Investment in ATC [Abstract] Investment in ATC [Abstract] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] ATC Summarized Financial Data [Table Text Block] Equity Method Investment, Schedule of Summarized Financial Data [Table Text Block] Tabular disclosure of summarized financial data reported by an equity method investment of the entity. 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] Project in Progress [Member] Project in Progress [Member] Project in Progress [Member] Completed Project [Member] [Member] Completed Project [Member] [Member] Completed Project [Member] [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] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [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. 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. Capital Cost Spent to Date Capital cost spent to date for project. 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. Investments [Abstract] Investments [Text Block] Investment [Text Block] Financial Covenants [Abstract] -- None. No documentation exists for this element. -- Debt Instrument, Required Indebtedness to Total Capital Ratio Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Debt Instrument, Actual Indebtedness to Total Capital Ratio Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Great Northern Transmission Line (GNTL) [Abstract] Great Northern Transmission Line (GNTL) [Abstract] Transmission [Axis] Transmission [Domain] Transmission [Line Items] Other Matters [Abstract] Other Matters [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Loss Contingency Nature [Axis] Loss Contingency Nature [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] Pending Litigation [Member] Pending Litigation [Member] Legal Proceedings [Line Items] Loss Contingencies [Line Items] Damages Sought Loss Contingency, Damages Sought, Value Acquisitions [Abstract] Acquisitions [Abstract] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Consolidated Statement of Cash Flows [Abstract] Consolidated Statement of Cash Flows [Abstract] Statement [Table] Scenario [Axis] Scenario, Unspecified [Domain] Statement [Line Items] 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 Sale of Assets / Investments 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 Debt Issuance Costs Amortization of Financing Costs 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 Cash Contributions to Defined Benefit Pension and Other Postretirement Benefit Plans Increase (Decrease) in Pension and Postretirement Obligations Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Investing Activities [Abstract] Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Investment in ATC Payments to Acquire Interest in Subsidiaries and Affiliates Changes to Other Investments Payments for (Proceeds from) Other Investing Activities Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Acquisition – Net of Cash Acquired Payments to Acquire Businesses, Net of Cash Acquired Changes in Restricted Cash Increase (Decrease) in Restricted Cash Proceeds from Sale of Assets 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 Reductions of Long-Term Debt Repayments of Long-term Debt Debt Issuance Costs Payments of Debt Issuance Costs Acquisition of Non-Controlling Interest Payments for Repurchase of Redeemable Noncontrolling Interest 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 Pension and Other Postretirement Benefit Plans [Abstract] Pension and Other Postretirement Benefit Plans [Abstract] Components of Net Periodic Benefit Expense [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] 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 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 April 2015) [Member] Long-term contract with Manitoba Hydro for the purchase of electric power. 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] Interstate Power and Light Company PPA (expires April 2019) [Member] Interstate Power and Light Company PPA (expires April 2019) [Member] Interstate Power and Light Company PPA (expires April 2019) [Member] Interstate Power and Light Company (expires June 2032) [Member] Interstate Power and Light Company (expires June 2032) [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 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. 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. 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. Regulatory Matters [Abstract] Regulatory Matters [Abstract] Utility Rates [Abstract] Electric Rates [Abstract] Electric Rates [Abstract] Rate Regulation [Table] Rate Regulation [Table] Rate Revision [Axis] Rate Revisions [Axis] Information about periodic rate revisions. Rate Revisions [Domain] Periodic rate revisions. 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] Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. 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 Bison 4 [Member] Bison 4 [Member] Bison 4 [Member] Rate Regulation [Line Items] [Line Items] for Rate Regulation [Table] Approved Rate of Return on Common Equity Rate of return on common equity approved by the regulatory commission through a rate case proceeding. Approved Percentage of Capital Structure Related to Equity Percentage of capital structure related to equity approved by the regulatory commission through a rate case proceeding. Number of Customers Number of Customers Number of Customers Length of Notice Required to Terminate (in Years) Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract (in years). Capital Cost Spent to Date Income Tax Expense [Abstract] Income Tax Expense [Abstract] Current Tax Expense (Benefit) [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 (Benefit) 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 Income Tax Expense (Benefit) Effective Income Tax Rate Effective Income Tax Rate Reconciliation, Percent Federal Statutory Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 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 Earnings Per Share and Common Stock [Abstract] Earnings Per Share and Common Stock [Abstract] Anti-dilutive 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 Net Income (Loss) Available to Common Stockholders, Basic Average Common Shares Weighted Average Number of Shares Outstanding, Basic Earnings Per Share Earnings Per Share, Basic Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Net Income (Loss) Available to Common Stockholders, Diluted Average Common Shares Weighted Average Number of Shares Outstanding, Diluted Earnings Per Share Earnings Per Share, Diluted Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment 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, Shares, Issued - Option Agreement Common Stock, Shares, Issued Proceeds from Issuance or Sale of Equity - Option Agreement Proceeds from Issuance or Sale of Equity Forward Contract Indexed to Issuer's Equity, Dilutive effect Forward Contract Indexed to Issuer's Equity, Dilutive effect Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of equity forward arrangements using the treasury stock method. Forward Contract Indexed to Issuer's Equity, Initial Fair Value Forward Contract Indexed to Issuer's Equity, Initial Fair Value Initial fair value of a forward contract indexed to the issuer's equity. Stock Issued During Period, Shares, Employee Benefit Plan Stock Issued During Period, Shares, Employee Benefit Plan Stock Issued During Period, Value, Employee Benefit Plan Stock Issued During Period, Value, Employee Benefit Plan Environmental Matters [Abstract] Environmental Matters [Abstract] Schedule of Utility Generating Facilities [Table] Schedule of Utility Generating Facilities [Table] Schedule of Generating Facilities owned by the Utility Boswell Unit 4 [Member] Boswell Unit 4 [Member] Boswell Unit 4 [Member] 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 Other Commitment, Due in Next Twelve Months Other Commitment, Due in Next Twelve Months Other Commitment, Due in Second Year Other Commitment, Due in Second Year Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Derivatives [Abstract] DERIVATIVES [Abstract] Derivative [Table] Derivative [Table] Derivative, by Nature [Axis] Derivative, by Nature [Axis] Derivative, Name [Domain] Derivative, Name [Domain] Interest Rate Swap [Member] Interest Rate Swap [Member] Interest Rate Swap Member Effective August 25, 2011 [Member] Interest Rate Swap Member Effective August 25, 2011 [Member] Forward based contracts in which two parties agree to swap periodic payments that are fixed at the outset of the swap contract with variable payments based on a market interest rate (index rate) over a specified period. ALLETE swap on $75 million term loan effective August 25, 2011, and maturing August 25, 2014. Interest Rate Swap Member Effective August 26, 2014 [Member] Interest Rate Swap Member Effective August 26, 2014 [Member] Forward based contracts in which two parties agree to swap periodic payments that are fixed at the outset of the swap contract with variable payments based on a market interest rate (index rate) over a specified period. ALLETE swap on $75 million term loan effective August 26, 2014, and maturing August 25, 2015. Derivative [Line Items] Derivative [Line Items] Interest Rate Swap Variable Rate Basis Derivative, Description of Variable Rate Basis Interest Rate Swap Fixed Interest Rate Derivative, Fixed Interest Rate Interest Rate Cash Flow Hedge Effectiveness Assessment and Measurement Discussion of Interest Rate Cash Flow Hedge Effectiveness Assessment and Measurement Interest Rate Swap Liability Interest Rate Cash Flow Hedge Liability at Fair Value Interest Rate Swap Liability, Other Non-Current Liabilities Interest Rate Cash Flow Hedge Liability at Fair Value, Noncurrent Fair value as of the balance sheet date of all interest rate derivative liabilities designated as cash flow hedging intstruments included in noncurrent liabilities. Interest Rate Swap Liability, Other Current Liabilities Interest Rate Cash Flow Hedge Liability at Fair Value, Current Fair value as of the balance sheet date of all interest rate derivative liabilities designated as cash flow hedging instruments included in current liabilities. Hydro Operations [Abstract] Hydro Operations [Abstract] Hydro Operations [Table] Hydro Operations [Table] Hydro Operations [Table] Hydro Operations [Axis] Hydro Operations [Axis] Hydro Operations [Axis] Hydro Operations [Domain] Hydro Operations [Domain] [Domain] for Hydro Operations [Axis] Hydro Project [Member] Hydro Project [Member] Hydro Project [Member] Hydro Operations [Line Items] Hydro Operations [Line Items] [Line Items] for Hydro Operations [Table] Fair Value [Abstract] Fair Value Recurring Fair Value Measures [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] 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] Recurring Fair Value Measures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Investments [Abstract] 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 Total Fair Value of Liabilities Liabilities, Fair Value Disclosure, Recurring Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Fair Value [Abstract] Fair Value Disclosures [Abstract] Fair Value Activity in Level 3 Fair Value Activity in Level 3 Fair Value Activity in Level 3 Fair Value Hierarchy Transfers, All Levels Fair Value Hierarchy Transfers, All Levels The amount of transfers in and out of levels 1, 2 and 3 of the fair value hierarchy during the period. Operations and Significant Accounting Policies [Abstract] Operations and Significant Accounting Policies [Abstract] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Fair Value [Text Block] Fair Value Disclosures [Text Block] Coal Rail and Shipping Contracts [Abstract] Coal Rail and Shipping Contracts [Abstract] Coal, Rail and Shipping Contracts [Table] Recorded Unconditional Purchase Obligation [Table] Coal, Rail and Shipping Contracts [Axis] Recorded 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 and Transportation [Member] Coal and Transportation [Member] Coal, Rail and Shipping Contracts [Line Items] Recorded Unconditional Purchase Obligation [Line Items] Minimum Annual Payment Obligation, Current Year Recorded Unconditional Purchase Obligation Due in Next Twelve Months Minimum Annual Payment Obligation, Year Two Recorded Unconditional Purchase Obligation Due in Second Year Reclassifications Out of Accumualted Other Comprehensive Income (Loss) [Abstract] Reclassifications Out of Accumualted Other Comprehensive Income [Abstract] Changes in Accumulated Other Comprehensive Loss by Component [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] Short-Term and Long-Term Debt [Abstract] SHORT-TERM AND LONG-TERM DEBT [Abstract] Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] ALLETE Properties SEC Schedule III, Real Estate Investment Property, Net Available-for-sale Securities Available-for-sale Securities, Noncurrent Other Other Investments Total Other Investments Other Long-term 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 Years or More 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). ALLETE Properties [Rollforward] SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward] Land Inventory Beginning Balance Land Available-for-sale Cost of Sales SEC Schedule III, Real Estate, Cost of Real Estate Sold Other ALE Real Estate Other Additions Amount of other additions including deeds to collateralized property and capitalized improvements to real estate investments for entities with a substantial portion of business acquiring and holding investment real estate. Land Inventory Ending Balance Long-Term Finance Reveivables (net of allowances of $0.6 and $0.6) Notes, Loans and Financing Receivable, Net, Noncurrent Other Other Real Estate Total Real Estate Assets Impairment of Real Estate Impairment of Real Estate Long-Term Finance Receivables, Allowance Allowance for Notes, Loans and Financing Receivable, Noncurrent Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Other Income (Expense) [Abstract] Other Income (Expense) [Abstract] Other Income (Expense) [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Investments Parentheticals [Abstract] Investments Parentheticals [Abstract] 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 Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Ownership Percentage Equity Method Investment, Ownership Percentage 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. 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. 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, 2013 Equity Method Investments Cash Investments Payments to Acquire Equity Method Investments Equity Earnings in ATC Income (Loss) from Equity Method Investments Distributed ATC Earnings Proceeds from Equity Method Investment, Dividends or Distributions Equity Investment Balance as of March 31, 2014 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 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] First Mortgage Bonds Issuance [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Consolidated Statement of Comprehensive Income Parentheticals [Abstract] Consolidated Statement of Comprehensive Income Parentheticals [Abstract] 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 Commitments, Guarantees and Contingencies [Abstract] Commitments, Guarantees and Contingencies [Abstract] North Dakota Wind Development [Table] North Dakota Wind Development [Table] North Dakota Wind Development [Table] DC Transmission Line from Square Butte [Member] DC Transmission Line from Square Butte [Member] DC Transmission Line from Square Butte [Member] North Dakota Wind Development [Line Items] North Dakota Wind Development [Line Items] [Line Items] for North Dakota Wind Development [Table] AFUDC - Equity Gain on Sale of Available-for-sale Securities Available-for-sale Securities, Gross Realized Gains Investments and Other Income Other Income Total Other Income Other Nonoperating Income (Expense) Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Storm Lake [Member] Storm Lake [Member] Storm Lake wind facility acquired from The AES Corporation [Member] Condon [Member] Condon [Member] Condon wind facility acquired from The AES Corporation [Member] Lake Benton [Member] Lake Benton [Member] Lake Benton wind facility acquired from The AES Corporation[Member] Armenia Mountain [Member] Armenia Mountain [Member] Armenia Mountain [Member] Purchase Option [Axis] Purchase Option [Axis] Purchase Option [Axis] Purchase Option [Domain] Purchase Option [Domain] [Domain] for Purchase Option [Axis] Business Acquisition [Line Items] Business Acquisition [Line Items] Business Acquisition, Effective Date of Acquisition Business Acquisition, Effective Date of Acquisition Business Acquisition, Name of Acquired Entity Business Acquisition, Name of Acquired Entity Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Payments to Acquire Businesses, Gross Assets [Abstract] Assets [Abstract] Cash and Cash Equivalents Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Other Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Property, Plant and Equipment - Net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Other Non-Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Total Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets Liabilities [Abstract] Liabilities [Abstract] Other Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other Long-term Debt Due Within One Year Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Long-term Debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Power Purchase Agreement Power Purchase Agreement - Noncurrent 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. Other Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Non-Controlling Interest Business Combination, Acquisition of Less than 100 Percent, Noncontrolling Interest, Fair Value Total Liabilities and Non-Controlling Interest Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Net Identifiable Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Purchase option