Goodwill and Intangible Assets [Abstract] Goodwill and Intangible Assets [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. Regulatory Matters [Abstract] Regulatory Matters [Abstract] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Acquisitions [Abstract] Acquisitions [Abstract] Acquisitions [Text Block] Business Combination Disclosure [Text Block] Consolidated Balance Sheet [Abstract] Consolidated Balance Sheet [Abstract] Accounts Receivable [Abstract] Accounts Receivable, Net, Current [Abstract] Accounts Receivable, Allowance Allowance for Doubtful Accounts Receivable, Current Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value Common Stock, No Par Value Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Outstanding Common Stock, Shares, Outstanding Power Purchase Agreements [Abstract] -- None. No documentation exists for this element. -- Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Power Purchase Agreements [Axis] Long-term Contract for Purchase of Electric Power [Axis] Power Purchase Agreements [Domain] Long-term Contract for Purchase of Electric Power [Domain] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Minnkota Power PPA [Member] Minnkota PPA [Member] Minnkota PPA [Member] Oliver Wind I PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. Oliver Wind II PPA [Member] Long-term contract with an affiliate of NextEra Energy, Inc. for the purchase of electric power. Manitoba Hydro PPA (expires May 2020) [Member] Manitoba Hydro PPA (expires May2020) [Member] Manitoba Hydro PPA (expires May2020) [Member] Manitoba Hydro PPA (expires April 2022) [Member] Manitoba Hydro PPA (expires April 2022) [Member] Long-term contract with Manitoba Hydro for the purchase of electric power. Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Manitoba Hydro PPA (expires 2040) [Member] Great River Energy and Capacity PPA (expires May 2020) [Member] Great River Energy and Capacity PPA (expires 2020) [Member] Great River Energy and Capacity PPA (expires 2020) [Member] Great River Capacity Only PPA (expires May 2020) [Member] Great River Capacity Only PPA (expires 2020) [Member] Great River Capacity Only PPA (expires 2020) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta Off-Peak Hours PPA (expires 2019) [Member] TransAlta On-Peak Hours PPA (expire 2019) [Member] TransAlta On-Peak Hours PPA (expire 2019) [Member] TransAlta On-Peak Hours PPA (expires 2019) Power Sales Agreements [Axis] Long-term Contract for Sale of Electric Power [Axis] Information by contract related to sales of electric power. Power Sales Agreements [Domain] Long-term Contract for Sale of Electric Power [Domain] [Domain] for Information by contract related to sales of electric power. Minnkota Power Sales Agreement [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [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 Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Generating Unit Capacity (MW) Generating Capacity (MW) Capacity of a generating facility, unit, or project in megawatts. Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased PPA Counterparty Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Annual Debt Service Long-term Contract for Purchase of Electric Power, Annual Minimum Debt Service Payment Required Power Purchased under Long-term Contracts Utilities Operating Expense, Purchased Power under Long-term Contracts Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges Output Being Purchased (MW) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Fixed Capacity Charges Long-Term Contract for Purchase of Electric Power, Fixed Capacity Carges Long-Term Contract for Purchase of Electric Power, Fixed Capacity Carges Output Being Purchased (MWh) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MWh) Share of plant output being purchased in megawatthours under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Term of Contract (Years) Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Business Segments [Abstract] Business Segments [Abstract] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Earnings Per Share and Common Stock [Abstract] Earnings Per Share and Common Stock [Abstract] Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Available-for-sale Securities [Abstract] Cost Available-for-sale Securities, Amortized Cost Basis Gross Unrealized Gain Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Loss Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Fair Value Available-for-sale Securities, Noncurrent Net Proceeds Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds Gross Realized Gain Available-for-sale Securities, Gross Realized Gains Gross Realized Loss Available-for-sale Securities, Gross Realized Losses Reclassifications Out of Accumualted Other Comprehensive Income (Loss) [Abstract] Reclassifications Out of Accumualted Other Comprehensive Income [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Unrealized Gains and Losses on Available-for-sale Securities [Member] Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] Defined Benefit Pension and Other Postretirement Items [Member] Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Gains and Losses on Cash Flow Hedge [Member] Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Other Income (Expense) [Member] Other Income [Member] Income Tax Expense [Member] Income Tax Expense (Benefit) [Member] Income Tax Expense (Benefit) [Member] Operating Expenses - Operating and Maintenance [Member] Operating Expense [Member] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Beginning Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss), Net of Tax Other Comprehensive Income Before Reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts Reclassified From Accumulated Other Comprehensive Income (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 Accumulated Other Comprehensive Loss Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income [Abstract] AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Before Tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax AOCI Reclassification, Unrealized Gains on Available-for-sale Securities, Tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Before Tax Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, before Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Prior Service Costs, Before Tax Other Comprehensive (Income) Loss, Amortization Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Prior Service Cost (Credit), before Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Actuarial Gains and Losses, Before Tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, for Net Gain (Loss), before Tax AOCI Reclassification, Amortization of Defined Benefit Pension and Other Postretirement Items, Tax Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Tax Regulated Operations [Abstract] Public Utilities General Disclosures [Table] Public Utilities General Disclosures [Table] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] MPUC 2010 Minnesota Rate Case [Member] 2010 Rate Case [Member] Retail rate increase request filed with the Minnesota Public Utilities Commission (MPUC). FERC-Approved Wholesale Rates [Member] Wholesale rates approved by the Federal Energy Regulatory Commission (FERC). PSCW 2012 Wisconsin Rate Case [Member] 2012 Wisconsin Rate Case [Member] 2012 retail rate increase requested filed with the Public Service Commission of Wisconsin (PSCW). Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Rate Information by Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Customer Informtaion by Customer Class [Domain] Customer Informtaion by Customer Class [Domain] Customer Informtaion by Customer Class [Domain] Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Long-term Contract for Sale of Electric Power [Axis] Long-term Contract for Sale of Electric Power [Domain] Wholesale Electric Contracts (Expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] North Dakota Wind Development [Axis] North Dakota Wind Development [Axis] Information about each wind project/transmission line in North Dakota. North Dakota Wind Development [Domain] North Dakota Wind Development [Domain] Name of wind project/transmission line in North Dakota. Bison Wind Energy Center [Member] Bison Wind Energy Center [Member] Bison Wind Energy Center [Member] includes all Bison Wind Facilities Public Utilities, General Disclosures [Line Items] Public Utilities, General Disclosures [Line Items] Approved Rate of Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Approved Percentage of Capital Structure Related to Equity Public Utilities, Approved Equity Capital Structure, Percentage Number of Customers Number of Customers Number of Customers Annual Change in Capacity Charge, Percent Change in Annual Capacity Charge Per Contract, Percentage The annual percentage change of the capacity charge specified in a power purchase, power sales, or electric sales agreement. Length of Notice Required to Terminate (Years) Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract (in years). Investments [Abstract] Other Investments [Table Text Block] Investment [Table Text Block] Available-for-sale Securities [Table Text Block] Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Related Party Transaction [Axis] Related Party Transaction [Axis] Related Party Transaction [Domain] Related Party Transaction [Domain] Related Party [Member] Related Party [Member] Related Party [Member] Business Segments [Axis] Segments [Axis] Business Segments [Domain] Segments [Domain] Regulated Operations [Member] [Member] Regulated Operations [Member] [Member] Operation of entities that are regulated by governmental organizations for example, but not limited to, local, state, county, country, or other regulatory organizations. ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] Corporate and Other [Member] Corporate and Other [Member] Business Segments [Line Items] Segment Reporting Information [Line Items] Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments Description of Effect on Previously Reported Segment Information for Change in Composition of Reportable Segments Number of Reportable Segments Number of Reportable Segments Number of Operating Segments Number of Operating Segments Land in Minnesota (in acres) Area of Land Operating Revenue Revenues Net Income (Loss) Attributable to ALLETE Net Income (Loss) Attributable to Parent Equity Method Investment, Ownership Percentage Equity Method Investment, Ownership Percentage Interest Expense Interest Expense Interest Income Investment Income, Interest Total Assets Assets Operations and Significant Accounting Policies [Abstract] Operations and Significant Accounting Policies [Abstract] Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Short-Term and Long-Term Debt [Abstract] SHORT-TERM AND LONG-TERM DEBT [Abstract] Short-Term and Long-Term Debt [Table] Schedule of Long-term Debt Instruments [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Armenia Mountain [Member] Armenia Mountain [Member] Armenia Mountain [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] ALLETE Term Loan Variable Rate Due 2017 [Member] ALLETE Term Loan Variable Rate Due 2017 [Member] ALLETE Term Loan Variable Rate Due 2017 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] ALLETE Bonds 2.80% Due September 2020 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] ALLETE Bonds 3.86% Due September 2030 [Member] Short-Term and Long-Term Debt [Line Items] Debt Instrument [Line Items] Short-Term Debt Outstanding Debt, Current Long-Term Debt Outstanding Long-term Debt, Excluding Current Maturities Business Combination, Long-Term Debt Assumed Including Current Portion Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Long-term Debt, Including current portion Business Combination, Current Portion of Long-Term Debt Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Principal Amount Debt Instrument, Face Amount Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate Debt Refinanced Repayments of Debt Required 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. Cross-Default Amount Debt Instrument, Cross-Default Amount Amount of other debt defaulted on triggering cross-default for this debt instrument. Proceeds from Issuance of First Mortgage Bonds Proceeds from Issuance of First Mortgage Bond Interest Rate Debt Instrument, Interest Rate, Stated Percentage Transmission [Abstract] Transmission [Abstract] Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Project Status [Axis] Project Status [Axis] Project Status: in progress or complete [Axis] Project Status [Domain] Project Status [Domain] [Domain] for Project Status: in progress or complete [Axis] Completed Projects - Current Year [Member] Completed Project - Current Year [Member] Completed Project - Current Year [Member] Completed Projects - Prior Years [Member] Completed Project - Prior Years [Member] Completed Project - Prior Years [Member] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Name of transmission investment. CapX2020 [Member] CapX2020 [Member] CapX2020 (all projects) [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Transmission [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Number of CapX2020 Projects Number of Projects Number of projects. Capital Expenditures Capital Cost Spent to Date Capital cost spent to date for project. 