Benefit Plan Amounts Recognized in Other Comprehensive Income (Loss) [Table Text Block] Information for Pension Plans with an Accumulated Benefit Obligation in Excess of Plan Assets [Table Text Block] Schedule of Accumulated Benefit Obligations in Excess of Fair Value of Plan Assets [Table Text Block] 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] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Estimated Future Benefit Payments [Abstract] Defined Benefit Plan, Expected Future Benefit Payment [Abstract] 2018 Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months 2019 Defined Benefit Plan, Expected Future Benefit Payment, Year Two 2020 Defined Benefit Plan, Expected Future Benefit Payment, Year Three 2021 Defined Benefit Plan, Expected Future Benefit Payment, Year Four 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year Five Years 2023 - 2027 Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter Acquisitions [Text Block] Business Combination Disclosure [Text Block] Valuation and Qualifying Accounts [Abstract] Valuation and Qualifying Accounts and Reserves [Table] Valuation and Qualifying Accounts Disclosure [Table] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves Type [Axis] Valuation Allowances and Reserves [Domain] Valuation Allowances and Reserves [Domain] Trade Accounts Receivable [Member] Allowance for Trade Receivables [Member] Finance Receivables – Long-Term [Member] Allowance for Losses on Finance Receivables [Member] Deferred Tax Assets [Member] Valuation Allowance of Deferred Tax Assets [Member] Valuation and Qualifying Accounts and Reserves [Line Items] Valuation and Qualifying Accounts Disclosure [Line Items] Valuation and Qualifying Accounts and Reserves [Roll Forward] Movement in Valuation Allowances and Reserves [Roll Forward] Balance at Beginning of Period Valuation Allowances and Reserves, Balance Additions, Charged to Income Valuation Allowances and Reserves, Additions for Charges to Cost and Expense Additions, Other Charges Valuation Allowances and Reserves, Additions for Charges to Other Accounts Deductions from Reserves Valuation Allowances and Reserves, Deductions Balance at End of Period 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) Transmission [Table] Public Utility, Property, Plant and Equipment [Table] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro PPA (expires May 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Transmission [Axis] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Transmission [Domain] Name of transmission investment. Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Transmission [Line Items] Public Utility, Property, Plant and Equipment [Line Items] Output Being Purchased (MW) 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. Estimated Capital Expenditures, Including Past Expenditures Estimated Capital Expenditures, Including Past Expenditures The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. Total Project Costs to Date - Incurred Capital Cost Spent to Date Capital cost spent to date for project. Total Project Costs to Date - Recovered Capital Costs Spent To Date, Recovered Capital Costs Spent To Date, Recovered 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 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 Goodwill and Intangible Assets [Text Block] Goodwill and Intangible Assets Disclosure [Text Block] Employee Stock and Incentive Plans [Text Block] Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Employee Stock Ownership Plan [Abstract] Share-based Arrangements with Employees and Nonemployees [Abstract] ESOP Employer Loan Employee Stock Ownership Plan (ESOP), Debt Structure, Employer Loan, Description ESOP Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense ESOP Shares, Allocated Employee Stock Ownership Plan (ESOP), Number of Allocated Shares ESOP Shares, Unallocated Employee Stock Ownership Plan (ESOP), Number of Suspense Shares ESOP Shares, Total Employee Stock Ownership Plan (ESOP), Shares in ESOP ESOP Shares, Fair Value of Unallocated Shares Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider [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] Total [Domain] Eliminations [Member] Consolidation, Eliminations [Member] Wind Construction Project [Member] Wind Construction Project [Member] Wind Construction Project [Member] Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments Number of Operating Segments Number of Operating Segments Total Operating Revenue Revenues Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent Depreciation and Amortization Depreciation, Depletion and Amortization Operating Expenses – Other Other Cost and Expense, Operating Interest Expense Interest Expense Equity Earnings in ATC Income (Loss) from Equity Method Investments Deferred Income Tax Expense (Benefit) due to the 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 Income Tax Expense (Benefit) Income Tax Expense (Benefit) Assets Assets Capital Expenditures Property, Plant and Equipment, Additions Schedule of Indefinite-Lived Intangible Assets [Table] Schedule of Indefinite-Lived Intangible Assets [Table] Indefinite-Lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-Lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] Trademarks and Trade Names [Member] Trademarks and Trade Names [Member] Indefinite-Lived Intangible Assets [Line Items] Indefinite-lived Intangible Assets [Line Items] Indefinite-Lived Intangible Assets [Roll Forward] Indefinite-lived Intangible Assets [Roll Forward] Beginning Balance Indefinite-Lived Intangible Assets (Excluding Goodwill) Additions Indefinite-lived Intangible Assets Acquired Ending Balance Schedule of Definite-Lived Intangible Assets [Table] 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] Customer Relationships [Member] Customer Relationships [Member] Developed Technology and Other [Member] Other Intangible Assets [Member] Weighted Average [Member] Weighted Average [Member] Definite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Definite-Lived Intangible Assets [Roll Forward] Finite-lived Intangible Assets [Roll Forward] Beginning Balance Finite-Lived Intangible Assets, Net Additions Finite-lived Intangible Assets Acquired Amortization Amortization of Intangible Assets Ending Balance Useful Life (Years) Finite-Lived Intangible Asset, Useful Life Definite-Lived Intangible Assets, Estimated Annual Amortization Expense [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] 2018 Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2022 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Intangible Assets [Abstract] Intangible Assets, Net (Excluding Goodwill) [Abstract] Total Intangible Assets Intangible Assets, Net (Excluding Goodwill) Total Intangible Assets, Additions Intangible Assets Acquired Amount of increase in assets, excluding financial assets and goodwill, lacking physical