Regulatory Matters [Abstract] Regulatory Matters [Abstract] Schedule of Regulatory Assets [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] PPACA Income Tax Deferral [Member] Medicare Part D Regulatory Asset [Member] Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). Conservation Improvement Program [Member] CIP Financial Incentive [Member] CIP financial incentive [Member] Other [Member] Other Regulatory Assets [Member] Discloses the amount of other regulatory assets. Regulatory Assets [Line Items] Regulatory Assets [Line Items] Current Regulatory Assets Regulatory Assets, Current Non-Current Regulatory Assets Regulatory Assets, Noncurrent Regulatory Assets Regulatory Assets Fair Value [Abstract] Fair Value Fair Value [Text Block] Fair Value Disclosures [Text Block] Commitments, Guarantees and Contingencies [Abstract] Commitments, Guarantees and Contingencies [Abstract] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] 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. DC Transmission Line from Square Butte [Member] Transmission line purchased as part of North Dakota Wind Development projects. Power Purchase Agreements [Axis] Long-term Contract for Purchase of Electric Power [Axis] Power Purchase Agreements [Domain] Long-term Contract for Purchase of Electric Power [Domain] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Minnkota Power Sales Agreement [Member] Long-term contract with Minnkota Power 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. Oliver Wind I and II PPA [Member] Long-term contracts 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] Transmission Line Purchase Date Transmission Line Purchase Date Date of purchase of a transmission line. PPA Start Date Long-term Contract for Purchase of Electric Power, Start Date of Contract Start date of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. PPA Expiration Date Long-Term Contract for Purchase of Electric Power, Date of Contract Expiration, Duration String Long-Term Contract for Purchase of Electric Power, Date of Contract Expiration, Duration String 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. 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 Transmission Line Capacity (kV) Transmission Line Capacity (kV) Capacity of a transmission line in kilovolts. 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. 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. 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 Anticipated Date of New Transmission Line Long-term Contract for Purchase of Electric Power, Anticipated Date of New Transmission Line Anticipated date that a new transmission line will be built as part of a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Date of MPUC Approval Long-term Contract for Purchase of Electric Power, Date of MPUC Approval Date of approval by Minnesota Public Utilities Commission (MPUC) of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Number of Wind PPAs Long-term Contract for Purchase of Electric Power, Number of Wind PPAs Number of long-term wind contracts to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Operations and Significant Accounting Policies [Abstract] Operations and Significant Accounting Policies [Abstract] 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] Investment in ATC [Abstract] Investment in ATC [Abstract] 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 - Anticipated 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. Equity Method Investment - Investments Made During Year Equity Method Investment - Additional Investment Amount Equity Method Investment - Additional Investment Amount 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, 2011 Equity Method Investments Cash Investments Payments to Acquire Equity Method Investments Equity Earnings in ATC Income (Loss) from Equity Method Investments Distributed ATC Earnings Equity Method Investment, Dividends or Distributions Equity Investment Balance as of March 31, 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 Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Recurring Fair Value Measures - Activity in Level 3 [Table Text Block] Fair Value, 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] 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] Statement [Table] Rate Revisions [Axis] Information about periodic rate revisions. Rate Revisions [Domain] Periodic rate revisions. 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). Non-Affiliated Municipal Customers, Private Utilities [Member] Non-Affiliated Municipal Customers, Private Utilities [Member] Non-Affiliated Municipal Customers, Private Utilities [Member] 2010 Wisconsin Rate Increase [Member] 2010 retail rate increase requested filed with the Public Service Commission of Wisconsin (PSCW). Unusual/Nonrecurring Items [Axis] Information about unusual/nonrecurring tax items. Unusual/Nonrecurring Items [Domain] Unusual/nonrecurring tax items. 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). 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. Medicare Part D [Member] Statement [Line Items] Electric Rates [Abstract] Public Utilities, General Disclosures [Abstract] Approved Rate of Return on Common Equity Rate of return on common equity approved by the state 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 state regulatory commission through a rate case proceeding. Tax Adjustments, Settlements, and Unusual Provisions Non-Affiliated Municipal Customers, Municipalities The number of municipalities that are non-affiliated municipal customers. Non-Affiliated Municipal Customers, Private Utilities The number of private utilities that are non-affiliated municipal customers. Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract in years. The Patient Protection and Affordable Care Act of 2010 (PPACA) [Abstract] Regulatory Assets and Liabilities, Other Disclosures [Abstract] 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. Leasing Agreements [Table] Schedule of Operating Leased Assets [Table] Leasing Agreements [Axis] Property, Plant and Equipment by 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. Leasing Agreement Expiration Date Leasing Agreement Expiration Date Expiration date of operating lease agreement. Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Operating Leases, Future Minimum Payments Due, Current Operating Leases, Future Minimum Payments Due, Current 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 Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Transmission [Axis] Information about each transmission investment. Transmission [Domain] Name of transmission investment. CapX2020 Fargo to St. Cloud Project [Member] Transmission investment project. 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. MPUC, Billing Factor, Approval Date MPUC, Billing Factor, Approval Date Date/expected date of approval by the Minnesota Public Utilities Commission (MPUC) of billing factor. MPUC, Updated Billing Factor, Filing Date MPUC, Updated Billing Factor, Filing Date Filing date of petition to the Minnesota Public Utilities Commission (MPUC) seeking updated billing factor approval. MPUC, Updated Billing Factor, Expected Approval Date MPUC, Updated Billing Factor, Approval Date Date/expected date of approval by the Minnesota Public Utilities Commission of updated billing factor. 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. 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. Capital Cost Spent to Date Capital cost spent to date for project. MPUC, Route Permit, Approval Date MPUC, Route Permit, Approval Date Date/expected date of approval by the Minnesota Public Utilities Commission (MPUC) of route permit. Project Completion Date Project Completion Date Date/expected date of project completion. In Service Date In Service Date Date/expected date an asset will be in service. Construction Start Date Construction Start Date Date/expected date of start of construction for a project. Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Business Segments [Abstract] Business Segments [Abstract] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Income Tax Expense [Abstract] Income Tax Expense [Abstract] 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. Income Tax Expense [Abstract] Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Tax Expense [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Tax Expense - Federal Current Federal Tax Expense (Benefit) Current Tax Expense - State Current State and Local Tax Expense (Benefit) Total Current Tax Expense Current Income Tax Expense (Benefit) Deferred Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Deferred Tax Expense - Federal Deferred Federal Income Tax Expense (Benefit) Deferred 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 Expense (Benefit) Income Tax Expense, Unusual/Nonrecurring Items [Abstract] Income Tax Reconciliation, Other Reconciling Items [Abstract] Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract] Effective Income Tax Rate, Continuing Operations Effective Income Tax Rate Reconciliation, Other Adjustments Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Unrecognized Tax Benefits Unrecognized Tax Benefits that Would Impact Effective Tax Rate Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value by Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] 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] Total [Member] Estimate of Fair Value, Fair Value Disclosure [Member] Recurring Fair Value Measures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Equity Securities Equity Method Investments, Fair Value Disclosure Available-for-sale Securities - Corporate Debt Securities Available-for-sale Securities, Fair Value Disclosure Money Market Funds Cash and Cash Equivalents, Fair Value Disclosure Total Fair Value of Assets Assets, Fair Value Disclosure Deferred Compensation Deferred Compensation Liability, Current and Noncurrent Derivatives - Interest Rate Swap Interest Rate Cash Flow Hedge Liability at Fair Value 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. Investment in ATC [Text Block] Equity Method Investments Disclosure [Text Block] Derivatives [Abstract] DERIVATIVES [Abstract] Derivative [Table] Derivative, by Nature [Axis] Derivative, Name [Domain] Derivatives [Line Items] Derivative [Line Items] Debt Instrument, Face Amount Debt Instrument, Description of Variable Rate Basis Debt Instrument, Basis Spread on Variable Rate Debt Instrument, Basis Spread on Variable Rate Description of Derivative Activity Volume Derivative, Description of Variable Rate Basis Derivative, Description of Variable Rate Basis Derivative, Fixed Interest Rate Discussion of Interest Rate Cash Flow Hedge Effectiveness Assessment and Measurement Derivative, Interest Rate Swap Liability Other Income (Expense) [Abstract] Other Income (Expense) [Abstract] Other Income (Expense) [Table Text Block] Schedule of Other Nonoperating Income (Expense) [Table Text Block] Environmental Costs [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Long-Term Debt, Including Current Portion - Carrying Amount Long-term Debt Long-term Debt, Including Current Portion - Fair Value Long-term Debt, Fair Value Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Short-Term and Long-Term Debt [Abstract] SHORT-TERM AND LONG-TERM DEBT [Abstract] Short-Term Debt Outstanding Debt, Current Long-Term Debt Outstanding Long-term Debt, Excluding Current Maturities 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 Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Statement [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Maximum [Member] Maximum [Member] Minimum [Member] Minimum [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] Statement [Line Items] Expected Capital Cost Expected Capital Cost Expected capital cost for each individual project. 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 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 Statement, Scenario [Axis] Scenario, Unspecified [Domain] Consolidated Statement of Cash Flows [Abstract] Statement of Cash Flows [Abstract] Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Allowance for Funds Used During Construction Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity Income from Equity Investments, Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Gain on Real Estate Foreclosure Gains (Losses) on Sales of Other Real Estate Gain on Sale of Assets Gain (Loss) on Sale of Other Assets Depreciation Expense Depreciation Amortization of Debt Issuance Costs Debt Related Commitment Fees and Debt Issuance 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 Other 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 Operating Capital 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 Operating Assets Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Operating 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 Changes in Notes Payable Proceeds from (Repayments of) Notes Payable Reductions of Long-term Debt Repayments of Long-term Debt Payments of Debt Issuance Costs Payments of Debt Issuance Costs Dividends on Common Stock Payments of Dividends, Common Stock Cash from (for) Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and Cash Equivalents at Beginning of Period Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents at End of Period Investments [Abstract] Land Held-for-sale [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] 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. Bison 1 Wind Project [Member] Wind project in North Dakota. Bison 1 Wind Project, Land Restoration and Upgrades [Member] Bison 1 Wind Project, Land Restoration and Upgrades [Member] Bison 1 Wind Project, Land Restoration and Transmission Upgrades After Project Completion [Member] Bison 1 Wind Project, Phase 1 [Member] Phase of wind project in North Dakota. Bison 1 Wind Project, Phase 2 [Member] Phase of 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. Bison 2 and 3 Wind Projects, Each [Member] Bison 2 and 3 Wind Projects [Member] Wind projects in North Dakota. 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. Number of Phases Number of Phases Number of phases for project. 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 Capital Cost Spent to Date MPUC, Updated Rates, Order Date MPUC, Updated Rates, Filing Date Filing date of petition to the Minnesota Public Utilities Commission (MPUC) seeking updated rates for additional investments and expenditures. MPUC, Project, Approval Date MPUC, Project, Approval Date Date/expected date of project approval by the Minnesota Public Utilities Commission (MPUC). NDPSC, Site Permit, Approval Date NDPSC, Site Permit, Approval Date Date/expected date of approval of site permit by the North Dakota Public Service Commission (NDPSC). 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 Cost of Domestic Regulated Electric 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 Other Nonoperating Income (Expense) 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 Weighted Average Number of Shares Outstanding, Basic Diluted Weighted Average Number of Shares Outstanding, Diluted Basic Earnings Per Share of Common Stock Earnings Per Share, Basic Diluted Earnings Per Share of Common Stock Earnings Per Share, Diluted Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Investment and Other Income (Expense) Other Income Total Other Income Loss Contingencies [Table] Loss Contingencies [Table] Loss Contingencies by Nature of Contingency [Axis] Loss Contingencies by Nature of Contingency [Axis] Loss Contingency, Nature [Domain] Loss Contingency, Nature [Domain] Pending or Threatened Litigation [Member] Pending or Threatened Litigation [Member] Legal Proceedings [Line Items] Loss Contingencies [Line Items] Loss Contingency, Damages Sought, Value Loss Contingency, Damages Sought, 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] Coal Contracts, Expiration Date Coal Contracts, Expiration Date Coal Contracts, Expiration Date Transportation Contracts, Expiration Date Transportation Contracts, Expiration Date Rail and Shipping Transportation Contracts, Expiration Date Minimum Annual Payment Obligation, Current Year Recorded Unconditional Purchase