Retirement Benefits [Abstract] Components of Net Periodic Benefit Expense (Income) [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Equity Method Investments and Joint Ventures [Abstract] Investment in ATC [Table] Schedule of Equity Method Investments [Table] Equity Method Investment [Axis] Investment, Name [Axis] Equity Method Investment [Domain] Investment, Name [Domain] ATC [Member] ATC [Member] ATC [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Return on Equity Complaint [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] Investment in ATC [Line Items] Schedule of Equity Method Investments [Line Items] Ownership Percentage Equity Method Investment, Ownership Percentage Expected Additional Investment in 2019 Equity Method Investment, Expected Additional Investment Additional investments expected to be made during the current fiscal year but after the filing date, not already separately disclosed as part of current period/year-to-date investments or as a subsequent event. ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Balance as of December 31, 2018 Equity Method Investments Cash Investments Payments to Acquire Equity Method Investments Equity in ATC Earnings Income (Loss) from Equity Method Investments Distributed ATC Earnings Proceeds from Equity Method Investment, Distribution Amortization of the Remeasurement of Deferred Income Taxes Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments Equity Investment Balance as of June 30, 2019 Authorized Return on Equity Public Utilities, Approved Return on Equity, Percentage Authorized Return on Equity, Including Incentive Adder Equity Method Investment, Approved Return on Common Equity, Including Incentive Adder The approved percentage return on common equity, including an incentive adder for participation in a regional transmission organization dedicated to the utility plant in the investee accounted for under the equity method of accounting. Proposed Return on Equity Equity Method Investment, Proposed Return on Common Equity The proposed return on common equity, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting. Proposed Return on Equity, Including Incentive Adder Equity Method Investment, Proposed Return on Common Equity, Including Incentive Adder The proposed return on common equity, including an incentive adder for participation in a regional transmission organization, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting. Fair Value Disclosures [Abstract] Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount Long-term Debt Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value Debt Disclosure [Abstract] Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Regulated Operations [Abstract] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Regulatory Matters [Table] Public Utilities General Disclosures [Table] CIP Consolidated Filing [Member] CIP Consolidated Filing [Member] CIP Consolidated Filing [Member] MPUC [Member] MPUC [Member] Minnesota Public Utilities Commission [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] CIP Financial Incentive Regulated Operating Revenue, Other Goodwill and Intangible Assets Disclosure [Abstract] Schedule of Intangible Assets [Table Text Block] Schedule of Intangible Assets and Goodwill [Table Text Block] Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] Schedule of Cash, Cash Equivalents and Restricted Cash [Table Text Block] Inventories – Net [Table Text Block] Schedule of Inventory, Current [Table Text Block] Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Supplemental Statement of Cash Flows Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Lease, Cost [Table Text Block] Lease, Cost [Table Text Block] Lessee, Operating Lease, Liability, Maturity [Table Text Block] Lessee, Operating Lease, Liability, Maturity [Table Text Block] Leases [Abstract] Lessee Disclosure [Abstract] Lessee Disclosure [Abstract] Operating Lease Residual Value Guarantee Lessee, Operating Lease, Residual Value Guarantee, Description Residual Value Guarantee Residual Value of Leased Asset Operating Lease Cost [Abstract] Income and Expenses, Lessee [Abstract] Operating Lease Cost Operating Lease, Cost Other Information [Abstract] Cash Flow, Operating Activities, Lessee [Abstract] Operating Cash Flows from Operating Leases Operating Lease, Payments Balance Sheet Information Related to Leases [Abstract] Assets and Liabilities, Lessee [Abstract] Operating Lease, Right-of-Use Asset Operating Lease, Right-of-Use Asset Other Non-Current Assets Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] Total Lease Obligations Operating Lease, Liability Operating Lease Liabilities Operating Lease, Liability, Current Other Current Liabilities Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] Operating Lease Liabilities Operating Lease, Liability, Noncurrent Other Non-Current Liabilities Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] Lease Description [Table] Lessee, Lease, Description [Table] Lease Arrangement, Type [Axis] Lease Arrangement, Type [Axis] Lease Arrangement, Type [Domain] Lease Arrangement, Type [Domain] Vehicles and Equipment [Member] Vehicles and Equipment [Member] Vehicles and Equipment [Member] Land and Other [Member] Land and Other [Member] Land and Other [Member] Lease Description [Line Items] Lessee, Lease, Description [Line Items] Weighted Average Remaining Lease Term Operating Lease, Weighted Average Remaining Lease Term Weighted Average Discount Rate Operating Lease, Weighted Average Discount Rate, Percent Fair Value [Text Block] Fair Value Disclosures [Text Block] Commitments and Contingencies Disclosure [Abstract] Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. 