substance, resulting from a business combination. Total Intangible Assets, Amortization Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Income Tax Disclosure [Abstract] Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Income Before Non-Controlling Interest and Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Taxes Computed at 35 Percent Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (Decrease) in Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Regulatory Differences for Utility Plant Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Change in Fair Value of Contingent Consideration Effective Income Tax Rate Reconciliation, Other Adjustments, Amount 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 Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Investments [Abstract] Investments [Text Block] Investment [Text Block] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Great River Energy Capacity and Energy PPA (expires May 2020) [Member] Great River Energy PPA (expires 2020) [Member] Great River Energy Capacity and Energy PPA (expires 2020) [Member] Great River Energy Capacity PPA Beginning June 2016 (expires May 2020) [Member] Great River Energy Capacity Only PPA Beginning 2016 (expires 2020) [Member] Great River Energy Capacity Only PPA (begins 2016/expires 2020) [Member] Great River Energy Capacity PPA Beginning June 2017 (expires May 2020) [Member] Great River Energy Capacity Only PPA Beginning 2017 (expires 2020) [Member] Great River Energy Capacity Only PPA Beginning 2017 (expires 2020) [Member] Manitoba Hydro PPA Beginning June 2015 (expires May 2020) [Member] Manitoba Hydro PPA (expires May2020) [Member] Manitoba Hydro PPA (expires May2020) [Member] Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member] Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member] Manitoba Hydro PPA Beginning June 2017 (expires May 2020) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Minnkota Power PPA (expires May 2020) [Member] Minnkota PPA [Member] Minnkota PPA [Member] Shell Energy PPA (expires December 2019) [Member] Shell Energy PPA [Member] Contract with Shell Energy North America, for the purchase of energy [Member] Manitoba Hydro PPA (expires April 2022) [Member] Manitoba Hydro PPA (expires April 2022) [Member] Long-term contract with Manitoba Hydro for the purchase of electric power. Oliver Wind I PPA (expires December 2031) [Member] Oliver Wind I PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. Oliver Wind II PPA (expires December 2032) [Member] Oliver Wind II PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. TransAlta Off-Peak Hours PPA (expires December 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta On-Peak Hours PPA (expire December 2019) [Member] TransAlta On-Peak Hours PPA (expire 2019) [Member] TransAlta On-Peak Hours PPA (expires 2019) Tenaska PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] Square Butte PPA (expires 2026) [Member] Great River Energy [Member] Great River Energy [Member] Great River Energy [Member] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Square Butte [Member] Square Butte [Member] Square Butte [Member] Oliver Wind [Member] Oliver Wind [Member] Oliver Wind [Member] Shell Energy [Member] Shell Energy [Member] Shell Energy [Member] TransAlta [Member] TransAlta [Member] TransAlta [Member] Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] Basin [Member] Basin [Member] Basin [Member] Silver Bay Power [Member] Silver Bay Power [Member] Silver Bay Power [Member] Square Butte Coal-fired Unit [Member] Silver Bay Power Self-Generation [Member] Silver Bay Power Coal, Liquid and Gaseous Fuels-fired Units [Member] Silver Bay Power Coal, Liquid and Gaseous Fuels-fired Units [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Basin PSA (expires April 2020) [Member] Basin Power Sales Agreement (expires April 2020) [Member] Basin Power Sales Agreement (expires April 2020) [Member] Basin PSA (expires 2018) [Member] Basin Power Sales Agreement (expires 2018) [Member] Basin Power Sales Agreement (expires 2018) [Member] Basin PSA (expires 2019) [Member] Basin Power Sales Agreement (expires 2019) [Member] Basin Power Sales Agreement (expires 2019) [Member] Minnkota Power PSA (expires December 2026) [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Silver Bay Power PSA through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Silver Bay Power Sales Agreement through 2031 (Years 2016-2019) [Member] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Generating Unit Capacity (MW) 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 2022 Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required Cost of Purchased Power Utilities Operating Expense, Purchased Power under Long-term Contracts Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges Output Being Purchased (MWh) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MWh) Share of plant output being purchased in megawatthours under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. 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. Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Output Being Sold (MW) Long-term Contract for Sale of Electric Power, Share of Plant Output Being Sold (MW) Share of plant output being purchased in megawatts under a long-term contract to sell electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Table] Document Information [Table] Document and Entity Information, Document [Axis] Document Information, Document [Axis] Document [Domain] Document [Domain] Document and Entity Information [Line Items] Document Information [Line Items] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Entity Tax Identification Number Entity Tax Identification Number Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Filer Category Entity Filer Category Entity Public Float Entity Public Float Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Common Stock and Earnings Per Share [Text Block] Stockholders' Equity Note Disclosure [Text Block] Fair Value Disclosures [Abstract] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Recurring Fair Value Measures - Activity in Level 3 [Table Text Block] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Stock-Based Compensation [Table] Stock-based Compensation [Line Items] 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) Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 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 Schedule II [Text Block] Schedule of Valuation and Qualifying Accounts Disclosure [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, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation Fair Value of Plan Assets Defined Benefit Plan, Pension Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Fair Value of Plan Assets Fair Value [Text Block] Fair Value Disclosures [Text Block] Employee Stock Ownership Plan [Table Text Block] Employee Stock