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 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. Coal Rail and Shipping Contracts [Abstract] Coal Rail and Shipping Contracts [Abstract] Coal, Rail and Shipping Contracts [Table] Unrecorded Unconditional Purchase Obligation [Table] Coal, Rail and Shipping Contracts [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Coal, Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Coal Supply and Transportation Agreements [Member] Coal and Transportation [Member] Coal and Transportation [Member] Coal, Rail and Shipping Contracts [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Minimum Annual Payment Obligation for Remainder of 2015 Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year Minimum Annual Payment Obligation in 2016 Unrecorded Unconditional Purchase Obligation, Due within Two Years Minimum Annual Payment Obligation in 2017 Unrecorded Unconditional Purchase Obligation, Due within Three Years Minimum Annual Payment Obligation in 2018 Unrecorded Unconditional Purchase Obligation, Due within Four Years Minimum Annual Payment Obligation in 2019 Unrecorded Unconditional Purchase Obligation, Due within Five Years Transmission [Abstract] Great Northern Transmission Line (GNTL) [Abstract] Great Northern Transmission Line (GNTL) [Abstract] Transmission [Axis] Transmission [Domain] Transmission [Line Items] Fair Value [Abstract] Fair Value Fair Value [Text Block] Fair Value Disclosures [Text Block] Other Matters [Abstract] Other Matters [Abstract] Schedule of 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 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 Community Development District Obligation [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. Palm Coast Park Community Development District Obligation [Member] Palm Coast Park Community Development District [Member] Community development district obligations for Palm Coast Park District. Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Letters of Credit [Member] Financial Standby Letter of Credit [Member] Surety Bonds [Member] Surety Bond [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Collateral Guarantor Obligations, Collateral Held Directly or by Third Parties, Amount Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Ownership Percentage of Benefited Property Community Development District Obligation, Ownership of Property Benefited Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. Annual Assessment Community Development District Obligation, Annual Assessment Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. Environmental Matters [Abstract] Environmental Matters [Abstract] Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Environmental Consent Decree [Axis] Environmental Consent Decree [Axis] Environmental Consent Decree [Axis] Environmental Consent Decree [Domain] Environmental Consent Decree [Domain] [Domain] for Environmental Consent Decree [Axis] NOV Consent Decree [Member] NOV Consent Decree [Member] NOV Consent Decree [Member] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Additional Wind Generating Capacity (MW) Estimated Capital Expenditures Estimated Capital Expenditures The estimated amount of certain future capital expenditures related to environmental compliance, regulatory matters, or other committments and/or contingencies. Estimated Capital Expenditures Capital Expenditures to Date Estimated Environmental Compliance Costs Site Contingency, Loss Exposure Not Accrued, Best Estimate Estimated Environmental Compliance Costs 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. Expected First Mortgage Bonds Issuance [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Schedule of Goodwill and Intangible Assets [Table] Schedule of Goodwill and Intangible Assets [Table] Schedule of Goodwill and Intangible Assets [Table] Definite-Lived Intangible Assets by Major Class [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] Developed Technology and Other [Member] Rights to developed technology, which can include the right to develop, use market, sell, or offer for sale products, compounds, or intellectual property. Other includes patents, non-compete agreements, and land easements. Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] Trademarks and Trade Names [Member] Trademarks and Trade Names [Member] Weighted Average [Member] Weighted Average [Member] Schedule of Goodwill and Intangible Assets [Line Items] Schedule of Goodwill and Intangible Assets [Line Items] [Line Items] for Schedule of Goodwill and Intangible Assets [Table] Goodwill [Roll Forward] Goodwill [Roll Forward] Balance as of December 31, 2014 Goodwill Acquired Goodwill Goodwill, Acquired During Period Balance as of September 30, 2015 Intangible Assets [Abstract] Intangible Assets, Net (Excluding Goodwill) [Abstract] Definite-Lived Intangible Assets Finite-Lived Intangible Assets, Net Definite-Lived Intangible Assets, Additions Finite-lived Intangible Assets Acquired Definite-Lived Intangible Assets, Amortization Amortization of Intangible Assets Definite-Lived Intangible Assets, Other Finite-Lived Intangible Assets, Other Changes Other changes to finite-lived intangible assets not disclosed separately. Indefinite-Lived Intangible Assets Indefinite-Lived Intangible Assets (Excluding Goodwill) Indefinite-Lived Intangible Assets, Additions Indefinite-lived Intangible Assets Acquired Indefinite-Lived Intangible Assets, Other Indefinite-Lived Intangible Assets, Other Changes Other changes to indefinite-lived intangible assets not disclosed separately. 