Investments [Abstract] Long-Term Investment Portfolio [Table Text Block] Investment [Table Text Block] Commitments and Contingencies Disclosure [Abstract] North Dakota Wind Development [Table] North Dakota Wind Development [Table] Presents detailed information about wind development projects in North Dakota. North Dakota Wind Development [Axis] Information about each wind project/transmission line in North Dakota. North Dakota Wind Development [Domain] Name of wind project/transmission line in North Dakota. Bison 1 Wind Project [Member] Wind project in North Dakota. Bison 2 Wind Project [Member] Wind project in North Dakota. Bison 3 Wind Project [Member] Wind project in North Dakota. DC Transmission Line from Square Butte [Member] Transmission line purchased as part of North Dakota Wind Development projects. North Dakota Wind Development [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. 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. Generating Unit Capacity (MW) Generating Unit Capacity (MW) Capacity of each generating unit/individual project in megawatts. Total Project Cost Total Project Cost Total Project Cost through Commissioning Expected Capital Cost Expected Capital Cost Expected capital cost for each individual project. Capital Cost Spent to Date Capital Cost Spent to Date Capital cost spent to date for project. ALLETE Properties Real Estate, Gross Available-for-sale Securities Available-for-sale Securities, Noncurrent Other Other Investments Total Investments Other Long-term Investments ALLETE Properties [Rollforward] Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward] Land Inventory Beginning Balance (January 1, 2012 and 2011, respectively) Land Available-for-sale Deeds to Collateralized Property Real Estate, Acquisitions Through Foreclosures Land Impairment Impairment of Real Estate Capitalized Improvements and Other Real Estate, Improvements Cost of Real Estate Sold Real Estate, Cost of Real Estate Sold Land Inventory Ending Balance Long-Term Finance Receivables (net of allowances of $0.6 and $0.6) Loans and Leases Receivable, Net Amount Other Other Real Estate Total Real Estate Assets Long-Term Finance Receivables, Allowance Allowance for Notes, Loans and Financing Receivable, Noncurrent Earnings Per Share and Common Stock [Abstract] Earnings Per Share and Common Stock [Abstract] Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share Reconciliation [Table Text Block] Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Antidilutive Shares Excluded from EPS Computation Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent Average Common Shares Weighted Average Number of Shares Outstanding, Basic Earnings Per Share Earnings Per Share, Basic Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent, 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 Income Tax Expense [Abstract] Income Tax Expense [Abstract] Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Operations and Significant Accounting Policies [Abstract] Operations and Significant Accounting Policies [Abstract] Inventories [Abstract] Inventory Disclosure [Abstract] Fuel Energy Related Inventory, Coal 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 (Received) 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 AFUDC - Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Accounts Receivable [Abstract] Accounts Receivable Additional Disclosures [Abstract] Pre-Bankruptcy Petition Receivable Bankruptcy Claims, Amount of Claims under Review by Management Other Income (Expense) [Abstract] Other Income (Expense) [Abstract] Other Income (Expense) [Text Block] Other Income and Other Expense Disclosure [Text Block] Investment in ATC [Abstract] Investment in ATC [Abstract] Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Short-Term and Long-Term Debt [Abstract] SHORT-TERM AND LONG-TERM DEBT [Abstract] Short-Term Debt Outstanding Debt, Current Short-Term Debt, Current Maturity, First Mortgage Bonds 4.86% Short-Term Debt, Current Maturity First Mortgage Bonds 4.86% Short-Term Debt, Current Maturity First Mortgage Bonds 4.86% Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] ALLETE First Mortgage Bonds 3.20% Due July2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] ALLETE Bonds 3.20% Due July2026 [Member] ALLETE First Mortgage Bonds 4.08% Due July2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] ALLETE Bonds 4.08% Due July2042 [Member] Industrial Development Revenue Bonds 6.5% Due 2025 [Member] Industrial Development Revenue Bonds 6.5% Due 2025 [Member] Industrial Development Revenue Bonds 6.5% Due 2025 [Member] ALLETE LOC Jan2014 [Member] ALLETE LOC Jan2014 [Member] ALLETE LOC Jan2014 [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Long-Term Debt Outstanding Long-term Debt, Excluding Current Maturities Proceeds from Issuance of First Mortgage Bonds Proceeds from Issuance of First Mortgage Bond Interest Rate Debt Instrument, Interest Rate, Stated Percentage Amount of Redeemed Bonds Debt Instrument, Decrease, Repayments Line of Credit, Amount Outstanding Line of Credit Facility, Amount Outstanding Line of Credit, Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity 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. 