Ownership Plan (ESOP) Disclosures [Table Text Block] Share-Based Compensation Expense [Table Text Block] Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan [Table Text Block] Non-Qualified Stock Options [Table Text Block] Share-based Compensation, Stock Options, Activity [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] Accounts Receivables [Abstract] Receivables [Abstract] Billed Accounts Receivable, Gross, Current Unbilled Unbilled Receivables, Current Less: Allowance for Doubtful Accounts Total Accounts Receivable Accounts Receivable, Net, Current Inventories – Net [Abstract] Inventory, Net [Abstract] Fuel Energy Related Inventory, Coal Materials and Supplies Inventory, Supplies, Net of Reserves Raw Materials Inventory, Raw Materials, Gross Work in Progress Inventory, Work in Process, Gross Finished Goods Inventory, Finished Goods, Gross Reserve for Obsolescence Inventory Valuation Reserves Total Inventories Inventory, Net Prepayments and Other Current Assets [Abstract] Prepaid Expense and Other Assets, Current [Abstract] Deferred Fuel Adjustment Clause Deferred Fuel Cost Restricted Cash Other Other Assets, Miscellaneous, Current Total Prepayments and Other Current Assets Other Assets, Current Property, Plant and Equipment [Abstract] Business Segments [Axis] 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] Goodwill [Line Items] Goodwill [Line Items] Impairment of Goodwill Goodwill, Impairment Loss Discount Rate Fair Value Inputs, Discount Rate Annual Revenue Growth Rate Fair Value Inputs, Long-term Revenue Growth Rate Revenue Growth Rate Related to Impact of Acquisition Fair Value Inputs, Revenue Growth Rate from Acquisition Fair Value Inputs, Revenue Growth Rate from Acquisition Terminal Growth Rate Terminal Growth Rate Terminal Growth Rate Definite-Lived Intangible Assets [Table] Indefinite-Lived Intangible Assets [Abstract] Indefinite-Lived Intangible Assets (Excluding Goodwill) [Abstract] Impairment of Intangible Assets Impairment of Intangible Assets (Excluding Goodwill) Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Contract Payment Contract Payment Non-Current Carrying value of a non-current deferred asset related to a payment made as part of a long-term contract. Financing Receivable Financing Receivable, Gross Restricted Cash Restricted Cash and Cash Equivalents, Noncurrent Other Other Assets, Miscellaneous, Noncurrent Total Other Non-Current Assets Other Assets, Noncurrent Real Estate Sale Consideration [Abstract] Real Estate Sale Consideration [Abstract] Real Estate Sale Consideration [Abstract] Total Consideration for Land Inventory Noncash or Part Noncash Divestiture, Amount of Consideration Received Down Payment of ALLETE Common Stock (Shares) Stock Repurchased and Retired During Period, Shares Down Payment of ALLETE Common Stock Stock Repurchased and Retired During Period, Value Finance Receivable Term (Years) Financing Receivable, Term Contractual term of a financing receivable. Other Current Liabilities [Abstract] Other Liabilities, Current [Abstract] PSAs 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 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] Charge to Fuel, Purchased Power and Gas - Utility Cost of Purchased Power Amortization of Power Sales Agreements Amortization of Power Purchase Agreements The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method. Minimum Cash Collections Requirement for Real Estate Sales (Percent) Real Estate Sales Revenue Recognition, Minimum Contract Price Cash Collections Requirement The minimum amount of cash collected on the contract price in order to fully recognize profit on the sale of real estate upon closing (company policy). Operating Expenses – Other [Abstract] Other Income and Expenses [Abstract] Change in Fair Value of Contingent Consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Total Operating Expenses – Other Other Operating Income (Expense), Net Income Taxes [Abstract] 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] 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] Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Decrease to Deferred Income Taxes Decrease to Deferred Income Taxes Decrease to Deferred Income Taxes 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 Short-Term and Long-Term Debt [Text Block] Debt 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] Pension Plan [Member] Employer Contributions to Defined Benefit Plans - Cash Defined Benefit Plan, Plan Assets, Cash Contributions by Employer Defined Benefit Plan, Plan Assets, Cash Contributions by Employer Employer Contributions to Pension - Shares Stock Issued During Period, Shares, Employee Benefit Plan Employer Contributions to Pension - Value Stock Issued During Period, Value, Employee Benefit Plan 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 2018 Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year Statement of Cash Flows [Abstract] Operating Activities Net Cash Provided by (Used in) Operating Activities [Abstract] Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Allowance for Funds Used During Construction – Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Increase (Decrease) in Deferred Fuel Costs Increase (Decrease) in Deferred Fuel Costs Gain on Sales of Investments and Property, Plant and Equipment Gain (Loss) on Disposition of Assets Depreciation Expense Depreciation Amortization of Power Sales Agreements Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Expense Share-Based 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 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 Acquired Payments to Acquire Businesses, Net of Cash Acquired Investment in ATC Payments to Acquire Equity Method Investments Changes to Other Investments Payments for (Proceeds from) Other Investing Activities Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Proceeds from Sale of Property, Plant and Equipment Proceeds from Sale of Productive Assets Changes in Restricted Cash Increase (Decrease) in Restricted Cash Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities Financing Activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from Issuance of Common Stock Proceeds from Issuance of Common Stock Proceeds from Issuance of Long-Term Debt Proceeds from Issuance of Long-term Debt Changes in Restricted Cash Proceeds from (Repayments of) Restricted Cash, Financing Activities Changes in Notes Payable Proceeds from (Repayments of) Notes Payable Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition of Non-Controlling Interest Payment for Contingent Consideration Liability, Financing Activities Payment for Contingent Consideration Liability, Financing Activities Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Other Financing Activities Proceeds from (Payments for) Other Financing Activities Cash from (for) Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and