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] Manitoba Hydro PPA [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Name of transmission investment. Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Transmission [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Output Being Purchased (MW) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. 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. Total Project Cost in the U.S. Estimated Capital Expenditures, Including Past Expenditures The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. Total Project Costs Incurred to Date Capital Cost Spent to Date Capital cost spent to date for project. Project Costs Recovered from Counterparty Capital Costs Spent To Date, Recovered Capital Costs Spent To Date, Recovered Segment Reporting [Abstract] Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Income Tax Disclosure [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Income Tax Expense (Benefit) [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Total Current Income Tax Expense Current Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Total Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Total Income Tax Expense (Benefit) Income Tax Expense (Benefit) Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Income Before Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Taxes Computed at Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (Decrease) in Income Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Regulatory Differences – Excess Deferred Tax Effective Income Tax Rate Reconciliation, Regulatory Differences Regulatory Differences contributing to difference in effective tax rate to federal statutory rate. U.S. Water Services Sale of Stock Basis Difference Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Share-Based Compensation Effective Income Tax Rate Reconciliation, Tax Expense (Benefit), Share-based Payment Arrangement, Amount Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Gross Unrecognized Tax Benefits Unrecognized Tax Benefits Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Investment in Nobles 2 [Table] Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Resource Package [Member] Resource Package [Member] Resource Package [Member] Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] Tenaska PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] Nobles 2 [Member] Nobles 2 [Member] Nobles 2 [Member] Investment in Nobles 2 [Line Items] Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Generating Capacity Counterparty Owned (MW) Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Equity Method Investment Balance Return of Capital From Equity Investments Proceeds from Equity Method Investment, Distribution, Return of Capital Statement of Cash Flows [Abstract] Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Net Income Net Income (Loss) Attributable to Parent AFUDC – Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Depreciation Expense Depreciation Amortization of PSAs Amortization of Power Purchase Agreements The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method. Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Benefit Share-Based and ESOP Compensation Expense Share-based Payment Arrangement, Noncash Expense Defined Benefit Pension and Postretirement Benefit Expense Pension and Other Postretirement Benefits Cost (Reversal of Cost) Payments / Provision for Interim Rate Refund Provision for Interim Rate Refund Provision for Interim Rate Refund Payments / Provision for Tax Reform Refund Provision for Tax Reform Refund Provision for Tax Reform Refund Bad Debt Expense Accounts Receivable, Credit Loss Expense (Reversal) Gain on Sale of U.S. Water Services Gain (Loss) on Disposition of Business 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 Plans Payment for Pension Benefits 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 Payments for Equity Investments Proceeds from Sale of U.S. Water Services - Net of Transaction Costs and Cash Retained Proceeds from Divestiture of Businesses Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Payments for (Proceeds from) Other Investing Activities Payments for (Proceeds from) Other Investing Activities Cash from (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 Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition-Related Contingent Consideration Payments Payment for Contingent Consideration Liability, Financing Activities Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Other Financing Activities Proceeds from (Payments for) Other Financing Activities Cash for Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash, Cash Equivalents and Restricted Cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Statement of Comprehensive Income [Abstract] Unrealized Gain (Loss) on Securities, Income