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. Investment and Other Income (Expense) Other Income Total Other Income Other Nonoperating Income (Expense) Investments [Text Block] Investment [Text Block] Consolidated Statement of Comprehensive Income [Abstract] Consolidated Statement of Comprehensive Income [Abstract] Statement [Table] Statement [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Statement [Line Items] Statement [Line Items] Comprehensive Income (Loss) [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Gain (Loss) on Securities, Net of Income Taxes of $0.5, $(1.1), $0.7 and $(0.3) Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Unrealized Loss on Derivatives, Net of Income Taxes of $(0.1), $(0.2), $(0.2) and $(0.2) 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, $0.3, $0.9 and $0.8 Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Other Comprehensive Income (Loss) Other Comprehensive Income (Loss), Net of Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Less: Non-Controlling Interest in Subsidiaries Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent 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 Commitments, Guarantees and Contingencies [Abstract] Commitments, Guarantees and Contingencies [Abstract] Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Transmission [Axis] Information about each transmission investment. Transmission [Domain] Name of transmission investment. CapX2020 Monticello to St. Cloud Project [Member] Transmission investment project. CapX2020 Bemidji to Grand Rapids Project [Member] Transmission investment project. CapX2020 Fargo to Monticello Projects [Member] Transmission investment projects. 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. Minimum Amount Expected to be Invested in Transmission Projects Minimum Amount Expected to be Invested in Transmission Projects The minimum amount expected to be invested in transmission line projects. Maximum Amount Expected to be Invested in Transmission Projects Maximum Amount Expected to be Invested in Transmission Projects The maximum amount expected to be invested in transmission line projects. Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Accounts Receivable, Net [Abstract] Accounts Receivable, Allowance Allowance for Doubtful Accounts Receivable, Current 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 [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. 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] 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] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Generating Unit Capacity (MW) Long-term Contract for Purchase of Electric Power, Generating Unit Capacity (MW) The generating capacity of a production plant constructed and financed by a public utility district, cooperative, or other organization with whom exists a long-term contract to purchase electric power. Share of Plant Output (Percent) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Total 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 Share of Plant Output (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 PPA (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 Carges Long-Term Contract for Purchase of Electric Power, Fixed Capacity Carges Long-Term Contract for Purchase of Electric Power, Fixed Capacity Carges Share of Plant Output (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. Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Derivatives [Abstract] DERIVATIVES [Abstract] Derivatives [Text Block] Derivative Instruments and Hedging Activities Disclosure [Text Block] 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] Derivative [Line Items] Derivative [Line Items] Percentage of Debt Hedged by Interest Rate Swap Percentage of Debt Hedged by Interest Rate Derivatives Interest Rate Swap Variable Rate Basis Derivative, Description of Variable Rate Basis Interest Rate Swap Fixed Interest Rate Derivative, Fixed Interest Rate Discussion of Interest Rate Cash Flow Hedge Effectiveness Assessment and Measurement Interest Rate Swap Liability Interest Rate Cash Flow Hedge Liability at Fair Value Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Other Comprehensive Income (Loss), Tax, Parenthetical Disclosures [Abstract] Unrealized Gain (Loss) on Securities, Tax Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Unrealized Loss 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 Debt Instruments [Abstract] Debt Instrument [Table] Term Loan [Member] Loans Payable [Member] Term Loan Principal Amount Debt Instrument, Face Amount Term Loan Variable Rate Basis Debt Instrument, Description of Variable Rate Basis Term Loan Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate 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 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 - Additional Investment for Year Equity Method Investment, Additional/Anticipated Investment The amount of additional or anticipated investments in the investee accounted for under the equity method of accounting. ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Beginning Balance 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 September 30, 2012 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, Operating Expense The amount of the operating expense reported by an equity method investment of the entity. Other Expense Equity Method Investment, Summarized Financial Information, Other Expense The amount of other expense reported by an equity method investment of the entity. Net Income Equity Method Investment, Summarized Financial Information, Net Income (Loss) ALLETE's Equity in Net Income Consolidated Statement of Income [Abstract] Income Statement [Abstract] Operating Revenue Regulated and Unregulated Operating Revenue Operating Expenses [Abstract] Cost of Revenue [Abstract] Fuel and Purchased Power Utilities Operating Expense, Fuel and Purchased Power Utilities operating expenses, total fuel and purchased power expenses 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 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 [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 Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] 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 Exploration and Production Equipment [Member] Leasing Agreements [Line Items] Operating Leased Assets [Line Items] Annual Lease Payment Annual Lease Payment Annual payment for operating lease agreement. Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Operating Leases, Future Minimum Payments Due, Next Twelve Months Operating Leases, Future Minimum Payments Due, Next Twelve Months Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Future Minimum Payments, Due in Two Years Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Future Minimum Payments, Due in Three Years Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Future Minimum Payments, Due in Four Years Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Future Minimum Payments, Due in Five Years Operating Leases, Future Minimum Payments, Due Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Income Tax Expense [Abstract] Income Tax Expense (Benefit), Continuing Operations [Abstract] 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 (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) Change in Valuation Allowance Valuation Allowance, Deferred Tax Asset, Change in Amount Investment Tax Credit Amortization Income Tax Credits and Adjustments Total Deferred Tax Expense Deferred Income Tax Expense (Benefit) Total Income Tax Expense Income Tax Reconciliation [Abstract] Income Tax Reconciliation, Other Reconciling Items [Abstract] Unusual or Infrequent Item [Table] Unusual or Infrequent Item [Axis] Unusual or Infrequent Item [Axis] Unusual or Infrequent Item [Domain] Unusual or Infrequent Item [Domain] MPUC Approval of Deferral of PPACA [Member] Income Tax Benefit, MPUC Approval of Deferral of PPACA [Member] Non-recurring income tax benefit related to the Minnesota Public Utilities Commission (MPUC) approval of a request to defer the retail portion of a tax charge resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). DTL Reversal, Revenue Receivable Forgone [Member] DTL Reversal, Revenue Receivable Forgone [Member] Non-recurring reversal of a deferred tax liability (DTL) related to a revenue receivable that was forgone as part of a stipulation and settlement agreement in a rate case. Unusual or Infrequent Item [Line Items] Unusual or Infrequent Item [Line Items] Deferred Income Tax Benefit Effective Income Tax Rate Reconciliation [Abstract] Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract] Effective Income Tax Rate Effective Income Tax Rate, Continuing Operations Effective Income Tax Rate Adjustment Effective Income Tax Rate Reconciliation, Other Adjustments Federal Statutory Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Gross Unrecognized Tax Benefits Unrecognized Tax Benefits Decrease in Unrecognized Tax Benefits Unrecognized Tax Benefits, Decreases Resulting from Settlements with Taxing Authorities Unrecognized Tax Benefits that Would Favorably Impact Effective Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Fair Value [Abstract] Fair Value Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Asset Class [Axis] Asset Class [Axis] Fair Value by Asset Class [Domain] Fair Value by Asset Class [Domain] Debt Securities Issued by the States of the United States (ARS) [Member] US Treasury and Government [Member] Recurring Fair Value Measures, Activity in Level 3 [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Balance as of December 31, 2011 and 2010, respectively Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Redeemed During the Period Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Balance as of September 30, 2012 and 2011, respectively 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. 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 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 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] Pension and Other Postretirement Benefit Plans Disclosures [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 Plans and Other Postretirement Benefit Plans [Domain] Pension [Member] Pension Plans, Defined Benefit [Member] Other Postretirement [Member] Other Postretirement Benefit Plans, 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 Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of Net Loss Defined Benefit Plan, Amortization of Gains (Losses) Amortization of Transition Obligation Defined Benefit Plan, Amortization of Transition Obligations (Assets) Net Periodic Benefit Expense Defined Benefit Plan, Net Periodic Benefit Cost Contributions Defined Benefit Plan, Contributions by Employer Estimated Future Contributions Defined Benefit Plan, Estimated Future Employer Contributions in Current Fiscal Year Prescription Drug Reimbursements Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received Business Segments [Abstract] Business Segments [Abstract] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Inventories [Policy Text Block] Inventory, Policy [Policy Text Block] Accounts Receivable [Policy Text Block] Receivables, Policy [Policy Text Block] Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] Land Inventory [Policy Text Block] Real Estate Held for Development and Sale, 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 of Financial Instruments [Policy Text Block] Fair Value of Financial Instruments, 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] Environmental Costs [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] ALLETE's Investment in ATC [Table Text Block] Schedule of 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. Regulatory Matters [Abstract] Regulatory Matters [Abstract] 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] Future Benefit Obligations Under Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension Costs [Member] Income Taxes [Member] Deferred Income Tax Charges [Member] Asset Retirement Obligation [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 [Member] Other Regulatory Assets [Member] Discloses the amount of other regulatory assets. Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Plant Removal Obligations [Member] Plant Removal Obligations Costs [Member] Costs provided for the removal of plant assets that are reasonably expected to be recovered in rates charged to customers. Regulatory asset or liability is recognized for differences in the timing of recognition of the period costs for financial reporting and rate-making purposes. Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Other [Member] Other Regulatory Liabilities [Member] Discloses the amount of other regulatory liabilties. Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] 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 Fair Value [Text Block] Fair Value Disclosures [Text Block] Investments Parentheticals [Abstract] Investments Parentheticals [Abstract] The Patient Protection and Affordable Care Act of 2010 (PPACA) [Abstract] The Patient Protection and Affordable Care Act of 2010 (PPACA) [Abstract] Unusual or Infrequent Item [Table] Elimination of Medicare Part D [Member] DTA Reversal, Elimination of Medicare Part D [Member] Non-recurring reversal of a deferred tax asset (DTA) related to the elimination of the tax deduction for expenses that are reimbursed under Medicare Part D. Reversal of Previously Recorded Tax Benefit Tax Adjustments, Settlements, and Unusual Provisions First Mortgage Bonds Issuance [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum [Member] Minimum [Member] Maximum [Member] Maximum [Member] Energy Center [Axis] Energy Center [Axis] Energy Center [Axis] Energy Center [Domain] Energy Center [Domain] Energy Center [Domain] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Boswell Unit 4 [Member] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Electric Rates [Abstract] Electric Rates [Abstract] Rate Revision [Axis] Rate Revisions [Axis] Information about periodic rate revisions. Rate Revisions [Domain] Periodic rate revisions. MPUC 2010 MN 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] Natural Gas Rates [Member] Natural Gas Rates [Member] Natural Gas Rates [Member] Water Rates [Member] Water Rates [Member] Water 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 Non-Affiliated Private Utility [Member] Non-Affiliated Private Utility [Member] Non-Affiliated Private Utility municipal customer [Member] Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. 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). Average Monthly Demand (in MWs) Average Monthly Demand (in MWs) The Average Monthly Demand (in MWs) Requested Rate Increase Requested Rate Increase The increase in rates requested from a regulatory commission through a rate case proceeding. Requested Rate of Return on Common Equity Requested Rate of Return on Common Equity Rate of return on common equity requested from a regulatory commission through a rate case proceeding. Requested Percentage of Capital Structure Related to Equity Requested Percentage of Capital Structure Related to Equity The percentage of capital structure related to equity reguested through a rate case proceeding. Requested Percentage of Capital Structure Related to Debt Requested Percentage of Capital Structure Related to Debt Percentage of capital structure related to debt requested through a rate case proceeding. Additional Revenue Generated from Rate Increase Additional Revenue Generated from Rate Increase The amount of incremental revenue that would be generated from requested rate increases as a result of rate case proceedings. Regulatory Matters [Text Block] Public Utilities Disclosure [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] Performance Guarantee [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. ALLETE Properties Performance and Maintenance Obligations [Member] Performance and maintenance obligations to governmental entities to construct improvements in various projects. Community Development Districts [Member] Community development district obligations. 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 Bonds [Abstract] Surety Bonds Outstanding Surety Bonds Outstanding Amount of surety bonds that are available for future payment obligations. Performance and Maintenance Obligations Performance and Maintenance Obligations Estimated costs remaining on performance and maintenance obligations. Performance and Maintenance Obligations, Land Purchaser Contractual Obligation Performance and Maintenance Obligations, Land Purchaser Contractual Obligation Performance and Maintenance Obligations, Portion that is the Land Purchaser's Contractual Obligation Total amount of letter of credit secured in addition to surety bonds for reclamation liability Total amount of letter of credit secured in addition to surety bonds for reclamation liability Amount of letter of credit issued in addition to surety bond to secure performance obligation. Total current estimated reclamation liability