Cash Equivalents at Beginning of Period Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents at End of Period Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount Long-term Debt, Gross Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Supplemental Cash Flow Information [Abstract] Cash Paid During the Period for Interest – Net of Amounts Capitalized Interest Paid, Net Cash Paid During the Period for Income Taxes Income Taxes Paid, Net Remeasurement of Deferred Income Taxes Resulting from the TCJA [Abstract] Remeasurement of Deferred Income Taxes Resulting from the TCJA [Abstract] 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 Capital Expenditures Incurred but Not yet Paid Capitalized Asset Retirement Costs Camp Ripley Solar Financing Liabilities Assumed AFUDC–Equity ALLETE Common Stock Contributed to the Defined Benefit Pension Plan Contingent Consideration Noncash or Part Noncash Acquisition, Value of Liabilities Assumed Noncash Consideration for Land Inventory [Table] Noncash or Part Noncash Divestitures [Table] Unique Name [Axis] Unique Name [Axis] Noncash Consideration for Land Inventory [Domain] Noncash or Part Noncash Divestiture, Name [Domain] ALLETE Common Stock Received for Land Inventory [Member] ALLETE Common Stock Received for Land Inventory [Member] ALLETE Common Stock Received for Land Inventory [Member] Long-Term Finance Receivable for Land Inventory [Member] Long-Term Finance Receivable for Land Inventory [Member] Long-Term Finance Receivable for Land Inventory [Member] Noncash Consideration for Land Inventory [Line Items] Noncash or Part Noncash Divestitures [Line Items] Consideration for Land Inventory Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Abstract] Jointly-Owned Facilities and Projects [Table] Jointly Owned Utility Plant Interests [Table] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Jointly-Owned Facilities and Projects [Domain] Transmission [Member] Jointly Owned Electricity Transmission and Distribution System [Member] CapX2020 Projects [Member] CapX2020 [Member] CapX2020 (all projects) [Member] Jointly-Owned Facilities and Projects [Line Items] Jointly Owned Utility Plant Interests [Line Items] Minnesota Power Ownership % Jointly Owned Utility Plant, Proportionate Ownership Share WPPI Energy Ownership % Jointly-Owned Electric Facility Percentage Not Owned Jointly-Owned Electric Facility Percentage Not Owned Number of Projects Number of Projects Number of projects. Plant in Service Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service Accumulated Depreciation Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation Construction Work in Progress Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress Statement of Stockholders' Equity [Abstract] Consolidated Statement of Equity [Table] Statement [Table] Equity Components [Axis] Total Equity [Domain] Retained Earnings [Member] Retained Earnings [Member] Accumulated Other Comprehensive Loss [Member] AOCI Attributable to Parent [Member] Unearned ESOP Shares [Member] Deferred Compensation, Share-based Payments [Member] 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 Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Other Comprehensive Income – Net of Tax Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Gain (Loss) on Securities Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Unrealized Gain on Derivatives Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Defined Benefit Pension and Other Postretirement Plans Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Common Stock Issued Stock Issued During Period, Value, New Issues Dividends Declared Dividends, Common Stock ESOP Shares Earned Stock Issued During Period, Value, Employee Stock Ownership Plan Common Stock Retired Acquisition of Non-Controlling Interest Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Ending Balance 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 Forward Sale Agreement and Issuance, Shares Stock Issued During Period, Shares, Forward Sale Number of shares issued during the period from the forward sales agreement, Forward Sales Agreement and Issuance, Equity Stock Issued During Period, Value, Forward Sale Value of shares issued during the period from the forward sales agreement. Contributions to 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 Received for Sale of Land Inventory, Equity Acquisition of Non-Controlling Interest, Shares Stock Issued During Period, Shares, Acquisitions Acquisition of Non-Controlling Interest, Equity 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 Leasing Agreements [Table] Schedule of Operating Leased Assets [Table] Leasing Agreements [Axis] Leasing Agreements [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 Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Cash and Cash Equivalents Accounts Receivable (Less Allowance of $2.1 and $3.1) Inventories – Net Prepayments and Other Total Current Assets Assets, Current Property, Plant and Equipment – Net Regulatory Assets Regulatory Assets, Noncurrent Investment in ATC Equity Method Investments Other Investments Goodwill and Intangible Assets – Net Intangible Assets, Net (Including Goodwill) Other Non-Current Assets Total Assets Liabilities and Equity [Abstract] Liabilities and Equity [Abstract] Liabilities [Abstract] Current Liabilities [Abstract] Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest Payable, Current Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Other Total Current Liabilities Liabilities, Current Long-Term Debt Long-term Debt, Excluding Current Maturities Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Regulatory Liability, Noncurrent 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, 51.1 and 49.6 Shares Issued and Outstanding Common Stock, Value, Issued Accumulated Other Comprehensive Loss Retained Earnings Retained Earnings (Accumulated Deficit) Total Shareholders' Equity Total Liabilities and Shareholders' Equity Liabilities and Equity Jointly-Owned Facilities and Projects [Table Text Block] Schedule of Jointly Owned Utility Plants [Table Text Block] 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 Schedule of Error Corrections and Prior Period Adjustment Restatement [Table] Schedule of Error Corrections and Prior Period Adjustment Restatement [Table] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Deferred Income Taxes [Member] Deferred Income Taxes [Member] Deferred Income Taxes [Member] Error Corrections and Prior Period Adjustments Restatement [Line Items] Error Corrections and Prior Period Adjustments Restatement [Line Items] Immaterial Revision Quantifying Misstatement in Current Year Financial Statements, Amount 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 Credit Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, 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 Goodwill [Roll Forward] Goodwill [Roll