Accounts Receivable (Less Allowance of $1.0 and $0.9) Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Loss Accumulated Depreciation Investment in ATC Cash Investments AFUDC - Equity Depreciation Expense Distributed ATC Earnings Accounts Receivable, Allowance Allowance for long-term receivables [Roll forward] Additional Reserve Asset Retirement Obligation Costs [Member] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent Balance sheet data [Abstract] ALLETE's Investment in ATC Basic Earnings Per Share of Common Stock Earnings Per Share - Basic (in dollars per share) Cash and Cash Equivalents Cash and Cash Equivalents at Beginning of Period Cash and Cash Equivalents at End of Period Interest - Net of Amounts Capitalized Inventories Increase (Decrease) in Inventories Prepayments and Other Changes in Operating Assets and Liabilities Accounts Payable Increase (Decrease) in Accounts Payable Common Stock Shares Authorized Common Stock Shares Outstanding Common Stock Without Par Value, 80.0 Shares Authorized, 36.5 and 35.8 Shares Outstanding Current Tax Expense (Benefit) [Abstract] Current Tax Expense (Benefit) Deferred Tax Expense [Abstract] Deferred Tax Expense Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Comprehensive Income Comprehensive Income (Loss) Note [Text Block] Construction and development costs Fuel and Purchased Power Fuel and Purchased Power Operating Expenses Cost of Revenue [Abstract] Federal - Current Total Current Tax Expense Total Current Tax Expense Total Current Liabilities Liabilities, Current Current Liabilities State - Current Damages from Product Defects [Member] Debt due within one year Short-Term and Long-Term Debt Debt Disclosure [Text Block] Debt instrument, restrictive covenant Debt Issuance Costs Payments of Debt Issuance Costs Amortization of Debt Issuance Costs Federal - Deferred Deferred Fuel Adjustment Clause Deferred Fuel Costs [Member] Deferred Income Tax Expense Total Deferred Tax Expense State - Deferred Deferred Income Taxes Deferred Income Tax Charges [Member] Defined Benefit Plans and Other Postretirement Benefit Plans [Domain] Guarantor Obligations, Nature [Domain] Guarantor Obligations by Nature [Axis] Guarantor Obligations [Line Items] Schedule of Guarantor Obligations [Table] Diluted Earnings Per Share of Common Stock Earnings Per Share -Diluted (in dollars per share) Equity Investments Equity Method Investments Disclosure [Text Block] Schedule of regulated assets and liabilities [Abstract] Equity Earnings in ATC Equity in ATC Earnings ALLETE's Equity in Net Income Effective tax rate Effective tax rate, statutory Share-Based Compensation Expense ESOP Compensation Expense Environmental Restoration Costs [Member] Investment in ATC Equity Investment Balance Percent ownership of equity investment Consilidated Statement of Income [Abstract] Income Statement [Abstract] Income Tax Expense Income Tax Disclosure [Text Block] Income Taxes Changes in Regulatory and Other Non-Current Assets Interest Expense Interest Expense Fuel Inventories Total Inventories Inventory, Net Investments [Abstract] Total Liabilities Liabilities Liabilities Liabilities [Abstract] Total Liabilities and Equity Liabilities and Equity Liabilities and Equity Liabilities and Equity [Abstract] Minimum annual payment obligations for purchase contracts, year two Minimum annual payment obligations for purchase contracts, year three Minimum annual payment obligations for purchase contracts, current year Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Long-Term Debt Due Within One Year Long-Term Debt Long-term debt outstanding Percentage output entitlement under the power purchase agreement (in hundredths) Portion of debt service obligation on debt for power cooperative corporation (in hundredths) Long-term debt, including current portion Long-Term Debt, Including Current Portion Loss Contingencies by Nature of Contingency [Axis] Loss Contingencies [Line Items] Loss Contingencies [Table] Loss Contingency, Nature [Domain] Loss on Reacquired Debt [Member] Available-for-sale Securities Available-for-sale Securities, Noncurrent Mine Reclamation And Closing Liability Noncurrent Non-Controlling Interest in Subsidiaries Money Market Funds Cash for Financing Activities Net Cash Provided by (Used in) Financing Activities Financing Activities Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities Investing Activities Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Operating Activities Changes in Notes Payable Net Income (Loss) Attributable to ALLETE Net Income (Loss) Attributable to ALLETE Net Income Attributable to ALLETE Change in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Allowance for long-term receivables Allowance for long-term receivables Loans and Leases Receivable, Allowance Long-Term Finance Receivables Loans and Leases Receivable, Net Reported Amount Total Other Expense Nonoperating Income (Expense) Other Income (Expense) Nonoperating Income (Expense) [Abstract] Notes Payable Notes Payable, Current Operating Income Operating Income (Loss) Other Other Accrued Liabilities, Noncurrent Defined Benefit Pension and Other Postretirement Plans Net of Income Taxes Comprehensive Income (Loss) Unrealized Gain on Securities Net of Income Taxes Income Taxes on Unrealized Gains on Securities Other Other Assets, Current Other Income (Expense) Other Income and Other Expense Disclosure [Text Block] Changes to Other Investments Payments for (Proceeds from) Other Investing Activities Other Investments Total Investments Other Long-term Investments Other Other Income Total Other Income Total Other Income Other Postretirement Benefit Plans, Defined Benefit [Member] Short-Term Investments Dividends on Common Stock Payments of Dividends, Common Stock Pension and Other Postretirement Benefit Plans Pension and Other Postretirement Benefits Disclosure [Text Block] Pension Plans, Defined Benefit [Member] Performance Guarantee [Member] Amortization of Net Loss Amortization of Prior Service Costs Amortization of Transition Obligation Components of Net Periodic Benefit Expense [Abstract] Amount contributed to benefit plan Expected Return on Plan Assets Expected Return on Plan Assets Interest Cost Net Periodic Benefit Expense Service Cost Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis] Prepayments and Other Prepaid Expense, Current Prescription drug reimbursements received Proceeds from Issuance of Common Stock Proceeds from Issuance of Long-Term Debt Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Assets Property, Plant and Equipment - Net Bad Debt Expense Amount of deferred tax liability reversed as part of agreement for retail rate increase Deferred tax adjustments Regulatory Matters Public Utilities Disclosure [Text Block] Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Operating Revenue Total regulatory assets Regulatory Assets Regulatory Asset [Axis] Regulatory Asset [Domain] Pension Costs [Member] Current assets Regulatory Assets Non-current assets Current liabilities Total regulatory liabilities Regulatory Liabilities Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liabilities Non-Current liabilities Payments on Long-Term Debt