Tax Expense Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block] Supplemental Cash Flow Information [Abstract] Cash Paid for Interest – Net of Amounts Capitalized Interest Paid, Excluding Capitalized Interest, Operating Activities Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Increase (Decrease) in Accounts Payable for Capital Additions to Property, Plant and Equipment Reclassification of Property, Plant and Equipment to Inventory Reclassification of Property, Plant and Equipment to Inventory Reclassification of Property, Plant and Equipment to Inventory Recognition of Right-of-use Assets and Lease Liabilities Recognition of Right-of-use Assets and Lease Liabilities Non Cash Impact From the Recognition of Right-of-use Assets and Lease Liabilities Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations AFUDC–Equity Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Customer Relationships [Member] Customer Relationships [Member] Developed Technology and Other [Member] Other Intangible Assets [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Definite-Lived Intangible Assets [Roll Forward] Finite-lived Intangible Assets [Roll Forward] Beginning Balance Finite-Lived Intangible Assets, Net Amortization Amortization of Intangible Assets Other Changes Finite-Lived Intangible Assets, Other Changes Other changes to finite-lived intangible assets not disclosed separately. Ending Balance Schedule of Indefinite-Lived Intangible Assets [Table] Schedule of Indefinite-Lived Intangible Assets [Table] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] Trademarks and Trade Names [Member] Trademarks and Trade Names [Member] Indefinite-lived Intangible Assets [Line Items] Indefinite-lived Intangible Assets [Line Items] Indefinite-Lived Intangible Assets [Roll Forward] Indefinite-lived Intangible Assets [Roll Forward] Beginning Balance Indefinite-lived Intangible Assets (Excluding Goodwill) Other Changes Indefinite-Lived Intangible Assets, Other Changes Other changes to indefinite-lived intangible assets not disclosed separately. Ending Balance Intangible Assets [Abstract] Intangible Assets, Net (Excluding Goodwill) [Abstract] Total Intangible Assets - Beginning Balance Intangible Assets, Net (Excluding Goodwill) Total Intangible Assets - Amortization Total Intangible Assets - Other Changes Intangible Assets, Other Changes Other changes to finite-lived and indefinite-lived intangible assets not disclosed separately. Total Intangible Assets - Ending Balance Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Domain] Guarantee Entity and Type [Domain] [Domain] for Guarantee Entity and Type [Axis] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] BNI Energy Reclamation Liability [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. ALLETE Properties Development and Maintenance Obligations [Member] ALLETE Properties Performance and Maintenance Obligations [Member] Performance and maintenance obligations to governmental entities to construct improvements in various projects. Town Center District Capital Improvement Bonds [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. Palm Coast Park District Special Assessment Bonds [Member] Palm Coast Park Community Development District [Member] Community development district obligations for Palm Coast Park District. Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Letters of Credit [Member] Financial Standby Letter of Credit [Member] Surety Bonds [Member] Surety Bond [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Collateral Guarantor Obligations, Collateral Held Directly or by Third Parties, Amount Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Ownership Percentage of Benefited Property Community Development District Obligation, Ownership of Property Benefited Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. Annual Assessment Community Development District Obligation, Annual Assessment Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. Financial Covenants [Table] Schedule of Long-term Debt Instruments [Table] Statistical Measurement [Axis] Statistical Measurement [Axis] Statistical Measurement [Domain] Statistical Measurement [Domain] Maximum [Member] Maximum [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Required Ratio of Indebtedness to Total Capitalization Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Actual Ratio of Indebtedness to Total Capitalization Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Unrealized Gain (Loss) on Securities Net of Income Tax Expense of $0.1, $–, $0.1 and $– Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $–, 0.1, $0.1 and $0.2 Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Total Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Recurring Fair Value Measures [Table] Fair Value, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Domain] Recurring Fair Value Measures [Member] Fair Value, Recurring [Member] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Domain] Contingent Consideration [Domain] [Domain] for Contingent Consideration [Axis] Payments [Member] Payments [Member] Payments made that decrease contingent consideration liability [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] Investments [Abstract] Available-for-sale – Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Available-for-sale – Corporate and Governmental Debt Securities (b) Debt Securities, Available-for-sale, Noncurrent Cash Equivalents Other Restricted Assets, Noncurrent Total Fair Value of Assets Assets, Fair Value Disclosure Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the first fiscal year through the third fiscal year following the latest fiscal year (maturing in more than one year to less than three years). Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the third fiscal year through the fifth fiscal year following the latest fiscal year (maturing in three years to less than five years). Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing in after the fifth fiscal year following the latest fiscal year (maturing in five years or more). 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. U.S. Water Services Contingent Consideration Business Combination, Contingent Consideration, Liability Total Fair Value of Liabilities Financial and Nonfinancial Liabilities, Fair Value Disclosure Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Schedule of Short-Term and Long-Term Debt [Table Text Block] Schedule of Debt [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Schedule of Long-term Debt Instruments [Table Text Block] Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Asset [Axis] Total Regulatory Asset [Domain] Regulatory Asset [Domain] Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension and Other Postretirement Plans Costs [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Boswell 1 & 2 [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] Cost Recovery Riders [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] PPACA Income Tax Deferral [Member] Medicare Part D Regulatory Asset [Member] Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). Other Other Regulatory Assets (Liabilities) [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Provision for Interim Rate Refund [Domain] Revenue Subject to Refund [Member] Transmission Formula Rates [Member] Transmission Formula Rates [Member] Transmission Formula Rates Provision for Tax Reform Refund [Domain] Provision for Tax Reform Refund [Domain] Provision for Tax Reform Refund [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Plant Removal Obligations [Member] Removal Costs [Member] North Dakota Investment Tax Credits [Member] North Dakota Investment Tax Credits Subject to Refund [Member] North Dakota Investment Tax Credits Subject to Refund [Member] CIP Financial Incentive [Member] CIP Financial Incentive [Member] Conservation Improvement Program [Member] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Regulatory Assets Currently Earning a Return Types of Net Regulatory Assets Earning Returns Non-Current Regulatory Assets Regulatory Assets, Noncurrent Current Regulatory Liabilities Regulatory Liability, Current Non-Current Regulatory Liabilities Regulatory Liability, Noncurrent Total Regulatory Liabilities Regulatory Liabilities Earnings Per Share [Abstract] Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Average Common Shares Weighted Average Number of Shares Outstanding, Basic Earnings Per Share Earnings Per Share, Basic Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Net Income (Loss) Available to Common Stockholders, Diluted Average Common Shares Weighted Average Number of Shares Outstanding, Diluted Earnings Per Share Earnings Per Share, Diluted Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Table] Document Information [Table] Document and Entity Information, Document [Axis] Document Information, Document [Axis] Document [Domain] Document [Domain] Document and Entity Information [Line Items] Document Information [Line Items] Document Type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name State of Incorporation Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Name of Exchange Security Exchange Name Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Shell Company Entity Shell Company Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Minimum [Member] Minimum [Member] SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Superior, WI [Member] Superior, WI [Member] Superior, WI Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Expected Period for Costs of Compliance Expected Period for Costs of Compliance Expected Period for Costs of Compliance Estimated Costs of Compliance (Accrued) Accrual for Environmental Loss Contingencies Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Business Segments [Axis] Segments [Axis] Business Segments [Domain] Segments [Domain] Regulated Operations [Member] Regulated Operations [Member] Regulated Operations [Member] U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] Corporate and Other [Member] Corporate and Other [Member] Revenue Streams [Axis] Contract with Customer, Sales Channel [Axis] Revenue Streams [Domain] Contract with Customer, Sales Channel [Domain] Residential [Member] Residential [Member] Revenue Recognized From Residential Customers Commercial [Member] Commercial [Member] Revenue Recognized From Commercial Customers Municipal [Member] Municipal [Member] Municipal Sales Channel Industrial [Member] Industrial [Member] Industrial Sales Channel [Member] Other Power Suppliers [Member] Other Power Suppliers [Member] Other Power Suppliers Sales Channel [Member] Other [Member] Other [Member] Regulated Operations Other Revenue Recognized From Other Sources Immaterial For Disaggregation Long-term PSA [Member] Long-term PSA [Member] Revenue Recognized From Long-term PSAs Other [Member] ALLETE Clean Energy Other [Member] ALLETE Clean Energy Other Revenue Recognized From Other Sources Immaterial For Disaggregation [Member] Point-in-Time [Member] Point-in-Time [Member] Revenue Recognized From Point-in-Time Sales Contract [Member] Contract [Member] Revenue Recognized by U.S. Water From Contract Sales Capital Project [Member] Capital Project [Member] Revenue Recognized From Capital Projects Long-term Contract [Member] Long-term Contract [Member] Long-term Contract Revenue Stream [Member] Other [Member] Corporate & Other - Other [Member] Revenue Recognized by Corporate & Other From Other Sources Immaterial for Disaggregation Business Segments [Line Items] Segment Reporting Information [Line Items] Number of Reportable Segments Number of Reportable Segments Number of Operating Segments Number of Operating Segments Land in Minnesota (Acres) Area of Land Operating Revenue Revenue from Contract with Customer, Excluding Assessed Tax Operating Revenue, Other Than Customer Revenue Revenue, Other Than Customer Revenue Non-cash Revenue Recognized From Amortization of PSAs Assumed Through Acquisitions Operating Revenue Revenues Net Income (Loss) Approximate Gain on Sale of U.S. Water Services, After-tax Gain on Sale of Affiliate, After-tax Gain on Sale of Affiliate, After-tax Assets Assets Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Square Butte [Member] Square Butte [Member] Square Butte [Member] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Power Purchase Agreement - Energy and Capacity [Axis] Power Purchase Agreement - Energy and Capacity [Domain] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Generating Facility [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Power Sales Agreements - Energy and Capacity [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Power Sales Agreements - Energy and Capacity [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Minnkota Power PSA [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] PPA Counterparty Total Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Cost of Power Purchased Utilities Operating Expense, Purchased Power under Long-term Contracts Expected Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges Schedule of Long-term Debt Instruments [Table] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility [Member] Revolving Credit Facility [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] ALLETE Bonds 4.08% Due March 2029 [Member] ALLETE Bonds 4.08% Due March 2029 [Member] ALLETE Bonds 4.08% Due March 2029 [Member] ALLETE Bonds 4.47% Due March 2049 [Member] ALLETE Bonds 4.47% Due March 2049 [Member] ALLETE Bonds 4.47% Due March 2049 [Member] Short-Term Debt - Principal Debt, Current, Gross Amount of short-term debt and current maturities of long-term debt before deduction of unamortized discount or premium and unamortized debt issuance costs. Short-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Debt, Current The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of short-term debt or the current maturities of long-term debt. Short-Term Debt - Total Debt, Current Long-Term Debt - Principal Long-term Debt, Gross Long-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Long-Term Debt Excluding Current Maturities The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of long-term debt excluding current maturities. Long-Term Debt - Total Long-term Debt, Excluding Current Maturities Total Debt - Principal Debt, Long-Term and Short-Term, Combined Amount, Gross Represents the aggregate of total long-term debt, including current maturities and short-term debt, before deduction of unamortized discount or premium and unamortized debt issuance expense. Total Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense Total Debt - Total Debt, Long-term and Short-term, Combined Amount Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility, Maximum Borrowing Capacity Line of Credit Facility Additional Borrowing Capacity Line of Credit Facility Additional Borrowing Capacity Line of Credit Facility Additional Borrowing Capacity Letters of Credit Maximum Issuances Letters of Credit Maximum Issuances Letters of Credit Maximum Issuances Letters of Credit Outstanding, Amount Letters of Credit Outstanding, Amount Line of Credit, Current Line of Credit, Current Proceeds from Issuance of First Mortgage Bond Proceeds from Issuance of First Mortgage Bond Interest Rate Debt Instrument, Interest Rate, Stated Percentage Natural Gas-fired [Member] Natural Gas Processing Plant [Member] Solar Energy Generation [Member] Solar Energy Generation [Member] Solar Energy Generation [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Combined-Cycle Natural Gas-Fired Generating Facility [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Solar Energy PPA [Member] Solar Energy PPA [Member] Solar Energy Provider [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Jointly Owned Electricity Generation Plant [Member] Jointly