Forward] Beginning Balance Acquired Goodwill Goodwill, Acquired During Period Impairment Charge Other Adjustments Ending Balance Equity Method Investments and Joint Ventures [Abstract] Investment in ATC [Table] Schedule of Equity Method Investments [Table] Investment in ATC [Member] Investment in ATC [Member] Investment in ATC [Member] Equity Method Investment [Axis] Investment, Name [Axis] Equity Method Investment [Domain] Investment, Name [Domain] ATC [Member] ATC [Member] ATC [Member] FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Return on Equity Complaint 2 [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] Investment in ATC [Line Items] Schedule of Equity Method Investments [Line Items] Ownership Percentage Equity Method Investment, Ownership Percentage Expected Additional Investment in 2018 Equity Method Investment, Expected Additional Investment 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 Distributed ATC Earnings Proceeds from Equity Method Investment, Distribution Remeasurement of Deferred Income Taxes 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. Other Investments [Table Text Block] Investment [Table Text Block] Net Loss Prior Service Credit Total Unrecognized Postretirement Health and Life Cost Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax Armenia Mountain [Member] Armenia Mountain [Member] Armenia Mountain [Member] Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Long-Term Debt Due Within One Year Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Long-Term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Other Non-Current Liabilities Power Sales Agreements - Current Asset Power Purchase Agreements - Current Asset The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current asset. Power Sales Agreements - Non-Current Asset Power Purchase Agreements - Noncurrent Asset The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent asset. Restricted Cash - Non-Current Concentration Risks, Types, No Concentration Percentage [Abstract] Concentration Risk [Table] Concentration Risk [Table] Concentration Risk Type [Axis] Concentration Risk Type [Axis] Concentration Risk Type [Domain] Concentration Risk Type [Domain] Customer Concentration Risk [Member] Customer Concentration Risk [Member] Customer Type [Axis] Customer Type [Axis] Customer Type [Domain] Customer Type [Domain] Large Power Customers [Member] Large Power Customer [Member] Large Power Customer [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Largest Customer [Member] Largest Customer [Member] Largest Customer [Member] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] Consolidated Operating Revenue [Member] Sales Revenue, Net [Member] Concentration Risk [Line Items] Concentration Risk [Line Items] Number of Large Power Customers Receivables from Large Power Customers Accounts Receivable, Gross Percent of Consolidated Operating Revenue Concentration Risk, Percentage Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] ALLETE Properties Real Estate Investments, Net Available-for-sale Securities Available-for-sale Securities, Noncurrent Cash Equivalents Other Restricted Assets, Noncurrent Other Other Long-term Investments Total Other Investments Available-for-sale Corporate Debt Securities, Maturities [Abstract] Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract] One Year or Less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value One Year to Less Than Three Years Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the first fiscal year through the third fiscal year following the latest fiscal year (maturing in more than one year to less than three years). Three Years to Less Than Five Years Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the third fiscal year through the fifth fiscal year following the latest fiscal year (maturing in three years to less than five years). Five or More Years Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing in after the fifth fiscal year following the latest fiscal year (maturing in five years or more). Impairment of Land Inventory Property, Plant and Equipment [Text Block] Property, Plant and Equipment Disclosure [Text Block] Income Statement [Abstract] Operating Revenue [Abstract] Regulated Operating Revenue [Abstract] Utility Regulated Operating Revenue Non-utility Unregulated Operating Revenue Total Operating Revenue Operating Expenses [Abstract] Costs and Expenses [Abstract] Fuel, Purchased Power and Gas - Utility Transmission Services - Utility Cost of Transmission Cost of Sales - Non-utility Cost of Goods and Services Sold Operating and Maintenance Operating Expenses Taxes Other Than Income Taxes Taxes, Other Other Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Other Income (Expense) [Abstract] Nonoperating Income (Expense) [Abstract] 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 Basic Earnings Per Share of Common Stock Earnings Per Share, Basic Diluted Earnings Per Share of Common Stock Earnings Per Share, Diluted Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared 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. New Accounting Standards [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Consolidated Statement of Income [Domain] New Accounting Standards [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Effect of the Adoption New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification Expected Minimum Lease Payment Due in Future Years Operating Leases, Future Minimum Payments Due Net Loss Total Net Pension Amounts Recognized in Consolidated Balance Sheet Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [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 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] Annual Vesting Percent Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 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) 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 Changes in Accumulated Other Comprehensive Loss [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] Regulated Operations [Abstract] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets [Table Text Block] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Liabilities [Table Text Block] Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Loss Contingencies [Table] Loss Contingencies [Table] Return on Equity Complaint 1 [Member] FERC Complaint 1 [Member] FERC Complaint 1 [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Requested Return on Equity Filed with the FERC by Third Party Requested Return on Common Equity Filed with Regulatory Organization The requested return on common equity, which would be dedicated to the utilities within the regulatory organizations jurisdiction. FERC Authorized Return on 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 Statement of Comprehensive Income [Abstract] Unrealized Gain (Loss) on Securities, Income Tax Expense (Benefit) Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Unrealized Gain on Derivatives, Income Tax Expense Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense (Benefit) Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Financial Statement Preparation [Policy Text Block] Basis of Accounting, Policy [Policy Text Block] Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] Principles of Consolidation [Policy Text Block] Consolidation, Policy [Policy Text Block] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Cash and Cash Equivalents [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] Accounts Receivable [Policy Text Block] Trade and Other Accounts Receivable, Policy [Policy Text Block] Long-Term Finance Receivables [Policy Text Block] Finance, Loan and Lease Receivables, Held-for-investment, Policy [Policy Text Block] Available-for-Sale Securities [Policy Text Block] Marketable Securities, Available-for-sale Securities, Policy [Policy Text Block] Inventories [Policy Text Block] Inventory, Policy [Policy Text Block] Property, Plant and Equipment [Policy Text Block] Property, Plant and Equipment, Policy [Policy Text Block] Impairment of Long-Lived Assets [Policy Text Block] Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Derivatives [Policy Text Block] Derivatives, Policy [Policy Text Block] Accounting for Stock-Based Compensation [Policy Text Block] Share-based Compensation, Option and Incentive Plans Policy [Policy Text Block] Goodwill [Policy Text Block] Goodwill and Intangible Assets, Goodwill, Policy [Policy Text Block] Intangible Assets [Policy Text Block] Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] Environmental Liabilities [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Revenue Recognition [Policy Text Block] Revenue Recognition, Policy [Policy Text Block] Revenue from Cost Recovery Riders [Policy Text Block] Revenue Recognition for Alternative Revenue Programs, Policy [Policy Text Block] Unamortized Discount and Premium on Debt [Policy Text Block] Debt, Policy [Policy Text Block] Income Taxes [Policy Text Block] Income Tax, Policy [Policy Text Block] Income Taxes, Effects of Regulation [Policy Text Block] Regulatory Income Taxes, Policy [Policy Text Block] Unrecognized Tax Benefits [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Excise Taxes [Policy Text Block] Excise Tax Policy [Policy Text Block] Policy regarding the collection and recording of excise taxes. Purchase Accounting [Policy Text Block] Business Combinations Policy [Policy Text Block] New Accounting Standards [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Asset Retirement Obligations [Policy Text Block] Asset Retirement 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] Land Inventory [Policy Text Block] Real Estate Held for Development and Sale, Policy [Policy Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Fair Value Transfers [Policy Text Block] Fair Value Transfer, Policy [Policy Text Block] Property, Plant and Equipment Impairment [Policy Text Block] Property, Plant and Equipment, Impairment [Policy Text Block] Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Pension and Other Postretirement Benefit Plans [Policy Text Block] Pension and Other Postretirement Plans, Policy [Policy Text Block] Employee Stock Ownership Plan [Policy Text Block] Employee Stock Ownership Plan (ESOP), Policy [Policy Text Block] Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Other Comprehensive Income (Loss) [Abstract] Unrealized Gain (Loss) on Securities - Net of Income Tax Expense (Benefit) of $0.7, $(0.2) and $(0.3) Unrealized Gain on Derivatives - Net of Income Tax Expense of $–, $– and $0.1 Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense (Benefit) of $2.2, $(2.4) and $(2.2) Total Other Comprehensive Income (Loss) Total Comprehensive Income Less: Non-Controlling Interest in Subsidiaries Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Total Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Average Common Shares Earnings Per Share Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Attributable to ALLETE, Diluted Net Income (Loss) Attributable to Parent, Diluted Average Common Shares Earnings Per Share Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Jointly-Owned Facilities and Projects [Text Block] Jointly-Owned Electric Facility [Text Block] Jointly-Owned Electric Facility [Text Block] Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Deferred Tax Assets and Liabilities [Table Text Block] Schedule of Deferred Tax Assets and Liabilities [Table Text Block] NOL and Tax Credit Carryforwards [Table Text Block] Summary of Tax Credit Carryforwards [Table Text Block] Gross Unrecognized Income Tax Benefits [Table Text Block] Summary of Income Tax Contingencies [Table Text Block] Operating Income Net Income Attributable to ALLETE Regulatory Matters [Table] Public Utilities General Disclosures [Table] Certificate of Need and Route Permit [Member] Great Northern Transmission Line Certificate of Need and Route Permit [Member] Great Northern Transmission Line Certificate of Need and Route Permit [Member] Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] First Mortgage Bonds - 1.83% Series Due 2018 [Member] ALLETE Bonds 1.83% Due April2018 [Member] ALLETE Bonds 1.83% Due April2018 [Member] First Mortgage Bonds - 8.17% Series Due 2019 [Member] 8.17% Series Due 2019 [Member] First Mortgage Bonds, 8.17% Series Due 2019 [Member] First Mortgage Bonds - 5.28% Series Due 2020 [Member] 5.28% Series Due 2020 [Member] First Mortgage Bonds, 5.28% Series Due 2020 [Member] First Mortgage Bonds - 2.80% Series Due 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] First Mortgage Bonds - 4.85% Series Due 2021 [Member] 4.85% Series Due 2021 [Member] First Mortgage Bonds, 4.85% Series Due 2021 [Member] First Mortgage Bonds - 3.02% Series Due 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] ALLETE Bonds 3.02% Due September 2021 [Member] First Mortgage Bonds - 3.40% Series Due 2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] First Mortgage Bonds - 6.02% Series Due 2023 [Member] 6.02% Series Due 2023 [Member] First Mortgage Bonds, 6.02% Series Due 2023 [Member] First Mortgage Bonds - 3.69% Series Due 2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] First Mortgage Bonds - 4.90% Series Due 2025 [Member] 4.90% Series Due 2025 [Member] First Mortgage Bonds, 4.90% Series Due 2025 [Member] First Mortgage Bonds - 5.10% Series Due 2025 [Member] 5.10% Series Due 2025 [Member] First Mortgage Bonds, 5.10% Series Due 2025 [Member] First Mortgage Bonds - 3.20% Series Due 2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] First Mortgage Bonds - 5.99% Series Due 2027 [Member] 5.99% Series Due 2027 [Member] First Mortgage Bonds, 5.99% Series Due 2027 [Member] First Mortgage Bonds - 3.30% Series Due 2028 [Member] ALLETE Bonds 3.30% Due October2028 [Member] ALLETE