Purchase option Purchase option to acquire entity. Goodwill Goodwill Power Purchase Agreement Power Purchase Agreement - Current 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. Business Combination, Acquisition Related Costs Business Combination, Acquisition Related Costs Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests 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 Other Income (Expense) [Text Block] Other Income and Other Expense Disclosure [Text Block] Business Segments [Abstract] Business Segments [Abstract] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Debt Instruments [Abstract] Debt Instrument [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Term Loan [Member] Loans Payable [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Term Loan Principal Amount Debt Instrument, Face Amount Percentage of Debt Hedged by Interest Rate Derivatives Percentage of Debt Hedged by Interest Rate Derivatives Earnings Per Share and Common Stock [Text Block] Earnings Per Share [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, Defined Benefit [Member] Other Postretirement [Member] Other Postretirement Benefit Plan, Defined Benefit [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 [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 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 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 Revenue Regulated and Unregulated Operating Revenue Operating Expenses [Abstract] Cost of Revenue [Abstract] Fuel and Purchased Power Cost of Purchased Power Operating and Maintenance Utilities Operating Expense, Maintenance and Operations Depreciation Utilities Operating Expense, Depreciation and Amortization Total Operating Expenses Utilities Operating Expense Operating Income Operating Income (Loss) Other Income (Expense) [Abstract] Nonoperating Income (Expense) [Abstract] Interest Expense Interest Expense 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 Net Income Less: Non-Controlling Interest in Subsidiaries Net Income (Loss) Attributable to Noncontrolling Interest Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent Average Shares of Common Stock [Abstract] Earnings Per Share [Abstract] Basic Diluted Basic Earnings Per Share of Common Stock Diluted Earnings Per Share of Common Stock Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Restricted Cash and Cash Equivalents, Noncurrent Restricted Cash and Cash Equivalents, Noncurrent Escrow Deposit Escrow Deposit Inventories [Abstract] Inventory, Net [Abstract] Fuel Energy Related Inventory Materials and Supplies Inventory, Raw Materials and Supplies, Net of 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 Other Other Assets, Current Total Prepayments and Other Current Assets Prepaid Expense and Other Assets, Current Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] Customer Deposits Customer Deposits, Current Other Other Accrued Liabilities, Current Total Other Current Liabilities Other Liabilities, Current Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Other Liabilities, Noncurrent Supplemental Statement of Cash Flows Information [Abstract] 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 Asset Retirement Obligation, Liabilities Incurred ALLETE Common Stock Contributed to the Pension Plan Pension Contributions Other Investments and ALLETE Properties [Table Text Block] Investment [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] Schedule of Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] 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 [Member] Community development district obligations for Town Center District. Palm Coast Park Community Development District [Member] Community development district obligations for Palm Coast Park District. Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Surety Bond Amount Environmental Loss Contingency, Surety Bond Amount of surety bonds that are available for future payment obligations related to environmental commitments and/or contingencies. Reclamation Letter of Credit Environmental Loss Contingency, Letters of Credit Amount of letters of credit that are available for the future payment of environmental commitments and/or contingencies. Total Current Estimated Reclamation Liability Mine Reclamation and Closing Liability, Noncurrent Surety Bond and Letters of Credit Environmental Loss Contingency, Surety Bond and Letters of Credit Amount of surety bonds outstanding and letters of credit to government entities that are available for future payment obligations related to environmental commitments and contingencies. Development and Maintenance Obligations Performance and Maintenance Obligations Estimated costs remaining on performance and maintenance obligations. Bond Amount Special Assessment Bond 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 of Property Benefited 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. Business Combination Disclosure [Text Block] Business Combination Disclosure [Text Block] 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 Unrealized Investment Gain (Loss) [Member] Defined Benefit Pension and Other Postretirement Items [Member] Accumulated Defined Benefit Plans Adjustment [Member] Gains and Losses on Cash Flow Hedge [Member] Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Other Income (Expense) - Other [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] Accumulated Other Comprehensive Loss 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 Income (Loss) Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Net Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax 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 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 Boswell Mercury Emissions Reduction Plan [Abstract] Boswell Mercury Emissions Reduction Plan [Abstract] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] 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] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [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] Plant