Owned Electricity Generation Plant [Member] Generating Capacity to be Counterparty Owned (MW) Output Being Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased Generating Capacity to be Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Generating Capacity Jointly Owned (MW) Expected Output Entitlement (Percent) 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 [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Pension [Member] Pension Plan [Member] Other Postretirement [Member] Other Postretirement Benefits Plan [Member] Pension and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Components of Net Periodic Benefit Cost (Income) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [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 (Loss) on Plan Assets Amortization of Prior Service Credits Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of Net Loss Defined Benefit Plan, Amortization of Gain (Loss) Net Periodic Benefit Cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Employer Contributions to Defined Benefit Plans Defined Benefit Plan, Plan Assets, Contributions by Employer Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Rate Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] PSCW [Member] PSCW [Member] Public Service Corporation of Wisconsin [Member] Minnesota Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] 2016 Minnesota General Rate Case [Member] 2016 Minnesota Rate Case [Member] [Member] 2016 Minnesota Rate Case [Member] [Member] 2018 Wisconsin General Rate Case [Member] 2018 Wisconsin General Rate Case [Member] 2018 retail rate increase request filed with the PSCW [Member] Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Customer Class [Domain] Customer Class [Domain] [Domain] for Customer Class [Axis] description of customer types or classes Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. Revenue from Cost Recovery Riders Approved Return on Common Equity Approved Equity Ratio Public Utilities, Approved Equity Capital Structure, Percentage Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Reserve for Interim Rate Refund Contract with Customer, Refund Liability, Current Approved Rate Increase Amount Public Utilities, Approved Rate Increase (Decrease), Amount Cash, Cash Equivalents and Restricted Cash [Abstract] Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract] Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Restricted Cash included in Prepayments and Other Restricted Cash, Current Restricted Cash included in Other Non-Current Assets Restricted Cash, Noncurrent Total Cash, Cash Equivalents and Restricted Cash Inventories – Net [Abstract] Inventory, Net [Abstract] Fuel Energy Related Inventory, Coal Materials and Supplies Inventory, Supplies, Net of Reserves Raw Materials (b) Inventory, Raw Materials, Gross Work in Progress (b) Inventory, Work in Process, Gross Finished Goods (b) Inventory, Finished Goods, Gross Reserve for Obsolescence (b) Inventory Valuation Reserves Total Inventories – Net Inventory, Net Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Contract Assets Contract with Customer, Asset, Net, Noncurrent Finance Receivable Financing Receivable, before Allowance for Credit Loss ALLETE Properties Real Estate Investments, Net Other Other Assets, Miscellaneous, Noncurrent Total Other Non-Current Assets Other Assets, Noncurrent Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] Provision for Interim Rate Refund Provision for Interim Rate Refunds Provision for Interim Rate Refund PSAs Power Purchase Agreement - Current Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability. Contract Liabilities Contract with Customer, Liability, Current Provision for Tax Reform Refunds Provision for Tax Reform Refunds Provision for Tax Reform Refunds Contingent Consideration Business Combination, Contingent Consideration, Liability, 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 PSAs Power Purchase Agreement - Noncurrent Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent liability. Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Other Liabilities, Noncurrent Income Statement [Abstract] Operating Revenues [Abstract] Revenues [Abstract] Contracts with Customers - Utility Regulated Operating Revenue Contracts with Customers - Non-utility Unregulated Operating Revenue Other - Non-utility Total Operating Revenue Operating Expenses [Abstract] Operating Expenses [Abstract] Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Fuel, Purchased Power and Gas - Utility Transmission Services - Utility Transmission Services - Utility Transmission Services - Utility Cost of Sales - Non-utility Cost of Goods and Services Sold Operating and Maintenance Operating Expenses Depreciation and Amortization Depreciation, Depletion and Amortization Taxes Other than Income Taxes Taxes, Other Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Other Income (Expense) [Abstract] Other Nonoperating Income (Expense) [Abstract] Interest Expense Interest Expense Equity Earnings Gain on Sale of U.S. Water Services Other Other Nonoperating Income (Expense) Total Other Income (Expense) Nonoperating Income (Expense) Income Before Income Taxes Income Tax Expense (Benefit) Net Income Earnings Per Share [Abstract] Basic (Shares) Diluted (Shares) Basic Earnings Per Share of Common Stock Diluted Earnings Per Share of Common Stock Goodwill Goodwill Statement of Financial Position [Abstract] Accounts Receivable [Abstract] Accounts Receivable, after Allowance for Credit Loss, Current [Abstract] Accounts Receivable, Allowance Accounts Receivable, Allowance for Credit Loss, Current Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value Common Stock, No Par Value Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Outstanding Common Stock, Shares, Outstanding Common Stock, Shares Issued Common Stock, Shares, Issued ALLETE Clean Energy Asset Acquisition [Abstract] ALLETE Clean Energy Asset Acquisition [Abstract] Generating Capacity Owned (MW) Generating Capacity Owned (MW) Generating Capacity Owned (MW) ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Lessee, Operating Lease, Liability, Payment, Due [Abstract] Lessee, Operating Lease, Liability, Payment, Due [Abstract] 2019 Lessee, Operating Lease, Liability, Payments, Due Next Twelve Months 2020 Lessee, Operating Lease, Liability, Payments, Due Year Two 2021 Lessee, Operating Lease, Liability, Payments, Due Year Three 2022 Lessee, Operating Lease, Liability, Payments, Due Year Four 2023 Lessee, Operating Lease, Liability, Payments, Due Year Five Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total Lease Payments Due Lessee, Operating Lease, Liability, Payments, Due Less: Imputed Interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Total Lease Obligations Less: Current Lease Obligations Long-term Lease Obligations Statement of Stockholders' Equity [Abstract] Consolidated Statement of Shareholders' Equity [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock [Member] Common Stock [Member] AOCI Attributable to Parent [Member] AOCI Attributable to Parent [Member] Retained Earnings [Member] Retained Earnings [Member] Consolidated Statement of Shareholders' Equity [Line Items] Statement [Line Items] Consolidated Statement of Shareholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Balance, Beginning Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Common Stock Issued Stock Issued During Period, Value, New Issues Unrealized Gain (Loss) on Debt Securities Defined Benefit Pension and Other Postretirement Plans Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Common Stock Dividends Balance, End of Period Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Goodwill and Intangible Assets [Text Block] Goodwill and Intangible Assets Disclosure [Text Block] Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Sale of U.S. Water Services [Abstract] Sale of U.S. Water Services [Abstract] Cash Purchase Price Cash Purchase Price for Sale of Affiliate Cash Purchase Price for Sale of Affiliate Proceeds from Sale of U.S. Water Services - Net of Transaction Costs and Cash Retained Approximate Proceeds from Divestiture of Business Approximate Proceeds from Divestiture of Business Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Unrecorded Unconditional Purchase Obligation [Table] Coal, Rail and Shipping Contracts [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Coal, Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Coal, Rail and Shipping Contracts [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Cash and Cash Equivalents, Policy [Policy Text Block] Cash and Cash Equivalents, Policy [Policy Text Block] Inventories – Net [Policy Text Block] Inventory, Policy [Policy Text Block] New Accounting Standards [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Leases [Policy Text Block] Lessee, Leases [Policy Text Block] Short-term Leases [Policy Text Block] Short-term Leases [Policy Text Block] CIP Financial Incentives [Policy Text Block] Revenue Recognition for Alternative Revenue Programs, 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 Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Environmental Accruals [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Income Tax [Policy Text Block] Income Tax, Policy [Policy Text Block] Uncertain Tax Positions [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Accounts Receivable (Less Allowance of $1.1 and $1.7) Accounts Receivable, after Allowance for Credit Loss, Current Inventories – Net Prepayments and Other Other Assets, Current Total Current Assets Assets, Current Property, Plant and Equipment – Net Property, Plant and Equipment, Net Regulatory Assets Equity Investments Goodwill and Intangible Assets – Net Intangible Assets, Net (Including Goodwill) Other Non-Current Assets Total Assets Liabilities and Shareholders' Equity [Abstract] 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 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 Liability, Defined Benefit Plan, Noncurrent Other Non-Current Liabilities Total Liabilities Liabilities Commitments, Guarantees and Contingencies (Note 7) Commitments and Contingencies Shareholders’ Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 51.7 and 51.5 Shares Issued and Outstanding Common Stock, Value, Issued Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained Earnings Retained Earnings (Accumulated Deficit) Total Shareholders’ Equity Stockholders' Equity Attributable to Parent Total Liabilities and Shareholders' Equity Liabilities and Equity