Total current estimated reclamation liability Total current estimated liability for performance obligation. Capital Improvement Revenue/Special Assessment Bonds [Abstract] Special Assessment Bond [Abstract] Amount Special Assessment Bond Interest Rate Capital Improvement Revenue/Special Assessment Bonds, Interest Rate Interest rate of municipal bonds used to fund a development project. Term (Years) Capital Improvement Revenue/Special Assessment Bonds, Term (Years) Term in years of municipal bonds used to fund a development project. Ownership Percentage of Benefited Property Capital Improvement Revenue/Special Assessment Bonds, Ownership Percentage of Benefited Property Ownership percentage of property benefited by a development project funded by municipal bonds. Annual Assessments Based on Ownership Percentage Capital Improvement Revenue/Special Assessment Bonds, Annual Assessments Based on Ownership Percentage Annual assessments based on ownership percentage of property benefited by a development project funded by municipal bonds. Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Axis] Business Segments [Axis] Business Segments [Domain] Segment [Domain] Regulated Operations [Member] Regulated Operation [Member] Investments and Other [Member] All Other Segments [Member] Business Segments [Line Items] Segment Reporting Information [Line Items] Land available-for-sale 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 (Expense) 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 Consolidated Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Allowance for 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 Gain (Loss) on Sale of Other Assets Depreciation Expense Depreciation Amortization of Debt Issuance Costs Amortization of Financing Costs Deferred Income Tax Expense 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 Pension and Other Postretirement Benefit Contributions 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 Businesses and Interest in 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 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 Proceeds (Payments) from (for) Notes Payable Proceeds from (Repayments of) Notes Payable Payments for Long-Term Debt Repayments of Long-term Debt 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 Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Recurring Fair Value Measures - Activity in Level 3 [Table Text Block] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Fair Value of Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] 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, Measurements, 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 Fair Value Disclosure, Equity Securities This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represeent Available-for-sale Securities which consist of all investments in certain equity securities neither classified as trading or held-to-maturity securities. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options and put options, but do not include convertible debt. Available-for-sale - Corporate Debt Securities Available-For-Sale Securities Fair Value Disclosure, Debt Securities This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. 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 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 Derivative Financial Instruments, Liabilities, Fair Value Disclosure Total Fair Value of Liabilities Liabilities, Fair Value Disclosure Total Net Fair Value of Assets (Liabilities) Net Assets and Liabilities Fair Value Disclosure This element represents the aggregate of the assets and liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. 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 Balance Sheet [Abstract] Statement of Financial Position [Abstract] Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Cash and Cash Equivalents Accounts Receivable (Less Allowance of $1.1 and $0.9) Accounts Receivable, Net, Current Inventories Prepayments and Other 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] Liabilities [Abstract] Current Liabilities [Abstract] Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest Payable, Current Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Notes Payable Notes Payable, Current 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 13) Commitments and Contingencies Equity [Abstract] Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] ALLETE's Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 38.8 and 37.5 Shares Outstanding Common Stock, Value, Issued Unearned ESOP Shares Unearned ESOP Shares Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained Earnings Retained Earnings (Accumulated Deficit) Total Equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Total Liabilities and Equity Liabilities and Equity Components of Net Periodic Benefit Expense [Table Text Block] Schedule of Defined Benefit Plans Disclosures [Table Text Block] Other Income (Expense) [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Solid and Hazardous Waste [Table] Schedule of Environmental Exit Cost [Table] Solid and Hazardous Waste [Axis] Environmental Exit Cost by Property [Axis] Solid and Hazardous Waste [Domain] Environmental Exit Costs, Name of Property [Domain] Manufactured Gas Plant Site [Member] Manufactured gas plant site that requires remediation as a result of unanticipated contamination of the assets. Solid and Hazardous Waste [Line Items] Environmental Exit Cost [Line Items] Remediation Costs Liability Environmental Exit Costs, Assets Previously Disposed, Liability for Remediation