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, Other Intangible Assets, Other Changes Other changes to finite-lived and indefinite-lived intangible assets not disclosed separately. Definite-Lived Intangible Assets, Useful Life (Years) Finite-Lived Intangible Asset, Useful Life Definite-Lived Intangible Assets, Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Definite-Lived Intangible Assets, Estimated Annual Amortization Expense [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] Definite-Lived Intangible Assets, Estimated Amortization Expense, Remainder of 2015 Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year Definite-Lived Intangible Assets, Estimated Amortization Expense, 2016 Finite-Lived Intangible Assets, Amortization Expense, Year Two Definite-Lived Intangible Assets, Estimated Amortization Expense, 2017 Finite-Lived Intangible Assets, Amortization Expense, Year Three Definite-Lived Intangible Assets, Estimated Amortization Expense, 2018 Finite-Lived Intangible Assets, Amortization Expense, Year Four Definite-Lived Intangible Assets, Estimated Amortization Expense, 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Five Definite-Lived Intangible Assets, Estimated Amortization Expense, Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Fair Value Disclosures [Abstract] Nonrecurring Fair Value Measures [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Nonrecurring Fair Value Measurements [Member] Fair Value, Measurements, Nonrecurring [Member] Nonrecurring Fair Value Measures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Equity Method Investment, Carrying Amount Equity Method Investments Equity Method Investment, Impairment Equity Method Investment, Other than Temporary Impairment Goodwill, Carrying Amount Goodwill, Impairment Goodwill, Impairment Loss Intangible Assets, Carrying Amount Intangible Assets, Impairment Impairment of Intangible Assets (Excluding Goodwill) Property, Plant and Equipment, Impairment Impairment of Long-Lived Assets Held-for-use Reason for Business Acquisitions Business Combination, Reason for Business Combination Pro Forma Impact of Business Acquisitions Business Acquisition, Pro Forma Information, Description Commitments, Guarantees and Contingencies [Abstract] Commitments, Guarantees and Contingencies [Abstract] Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Income Tax Expense [Abstract] Income Tax Expense [Abstract] Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Other Income (Expense) [Abstract] Other Income (Expense) [Abstract] Other Income (Expense) [Text Block] Other Income and Other Expense Disclosure [Text Block] Consolidated Statement of Cash Flows [Abstract] Consolidated Statement of Cash Flows [Abstract] Operating Activities [Abstract] 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 Gain on Sales of Investments and Property, Plant and Equipment Gains Loss on Sale of Assets / Investments Gain or loss on sale or disposal of other assets. Also included are investments. This element represents the undistributed income (or loss) of equity method investments, net of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporations; such investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities. Depreciation Expense Depreciation Amortization of Power Purchase Agreements Amortization of Power Purchase Agreements The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method. Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Expense Deferred Income Tax Expense (Benefit) Share-Based Compensation Expense Share-based Compensation ESOP Compensation Expense Employee Stock Ownership Plan (ESOP), Compensation Expense Defined Benefit Pension and Postretirement Benefit Expense Pension and Other Postretirement Benefit Expense Bad Debt Expense Provision for Doubtful Accounts Changes in Operating Assets and Liabilities [Abstract] Increase (Decrease) in Operating Capital [Abstract] Accounts Receivable Increase (Decrease) in Accounts and Other Receivables Inventories Increase (Decrease) in Inventories Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Accounts Payable Increase (Decrease) in Accounts Payable Other Current Liabilities Increase (Decrease) in Other Current Liabilities Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Investing Activities [Abstract] Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities 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 Cash in Escrow for Acquisition Increase (Decrease) in Restricted Cash Proceeds from Sale of Property, Plant and Equipment Proceeds from Sale of Productive Assets Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities Financing Activities [Abstract] Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from Issuance of Common Stock Proceeds from Issuance of Long-Term Debt Proceeds from Issuance of Long-term Debt Changes in Restricted Cash Proceeds from (Repayments of) Restricted Cash, Financing Activities Changes in Notes Payable Proceeds from (Repayments of) Notes Payable Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition of Non-Controlling Interest Payments for Repurchase of Redeemable Noncontrolling Interest Debt Issuance Costs Payments of Debt Issuance Costs Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Cash from Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and Cash Equivalents at Beginning of Period Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents at End of Period Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Wind Development Project [Member] Wind Development Project [Member] Wind Development Project [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Date of Acquisition Business Acquisition, Effective Date of Acquisition Payments to Acquire Business Payments to Acquire Businesses, Gross Number of Wind Turbines Number of Wind Turbines Number of wind turbines at a wind energy facility or planned for a wind energy facility. Contractual Sales Price Contractual Sales Price Contractual Sales Price Assets [Abstract] Assets [Abstract] Cash and Cash Equivalents Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Construction Costs Construction in Progress, Gross Goodwill Liabilities [Abstract] Liabilities [Abstract] Total Liabilities Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Billings in Excess of Costs and Estimated Earnings [Abstract] Billings in Excess of Cost [Abstract] Contract Billings Customer Advances, Current Estimated Earnings Gross Profit Billings in Excess of Costs and Estimated Earnings Billings in Excess of Cost, Current Revenue Contracts Revenue Cost of Sales Contract Revenue Cost Name of Acquired Entity Business Acquisition, Name of Acquired Entity Consideration Transferred Business Combination, Consideration Transferred 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 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 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 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 Net Identifiable Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Fair Value Adjustments for Work in Process and Finished Goods 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 Expected Tax Deductible Amount of Goodwill Business Acquisition, Goodwill, Expected Tax Deductible Amount Acquisition Related Costs Business Combination, Acquisition Related Costs Conservation Improvement Program [Abstract] Conservation Improvement Program [Abstract] Conservation Improvement Program [Abstract] CIP Energy-Savings Goal CIP Energy-Savings Goal Minnesota's Next Generation Act of 2007 introduced an energy-savings goal of 1.5 percent of net gross annual retail electric energy sales beginning with the 2010 program year. CIP Financial Incentive CIP Financial Incentive Conservation improvement program financial incentive approved by the public utilities commission. AFUDC–Equity Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity Gain on Sale of Available-for-sale Securities Investments and Other Income Other Income Investments and Other Expense Other Expenses Total Other Income Other Nonoperating Income (Expense) Loss Contingencies [Table] Loss Contingencies [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] FERC Complaint [Axis] FERC Complaint [Axis] FERC Complaint [Axis] FERC Complaint [Domain] FERC Complaint [Domain] [Domain] for FERC Complaint [Axis] FERC Complaint 1 [Member] FERC Complaint 1 [Member] FERC Complaint 1 [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Proposed Return on Common Equity Equity Method Investment, Proposed Return on Common Equity The proposed return on common equity, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting. Estimated Refund for MISO Revenue Estimated Refund Obligation Estimated Refund Obligation Estimated Refund for MISO Transmission Expense Estimated Refund Estimated Refund Reserve Loss Contingency Accrual Reserve, Portion Attributable to Prior Years Loss Contingency Accrual, Attributable to Prior Years Amount of a loss contingency accrual recorded in the current current but attributable to prior years. Pension and Other Postretirement Benefit Plans [Abstract] Pension and Other Postretirement Benefit Plans [Abstract] Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Schedule of Business Acquisitions [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Billings in Excess of Costs and Estimated Earnings [Table Text Block] Costs in Excess of Billings and Billings in Excess of Costs [Table Text Block] Leasing Agreements [Abstract] Leasing Agreements [Abstract] Leasing Agreements [Table] Schedule of Operating Leased Assets [Table] Leasing Agreements [Axis] Property, Plant and Equipment, Type [Axis] Leasing Agreements [Domain] Property, Plant and Equipment, Type [Domain] BNI Coal Dragline Lease [Member] Exploration and Production Equipment [Member] Leasing Agreements [Line Items] Operating Leased Assets [Line Items] Operating Leases, Minimum Payments Due in 2015 Operating Leases, Future Minimum Payments Due, Next Twelve Months Operating Leases, Minimum Payments Due in 2016 Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Minimum Payments Due in 2017 Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Minimum Payments Due in 2018 Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Minimum Payments Due in 2019 Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Minimum Payments Due Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Goodwill and Intangible Assets [Text Block] Goodwill and Intangible Assets Disclosure [Text Block] Components of Net Periodic Benefit Expense (Income) [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Changes in Accumulated Other Comprehensive Income (Loss) [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Amount Reclassified from Accumulated Other Comprehensive Loss [Table Text Block] Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Reclassifications [Abstract] Reclassifications [Abstract] Reclassifications [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. Consolidated Balance Sheet [Axis] Balance Sheet Location [Axis] Consolidated Balance Sheet [Domain] Balance Sheet Location [Domain] Property, Plant and Equipment - Net to Goodwill and Intangible Assets - Net [Member] Property, Plant and Equipment [Member] Other Non-Current Assets to Goodwill and Intangible Assets - Net [Member] Other Noncurrent Assets [Member] Total Assets [Member] Assets, Total [Member] Consolidated Statement of Income [Axis] Consolidated Statement of Income [Domain] Operating and Maintenance Expenses to Cost of Sales [Member] Operating and Maintenance Expenses to Transmission Services [Member] Operating Expense - Transmission Services [Member] Operating Expense - Transmission Services [Member] Operating and Maintenance Expenses to Taxes Other than Income Taxes [Member] Operating Expense - Taxes Other than Income Taxes [Member] Operating Expense - Taxes Other than Income Taxes [Member] Operating Income [Member] Operating Income (Loss) [Member] Net Income [Member] Net Income Before Non-Controlling Interest in Subsidiaries [Member] Net Income Before Non-Controlling Interest in Subsidiaries [Member] Net Income Attributable to ALLETE [Member] Net Income Attributable to Parent [Member] Net Income Attributable to Parent [Member] Reclassifications [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. Reclassification Adjustment Prior Period Reclassification Adjustment 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] Consolidated Balance Sheet [Abstract] Statement of Financial Position [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Cash and Cash Equivalents Accounts Receivable (Less Allowance of $1.3 and $1.1) Accounts Receivable, Net, Current Inventories Inventory, Net Prepayments and Other Other Assets, Current Deferred Income Taxes Deferred Tax Assets, Net, Current Total Current Assets Assets, Current Property, Plant and Equipment – Net Property, Plant and Equipment, Net Regulatory Assets Regulatory Assets, Noncurrent Investment in ATC Other Investments Long-term Investments Goodwill and Intangible Assets – Net Intangible Assets, Net (Including Goodwill) Other Non-Current Assets Other Assets, Noncurrent Total Assets Liabilities and Equity [Abstract] Current Liabilities [Abstract] Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest and Dividends Payable, Current Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Notes Payable Notes Payable, Current Other Other Liabilities, Current Total Current Liabilities Liabilities, Current Long-Term Debt Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Regulatory Liability, Noncurrent Defined Benefit Pension and Other Postretirement Benefit Plans Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent Other Non-Current Liabilities Other Liabilities, Noncurrent Total Liabilities Liabilities Commitments, Guarantees and Contingencies (Note 15) Commitments and Contingencies Equity [Abstract] Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 49.0 and 45.9 Shares Outstanding Common Stock, Value, Issued Unearned ESOP Shares Unearned