Obligation Due within One Year Minimum Annual Payment Obligation, Year Two Recorded Unconditional Purchase Obligation Due in Second Year 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 Reported Amount Other Other Real Estate Total Real Estate Assets Long-Term Finance Receivables, Allowance Allowance for Notes, Loans and Financing Receivable, Noncurrent Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Axis] Statement, 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] Number of acres of land available-for-sale in Minnesota 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 (Loss) Income (Loss) Before Non-Controlling Interest and Income Taxes Income Tax Expense (Benefit) Net Income Net Income 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 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] Common Shares Earnings Per Share Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent, Diluted Common Shares Earnings Per Share Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] 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 Defined Benefit Plans Disclosures [Table Text Block] Derivatives [Text Block] Derivative Instruments and Hedging Activities Disclosure [Text Block] Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Investments [Text Block] Investment [Text Block] Equity Method Investment [Policy Text Block] Equity Method Investments, Policy [Policy Text Block] Inventories [Table Text Block] Schedule of Inventory, Current [Table Text Block] Prepayments and Other Current Assets [Table Text Block] Schedule of Other 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] 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 Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Consolidated Statement of Comprehensive Income [Abstract] Consolidated Statement of Comprehensive Income [Abstract] Statement, Scenario [Axis] Scenario, Unspecified [Domain] Unrealized Gain on Securities, Net of Income Taxes of $0.7 and $0.6 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) 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 $0.3 and $0.3 Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Other Comprehensive Income 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 Fair Value Transfers [Policy Text Block] Fair Value Transfer, Policy [Policy Text Block] Fair Value of Financial Instruments [Policy Text Block] Fair Value of Financial Instruments, Policy [Policy Text Block] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis] 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] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [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 Schedule of Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantor Obligations by Nature [Axis] Guarantor Obligations by 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] Date Issued Capital Improvement Revenue/Special Assessment Bonds, Date Issued Issue date of municipal bonds used to fund a development project. 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] Derivatives [Policy Text Block] Derivatives, Policy [Policy Text Block] Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Other Income (Expense) [Text Block] Other Income and Other Expense Disclosure [Text Block] Long-Term Investment Portfolio [Table Text Block] Investment [Table Text Block] Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Schedule of Regulatory Liabilities [Table] Schedule of Regulatory Liabilities [Table] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Income Taxes [Member] 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. Other [Member] Other Regulatory Liabilities [Member] Discloses the amount of other regulatory liabilties. Regulatory Liabilities [Line Items] Regulatory Liabilities [Line Items] Non-Current Regulatory Liabilties Regulatory Liability, Noncurrent Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value by Asset Class [Axis] Fair Value by Asset Class [Axis] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, by Asset Class [Domain] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, by Asset Class [Domain] Debt Securities Issued by States of the United States (ARS) [Member] US Treasury and Government [Member] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [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 Settled During the Period Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Redeemed During the Period Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales Balance as of March 31, 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. Consolidated Balance Sheet [Abstract] Statement of Financial Position [Abstract] Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Cash and Cash Equivalents Short-Term Investments Other Short-term Investments Accounts Receivable (Less Allowance of $1.0 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, 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 Shares Authorized, 38.1 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 Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Entities [Table] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Entity Information [Line Items] Entity Information [Line Items] 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 Other Comprehensive Income (Loss), Tax [Abstract] Unrealized Gain 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 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