Bonds 3.30% Due October2028 [Member] First Mortgage Bonds - 3.74% Series Due 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] ALLETE Bonds 3.74% Due September 2029 [Member] First Mortgage Bonds - 3.86% Series Due 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] First Mortgage Bonds - 5.69% Series Due 2036 [Member] 5.69% Series Due 2036 [Member] First Mortgage Bonds, 5.69% Series Due 2036 [Member] First Mortgage Bonds - 6.00% Series Due 2040 [Member] 6.00% Series Due 2040 [Member] First Mortgage Bonds, 6.00% Series Due 2040 [Member] First Mortgage Bonds - 5.82% Series Due 2040 [Member] 5.82% Series Due 2040 [Member] First Mortgage Bonds, 5.82% Series Due 2040 [Member] First Mortgage Bonds - 4.08% Series Due 2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] First Mortgage Bonds - 4.21% Series Due 2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] ALLETE Bonds 4.21% Due October2043 [Member] First Mortgage Bonds - 4.95% Series Due 2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] First Mortgage Bonds - 5.05% Series Due 2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] First Mortgage Bonds - 4.39% Series Due 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] ALLETE Bonds 4.39% Due September 2044 [Member] Unsecured Term Loan Variable Rate Due 2017 [Member] ALLETE Term Loan Variable Rate Due 2017 [Member] ALLETE Term Loan Variable Rate Due 2017 [Member] Senior Unsecured Notes 5.99% Due 2017 [Member] Senior Unsecured Notes 5.99% Due 2017 Variable Demand Revenue Refunding Bonds Series 1997 A Due 2020 [Member] Variable Demand Revenue Refunding Bonds Series 1997 ABC Due 2013-2020 [Member] Variable Demand Revenue Refunding Bonds Series 1997 A, B, and C Due 2013-2020 [Member] ALLETE Term Loan Variable Rate Due 2020 [Member] ALLETE Term Loan Variable Rate Due 2020 [Member] ALLETE Term Loan Variable Rate Due 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] 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] 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] SWL&P First Mortgage Bonds 4.15% Series Due 2028 [Member] A415SeriesDue2028 [Member] A415SeriesDue2028 [Member] Other Long-Term Debt, 3.11% – 5.37% Due 2018 – 2037 [Member] Other Long-Term Debt [Member] Other long-term debt instruments not specifically identified elsewhere Debt Instrument [Line Items] Interest Rate Debt Instrument, Interest Rate, Stated Percentage Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate 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 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. Fuel, Purchased Power and Gas – Utility [Member] Fuel, Purchased Power and Gas – Utility [Member] Cost of electricity purchased as well as fuel used to generate electricity used in operations and sold during the period. 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 Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Tonka Water [Member] Tonka Water [Member] Tonka Equipment Company [Member] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Total Regulatory Assets [Domain] Deferred Fuel Adjustment Clause [Member] Deferred Fuel Costs [Member] Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension and Other Postretirement Plans Costs [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligation [Member] Asset Retirement Obligation Costs [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] PPACA Income Tax Deferral [Member] Medicare Part D Regulatory Asset [Member] Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). Conservation Improvement Program [Member] Conservation Improvement Program [Member] Conservation Improvement Program [Member] Cost Recovery Riders [Member] Regulatory Clause Revenues, under-recovered [Member] Other [Member] Other Regulatory Assets (Liabilities) [Member] Regulatory Liabilities [Axis] Regulatory Liability [Axis] Total Regulatory Liabilities [Domain] Regulatory Liability [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] 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 [Domain] Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. 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 Current Regulatory Assets Regulatory Assets, Current Non-Current Regulatory Assets Total Regulatory Assets Regulatory Assets Non-Current Regulatory Liabilities Discount Provided to EITE Customers Discount Provided to EITE Customers Discount Provided to EITE Customers Alternative Revenue Program, Required Collection Period (Years) Accounts Receivable Business Combination, Acquired Receivable, Fair Value Trade Names Estimated Minimum Annual Payments for Certain Long-Term Commitments [Table Text Block] Unrecorded Unconditional Purchase Obligations Disclosure [Table Text Block] Property, Plant and Equipment and Estimated Useful Lives of Property, Plant and Equipment [Table Text Block] Property, Plant and Equipment [Table Text Block] Asset Retirement Obligations [Table Text Block] Schedule of Change in Asset Retirement Obligation [Table Text Block] Camp Ripley Financing [Member] Camp Ripley Financing [Member] Camp Ripley Financing [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] 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 2018 Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months Long-Term Debt Maturing in 2019 Long-term Debt, Maturities, Repayments of Principal in Year Two Long-Term Debt Maturing in 2020 Long-term Debt, Maturities, Repayments of Principal in Year Three Long-Term Debt Maturing in 2021 Long-term Debt, Maturities, Repayments of Principal in Year Four Long-Term Debt Maturing in 2022 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, in 'PnYnMnDTnHnMnS' format. Purchase Option Purchase Option, Asset Purchase option to acquire an asset. Proceeds from Issuance of Unsecured Debt Proceeds from Issuance of Unsecured Debt Expected Proceeds from Future Issuance of Senior First Mortgage Bonds Expected Proceeds from Future Issuance of First Mortgage Bonds Expected Proceeds from Future Issuance of First Mortgage Bonds Expected Future Interest Rate Future Debt Instrument, Interest Rate, Stated Percentage Contractual interest rate for funds expected to be borrowed, under the debt agreement. Recurring Fair Value Measurements [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value Measures [Member] Fair Value, Measurements, Recurring [Member] 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 Available-for-sale Securities, Debt Securities, Noncurrent Total Fair Value of Assets Assets, Fair Value Disclosure, Recurring Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Deferred Compensation Deferred Compensation Liability, Classified, Noncurrent U.S. Water Services Contingent Consideration Total Fair Value of Liabilities Liabilities, Fair Value Disclosure, Recurring Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Activity in Level 3 [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning Balance Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value Activity in Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Ending Balance Fair Value Hierarchy Transfers, All Levels Fair Value Transfers Between Levels, All Levels The amount of transfers in and out of levels 1, 2, and 3 of the fair value hierarchy during the period. 