ESOP Shares Accumulated Other Comprehensive Loss Retained Earnings Retained Earnings (Accumulated Deficit) Total ALLETE Equity Stockholders' Equity Attributable to Parent Non-Controlling Interest in Subsidiaries Stockholders' Equity Attributable to Noncontrolling Interest Total Equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Total Liabilities and Equity Liabilities and Equity Recurring Fair Value Measures [Table] Recurring Fair Value Measurements [Member] Fair Value, Measurements, Recurring [Member] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Domain] Contingent Consideration [Domain] [Domain] for Contingent Consideration [Axis] Recognition of U.S. Water Services Contingent Consideration [Member] Acquired in Business Combination [Member] Acquired in Business Combination [Member] Accretion Expense [Member] Accretion [Member] Accretion expense [Member] Payments [Member] Payments [Member] Payments made that decrease contingent consideration liability [Member] Recurring Fair Value Measures [Abstract] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Investments [Abstract] Available-for-sale - Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Available-for-sale - Corporate Debt Securities Available-for-sale Securities, Debt Securities, Noncurrent Cash Equivalents Investments, Cash and Cash Equivalents, Fair Value Disclosure This element represents the portion of the balance sheet Investments line item's cash and cash equivalents, valued at fair value by the entity. Total Fair Value of Assets Assets, Fair Value Disclosure, Recurring Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Deferred Compensation Deferred Compensation, Fair Value Disclosure This element represents the portion of the balance sheet assertion, Deferred Compensation, valued at fair value by the entity. Derivatives - Interest Rate Swap 2015 Interest Rate Derivative Instruments Not Designated as Hedging Instruments, Liability at Fair Value Derivatives - Interest Rate Swap 2014 Interest Rate Cash Flow Hedge Liability at Fair Value U.S. Water Services Contingent Consideration Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value 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] Balance as of December 31, 2014 Activity in Level 3, Period Increase (Decrease) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Balance as of September 30, 2015 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. Consolidated Statement of Comprehensive Income Parentheticals [Abstract] Consolidated Statement of Comprehensive Income Parentheticals [Abstract] Unrealized Gain on Securities, Tax Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Unrealized Gain on Derivatives, Tax Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Tax Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax Inventories [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 Prepayments and Other Current Assets [Abstract] Prepaid Expense and Other Assets, Current [Abstract] Deferred Fuel Adjustment Clause Deferred Fuel Cost Construction Costs for Development Project Restricted Cash Other Other Assets, Miscellaneous, Current Total Prepayments and Other Current Assets Prepaid Expense and Other Assets, Current Goodwill and Intangible Assets [Abstract] Intangible Assets, Net (Including Goodwill) [Abstract] Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Statement [Table] Statement [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Debt Service and Other Requirements [Member] Debt Service and Other Requirements [Member] Restricted cash related to debt service and other requirements [Member] Statement [Line Items] Statement [Line Items] Restricted Cash, Noncurrent Restricted Cash and Cash Equivalents, Noncurrent Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] Customer Deposits Customer Deposits, Current Power Purchase Agreements Power Purchase Agreement - Current Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability. Construction Deposits Received for Development Project Other Other Accrued Liabilities, Current Total Other Current Liabilities Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent Power Purchase Agreements Power Purchase Agreement - Noncurrent Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent liability. Contingent Consideration Business Combination, Contingent Consideration, Liability, Noncurrent Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Inventories [Table Text Block] Schedule of Inventory, Current [Table Text Block] Prepayments and Other Current Assets [Table Text Block] Schedule of Other Current Assets [Table Text Block] Other Current and Non-Current Liabilities [Table Text Block] Schedule of Other Assets and Other Liabilities [Table Text Block] Supplemental Statement of Cash Flows Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Consolidated Statement of Income [Abstract] Consolidated Statement of Income [Abstract] Operating Expenses [Abstract] Cost of Revenue [Abstract] Fuel and Purchased Power Cost of Purchased Power Transmission Services Cost of Transmission Cost of Sales Cost of Goods and Services Sold Operating and Maintenance Operating Expenses Depreciation and Amortization Depreciation, Depletion and Amortization Taxes Other than Income Taxes Taxes, Other Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Other Income (Expense) [Abstract] Nonoperating Income (Expense) [Abstract] Interest Expense Equity Earnings in ATC Income (Loss) from Equity Method Investments Other Total Other Expense Nonoperating Income (Expense) Income Before Non-Controlling Interest and Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest Income Tax Expense Income Tax Expense (Benefit) 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 (in shares) Weighted Average Number of Shares Outstanding, Basic Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Basic Earnings Per Share of Common Stock Earnings Per Share, Basic Diluted Earnings Per Share of Common Stock Earnings Per Share, Diluted Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Storm Lake I [Member] Storm Lake I [Member] Storm Lake I [Member] Power Purchase Agreements, Non-Current Liability Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Restricted Cash, Non-Current Restricted Cash and Investments, Noncurrent Goodwill Goodwill Description Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Power Purchase Agreements, Current Liability Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets Long-term Debt Due Within One Year Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Other Non-Current Liabilities Power Purchase 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 Purchase 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. Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Supplemental Cash Flow Elements [Abstract] Cash Paid During the Period for Interest - Net of Amounts Capitalized Interest Paid, Net Cash Paid During the Period for Income Taxes Income Taxes Paid, Net Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment Capital Expenditures Incurred but Not yet Paid Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations AFUDC–Equity ALLETE Common Stock Contributed to the Pension Plan Stock Issued During Period, Value, Employee Benefit Plan Contingent Consideration Noncash or Part Noncash Acquisition, Value of Liabilities Assumed Consolidated Statement of Comprehensive Income [Abstract] Consolidated Statement of Comprehensive Income [Abstract] Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Other Comprehensive Income [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Gain (Loss) on Securities, Net of Income Taxes of $(0.4), $–, $(0.3) and $0.1 Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Unrealized Gain on Derivatives, Net of Income Taxes of $–, $–, $0.1 and $0.1 Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Net of Income Taxes of $0.3, $0.2, $0.7 and $0.6 Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Other Comprehensive Income (Loss) Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Less: Non-Controlling Interest in Subsidiaries Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Investment in ATC [Abstract] Investment in ATC [Abstract] Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Financial Covenants [Abstract] 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. Reclassifications Out of Accumulated Other Comprhensive Income (Loss) [Text Block] Disclosure of Reclassification Amount [Text Block] Pension and Other Postretirement Benefit Plans [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Pension and Other Postretirement Benefit Plans [Axis] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Pension and Other Postretirement Benefit Plans [Domain] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Pension [Member] Pension Plan [Member] Other Postretirement [Member] Other Postretirement Benefit Plan [Member] Pension and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Components of Net Periodic Benefit Expense (Income) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] Service Cost Defined Benefit Plan, Service Cost Interest Cost Defined Benefit Plan, Interest Cost Expected Return on Plan Assets Defined Benefit Plan, Expected Return on Plan Assets Amortization of Prior Service Costs (Credits) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of Net Loss Defined Benefit Plan, Amortization of Gains (Losses) Net Periodic Benefit Expense (Income) Defined Benefit Plan, Net Periodic Benefit Cost Contributions Defined Benefit Plan, Contributions by Employer Estimated Future Contributions Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Asset [Axis] Regulatory Asset [Domain] Deferred Fuel [Member] Deferred Fuel Costs [Member] Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension and Other Postretirement Plans Costs [Member] Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] PPACA Income Tax Deferral [Member] Medicare Part D Regulatory Asset [Member] Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). Other Other Regulatory Assets (Liabilities) [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Plant Removal Obligations [Member] Removal Costs [Member] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities 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 FERC Complaints [Abstract] FERC Complaints [Abstract] ALLETE Transmission Holdings [Member] ALLETE Transmission Holdings [Member] ALLETE Transmission Holdings [Member] Reserve Reflected in Equity Earnings in ATC Equity Method Investment, Proposed Return on Common Equity Approved Return on Common Equity Equity Method Investment, Approved Return on Common Equity The approved percentage return on common equity dedicated to the utility plant in the investee accounted for under the equity method of accounting. Sensitivity Analysis [Table] Sensitivity Analysis [Table] Sensitivity Analysis [Table] Expense Presentation [Axis] Expense Presentation [Axis] Expense Presentation [Axis] Expense Presentation [Domain] Expense Presentation [Domain] [Domain] for Expense Presentation [Axis] After-tax [Member] After-tax [Member] After-tax [Member] Pre-tax [Member] Pre-tax [Member] Pre-tax [Member] Sensitivity Analysis [Axis] Sensitivity Analysis [Axis] The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Domain] Sensitivity Analysis [Domain] [Domain] for The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Member] Sensitivity Analysis [Member] [Member] for The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Line Items] Sensitivity Analysis [Line Items] [Line Items] for Sensitivity Analysis [Table] Basis Point Reduction on Approved Rate of Return on Common Equity Basis Point Reduction on Approved Rate of Return on Common Equity A sensitivity analysis using basis point reduction. Annual Effect on Future Equity Earnings in ATC Effect on Future Earnings, Amount Antidilutive Shares Excluded from Diluted EPS Computation Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Available to Common Stockholders, Basic Average Common Shares Earnings Per Share Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income to Attributable ALLETE Net Income (Loss) Available to Common Stockholders, Diluted Average Common Shares Earnings Per Share Dilutive Securities (in shares) Weighted Average Number Diluted Shares Outstanding Adjustment Proceeds from Issuance of Common Stock - Pension Plan Common Stock Issued - Pension Plan (in shares) Stock Issued During Period, Shares, Employee Benefit Plan Forward Contract Indexed to Issuer's Equity [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 Contracts [Member] Forward Contracts [Member] Options Held [Member] Options Held [Member] Forward Contract Indexed to Issuer's Equity [Line Items] Forward Contract Indexed to Issuer's Equity [Line Items] Forward Contract Indexed to Issuer's Equity, Indexed Shares Forward Contract Indexed to Issuer's Equity, Indexed Shares Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share Common Stock Issued - Forward Contract (in shares) Common Stock, Shares, Issued Proceeds from Issuance of Common Stock - Forward Contract Proceeds from Issuance or Sale of Equity Current Tax Expense [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Total Current Tax Expense Current Income Tax Expense (Benefit) Deferred Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Total Deferred Tax Expense Total Income Tax Expense Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Combined Federal and State Statutory Tax Rate Effective Income Tax Rate Reconciliation, Combined Federal and State Statutory Rate, Percent Percentage of domestic combined federal and state statutory tax rate applicable to pretax income (loss). Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Income Before Non-Controlling Interest and Income Taxes Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Taxes Computed at 35 percent Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Regulatory Differences for Utility Plant Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Gross Unrecognized Tax Benefits Unrecognized Tax Benefits Unrecognized Tax Benefits That Would Favorably Impact Effective Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Reclassification [Policy Text Block] Reclassification, Policy [Policy Text Block] Inventories [Policy Text Block] Inventory, 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] Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] New Accounting Standards [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Land Inventory [Policy Text Block] Real Estate Held for Development and Sale, Policy [Policy Text Block] Acquisition [Policy Text Block] Business Combinations Policy [Policy Text Block] Percentage of Completion Method of Accounting for Contracts [Policy Text Block] Revenue Recognition, Percentage-of-Completion Method [Policy Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Equity Method Investments [Policy Text Block] Equity Method Investments, Policy [Policy Text Block] Income Tax [Policy Text Block] Income Tax, Policy [Policy Text Block] Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Environmental Accruals [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Generation [Table] Generation [Table] 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] Generation [Line Items] Generation [Line Items] [Line Items] for Generation [Table] Boswell Mercury Emissions Reduction Plan [Abstract] Boswell Mercury Emissions Reduction Plan [Abstract] Schedule of Utility Generating Facilities [Table] Schedule of Utility Generating Facilities [Table] Schedule of Generating Facilities owned by the Utility Schedule of Utility Generating Facilities [Line Items] Schedule of Utility Generating Facilities [Line Items] [Line Items] for Schedule of Generating Facilities owned by the Utility Ownership Percentage Cash Investments Equity Method Investment, Expected Additional Investment Equity Method Investment, Expected Additional Investment Additional investments expected to be made during the current fiscal year but after the filing date, not already separately disclosed as part of current period/year-to-date investments or as a subsequent event. Subsequent Events [Abstract] Subsequent Events [Abstract] Subsequent Event [Table] Subsequent Event [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Balance as of December 31, 2014 Distributed ATC Earnings Proceeds from Equity Method Investment, Dividends or Distributions Equity Investment Balance as of September 30, 2015 ATC Summarized Financial Data [Abstract] Equity Method Investment, Summarized Financial Information [Abstract] Revenue Equity Method Investment, Summarized Financial Information, Revenue Operating Expense Equity Method Investment, Summarized Financial Information, Cost of Sales 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 Other Income (Expense) [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Acquisition [Abstract] Acquisition [Abstract] ACE Wind [Member] ACE Wind [Member] ACE Wind [Member] Lake Benton [Member] Lake Benton [Member] Lake Benton wind facility acquired from The AES Corporation[Member] Storm Lake II [Member] Storm Lake II [Member] Storm Lake II [Member] Condon [Member] Condon [Member] Condon wind facility acquired from The AES Corporation [Member] Purchase Option [Axis] Purchase Option [Axis] Purchase Option [Axis] Purchase Option [Domain] Purchase Option [Domain] [Domain] for Purchase Option [Axis] Number of Wind Energy Facilities Acquired Number of Wind Energy Facilities Acquired Number of Wind Energy Facilities Acquired Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Non-Controlling Interest Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Total Liabilities and Non-Controlling Interest Assumed Purchase option Purchase option Purchase option to acquire entity. Non-Controlling Interest, Decrease from Purchase of Interests Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Document and Entity Information [Abstract] Document and Entity Information [Abstract] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Filer Category Entity Filer Category Entity 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 Chanarambie/Viking [Member] Chanarambie/Viking [Member] Chanarambie/Viking [Member] 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] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Recurring Fair Value 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] Financial Instruments [Abstract] Financial Instruments [Abstract] Long-Term Debt, Including Current Portion - Carrying Amount Long-term Debt Long-Term Debt, Including Current Portion - Fair Value Long-term Debt, Fair Value ALLETE Properties Real Estate Investments, Net Available-for-sale Securities Other Other Long-term Investments Total Other Investments Available-for-sale Corporate Debt Securities, Maturities, One Year or Less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Available-for-sale Corporate Debt Securities, Maturities, More than One Year to Less than Three Years Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the first fiscal year through the third fiscal year following the latest fiscal year (maturing in more than one year to less than three years). Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the third fiscal year through the fifth fiscal year following the latest fiscal year (maturing in three years to less than five years). Available-for-sale Corporate Debt Securities, Maturities, Five or More Years Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing in after the fifth fiscal year following the latest fiscal year (maturing in five years or more). Impairment of Land Inventory Impairment of Real Estate Investments [Text Block] Investment [Text Block] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block]