Removal Obligations [Member] Removal Costs [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Total Current Regulatory Assets Regulatory Assets, Current Non-Current Regulatory Assets Regulatory Assets, Noncurrent Total Regulatory Assets Regulatory Assets Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] 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 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 Long-term Debt Instruments [Table] ALLETE Bonds 4.86% Series Due 2013 [Member] 4.86% Series Due 2013 [Member] First Mortgage Bonds, 4.86% Series Due 2013 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 3.69% Due March2024 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 4.95% Due March2044 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 3.40% Due July2022 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] ALLETE Bonds 5.05% Due July2044 [Member] Short-Term Debt Outstanding Debt, Current Short-Term Debt Outstanding, Subsequently Matured Long-term Debt, Current Maturities Long-Term Debt Outstanding Long-term Debt, Excluding Current Maturities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt 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, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Long-term Debt Expected Proceeds from Future Issuance of First Mortgage Bonds Expected Proceeds from Future Issuance of First Mortgage Bonds The expected cash inflow from amounts received from a future long-term debt instrument issuance secured by a first mortgage deed of trust, containing a pledge of real property. The lender has the highest claim on the property in case of default. Proceeds from Issuance of First Mortgage Bonds Proceeds from Issuance of First Mortgage Bond Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Future Debt Instrument, Interest Rate, Stated Percentage Future Debt Instrument, Interest Rate, Stated Percentage Contractual interest rate for funds borrowed, under the debt agreement, which will take place in the future. Inventories [Policy Text Block] Inventory, 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] Investment, Policy [Policy Text Block] Long-Term Finance Receivables [Policy Text Block] Finance, Loan and Lease Receivables, Held-for-investment, Policy [Policy Text Block] Derivatives [Policy Text Block] Derivatives, 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] 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] Earnings Per Share, Policy [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Environmental Costs [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Axis] Segments [Axis] Business Segments [Domain] Segments [Domain] Regulated Operations [Member] Regulated Operation [Member] Investments and Other [Member] Other Segments [Member] Business Segments [Line Items] Segment Reporting Information [Line Items] Land in Minnesota (in acres) Number of acres of land available-for-sale (in acres) Number of acres of land available-for-sale. Fuel and Purchased Power Expense Operating and Maintenance Expense Depreciation Expense Operating Income (Loss) Interest Expense Other Income Income (Loss) Before Non-Controlling Interest and Income Taxes Income Tax Expense (Benefit) Net Income (Loss) Net Income (Loss) Attributable to ALLETE Total Assets Assets Property, Plant and Equipment - Net Property, Plant and Equipment, Net Accumulated Depreciation Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Capital Additions Property, Plant and Equipment, Additions Derivatives [Text Block] Derivative Instruments and Hedging Activities Disclosure [Text Block] Reclassifications Out of Accumulated Other Comprhensive Income (Loss) [Text Block] Disclosure of Reclassification Amount [Text Block] Consolidated Statement of Comprehensive Income [Abstract] Consolidated Statement of Comprehensive Income [Abstract] Statement [Table] Scenario [Axis] Scenario, Unspecified [Domain] Statement [Line Items] 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 on Derivatives, Net of Income Taxes of $0.1 and $- 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 $0.2 and $0.2 Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Other Comprehensive Income 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 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] Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Operating Leases, Future Minimum Payments, Due in Current Year Operating Leases, Future Minimum Payments Due, Next Twelve Months Operating Leases, Future Minimum Payments, Due in Year Two Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Future Minimum Payments, Due in Year Three Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Future Minimum Payments, Due in Year Four Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Future Minimum Payments, Due in Year Five Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Future Minimum Payments, Due Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Consolidated Balance Sheet [Abstract] Statement of Financial Position [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Cash and Cash Equivalents Accounts Receivable (Less Allowance of $1.0 and $1.1) Accounts Receivable, Net, Current Inventories Prepayments and Other Deferred Income Taxes Deferred Tax Assets, Net, Current Total Current Assets Assets, Current Regulatory Assets Investment in ATC Other Investments 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 Payable, Current Long-Term Debt Due Within One Year Other Total Current Liabilities Liabilities, Current Long-Term Debt Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Defined Benefit Pension and Other Postretirement Benefit Plans Pension and Other Postretirement and Postemployment Benefit Plans, Liabilities, Noncurrent Other Non-Current Liabilities 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, 42.2 and 41.4 Shares Outstanding Common Stock, Value, Issued Unearned ESOP Shares Unearned ESOP Shares 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