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] Number of Options [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Outstanding as of January 1 Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Forfeited Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period Outstanding as of December 31 Exercisable as of December 31 Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Weighted-Average Exercise Price [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Outstanding as of January 1 Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Exercised Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Forfeited Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Outstanding as of December 31 Exercisable as of December 31 Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Cash Received from Non-Qualified Stock Options Exercised Proceeds from Stock Options Exercised Total Intrinsic Value of Options Exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value 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). 2016 Wisconsin General Rate Case [Member] 2016 Wisconsin Rate Case [Member] 2016 retail rate increase request filed with the PSCW [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contracts (expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Boswell Energy Center [Member] Boswell Energy Center [Member] Boswell Energy Center [Member] Revenue from Cost Recovery Riders Regulated Operating Revenue, Other Requested Rate Increase (Decrease) Public Utilities, Requested Rate Increase (Decrease), Percentage Requested Return on Equity Public Utilities, Requested Return on Equity, Percentage Requested Equity Ratio Public Utilities, Requested Equity Capital Structure, Percentage Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Annual Additional Revenue Generated from Requested Interim Rate Increase Public Utilities, Interim Rate Increase (Decrease), Amount Requested Rate Increase, Amended, Amount Public Utilities, Requested Rate Increase (Decrease), Amended, Amount Approved Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Approved Equity Ratio Public Utilities, Approved Equity Capital Structure, Percentage Approved Rate Increase - Amount Public Utilities, Approved Rate Increase (Decrease), Amount Reserve for Interim Rate Refund Reserve for Interim Rate Refund Reserve for Interim Rate Refund 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 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. Reduction in Operating Revenue due to Regulatory Outcome After-Tax Change for North Dakota Investment Tax Credits Reversed Tax Adjustments, Settlements, and Unusual Provisions Approved Rate Increase (Decrease), Percentage Public Utilities, Approved Rate Increase (Decrease), Percentage Solar Energy Generation [Member] Solar Energy Generation [Member] Solar Energy Generation [Member] Integrated Resource Plan [Member] Integrated Resource Plan [Member] Integrated Resource Plan which contains a utility's analysis of existing and future energy resource alternatives and a projection of customer cost impact by class [Member] Solar Energy PPA [Member] Solar Energy PPA [Member] Solar Energy Provider [Member] Output Purchased (Percent) 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 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) 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 Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Table] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] Fair Value, Measurements, Nonrecurring [Member] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items] Recurring Fair Value Measure, Unobservable Inputs, Period Increase (Decrease) Equity Method Investment, Carrying Amount Equity Method Investment, Indicators of Impairment Equity Method Investment, Other than Temporary Impairment Goodwill, Carrying Amount Goodwill, Impairment Charge Intangible Assets, Carrying Amount Intangible Assets, Indicators of Impairment Property, Plant and Equipment, Indicators of Impairment Impairment of Long-Lived Assets Held-for-use Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Text Block] Disclosure of Reclassification Amount [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] Solar Energy Standard Mandate - Overall Mandate Percentage Solar Energy Standard Mandate, Overall Mandate Percentage Minnesota's solar energy mandate for total retail electric sales, excluding sales to certain industrial customers, by the end of 2020. Solar Energy Standard Mandate - Small Scale Solar Mandate Percentage Solar Energy Standard Mandate, Small Scale Solar Mandate Percentage Minnesota's solar energy mandate for small scale solar with photovoltaic devices with a nameplate capacity of 20 kilowatts or less by the end of 2020. Expressed as a percentage of the overall mandate. Solar Energy Standard Mandate - Qualifying Capacity for Small Scale Solar Mandate (MW) Solar Energy Standard Mandate, Qualifying Capacity for Small Scale Solar Mandate The maximum amount of nameplate capacity of a solar device to qualify for counting towards the small scale solar mandate of Minnesota's solar energy standard. Quarterly Financial Data (Unaudited) [Table Text Block] Quarterly Financial Information [Table Text Block] 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 [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) Federal - 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 Long-term Debt [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] CIP Triennial Filing [Member] CIP Triennial Filing [Member] Conservation improvement program triennial filing [Member] CIP Consolidated Filing [Member] CIP Annual Filing [Member] Conservation Improvement Program Annual Filing [Member] CIP Spending Minimum - Percentage CIP Spend Requirement Minnesota's Next Generation Act of 2007 introduced a spend goal of 1.5% of net gross operating revenues from service provided in the state on energy CIPs each year. CIP Annual Energy-Savings Goal - Percentage CIP Energy-Savings Goal Minnesota's Next Generation Act of 2007 introduced an energy-savings goal of 1.5 percent of net gross annual retail electric energy sales beginning with the 2010 program year. 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 2018 CIP Investment Goal, Year Two Conservation improvement program Investment Goal for year two approved by the MPUC. CIP Investment Goal in 2019 CIP Investment Goal, Year Three Conservation improvement program Investment Goal for year three approved by the MPUC. CIP Financial Incentive 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