Proceeds from sale of business Proceeds from Divestiture of Businesses Finite-Lived Intangible Assets, Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Revision of Prior Period [Axis] Revision of Prior Period [Axis] Goodwill impairment Effective Income Tax Rate Reconciliation, Disposition of Business, Amount Accounts Payable Accounts Payable [Member] Debt Instrument, Redemption, Period Four Debt Instrument, Redemption, Period Four [Member] Commodity Contract Commodity Contract [Member] Measurement Period Adjustments, Other current liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Liabilities Purchases Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase for Purchase Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Estimated Future Benefit Payments Schedule of Expected Benefit Payments [Table Text Block] 2030 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Five Award Timing Predetermined Award Timing Predetermined [Flag] Entity File Number Entity File Number Capital in Excess of Par Value of Shares Additional Paid-in Capital [Member] Tax credits Effective Income Tax Rate Reconciliation, Tax Credit, Amount Fair value of option acquired in connection with Stelco Acquisition Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs, Business Combination Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs, Business Combination Tabular List, Table Tabular List [Table Text Block] 2026 Target Allocation Defined Benefit Plan, Plan Assets, Target Allocation, Percentage Supplementary Financial Statement Information Additional Financial Information Disclosure [Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Other Other [Member] Other [Member] Surety-backed letters of credit and surety bonds outstanding Guarantor Obligations, Maximum Exposure, Undiscounted Steelworkers Pension Trust Steelworkers Pension Trust [Member] Steelworkers Pension Trust Undistributed earnings of foreign subsidiaries Undistributed Earnings of Foreign Subsidiaries Share-based Payment Arrangement Share-Based Payment Arrangement [Member] Goodwill [Line Items] Goodwill [Line Items] Measurement Basis [Axis] Measurement Basis [Axis] Adjustment to Compensation: Adjustment to Compensation [Axis] Named Executive Officers, Footnote Named Executive Officers, Footnote [Text Block] Pension and OPEB liability, non-current Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Non-Current Liabilities, Pension and OPEB liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Non-Current Liabilities, Pension and OPEB liabilities Revenues Revenue from Contract with Customer [Text Block] Capital Stock [Table] Stock, Class of Stock [Table] Current Income Tax Expense (Benefit) Current Income Tax Expense (Benefit) Pension and OPEB payments and contributions Payment for Pension and Other Postretirement Benefits Federal & state provision to return Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount Adjustment to Compensation, Amount Adjustment to Compensation Amount Pension Benefits Pension Plan [Member] Unamortized discounts and issuance costs Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net Basis of Consolidation Consolidation, Policy [Policy Text Block] Ohio OHIO Options Share-Based Payment Arrangement, Option [Member] Cleveland-Cliffs Inc. Cleveland-Cliffs Inc. [Member] Cleveland-Cliffs Inc. [Member] Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Income taxes paid Income Taxes Paid Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] Finance Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration] Grantee Status [Domain] Grantee Status [Domain] Mortality Changes in Assumptions for Defined Benefit Plans, Mortality [Member] Changes in Assumptions for Defined Benefit Plans, Mortality [Member] Effective Income Tax Rate Reconciliation, Percent [Abstract] Effective Income Tax Rate Reconciliation, Percent [Abstract] Antidilutive Securities [Axis] Antidilutive Securities [Axis] Actual return on plan assets: Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement [Abstract] Ultimate health care cost trend rate Defined Benefit Plan, Ultimate Health Care Cost Trend Rate EBITDA Calculation EBITDA Calculation [Member] EBITDA Calculation Total Cliffs shareholders' equity Equity, Attributable to Parent Other comprehensive income (loss): Other Comprehensive Income (Loss), Net of Tax [Abstract] Insider Trading Policies and Procedures [Line Items] Summary of Estimated Useful Lives Summary of Carrying Value of Each Major Class of Depreciable Assets Property, Plant and Equipment [Table Text Block] Common shares, authorized (in shares) Common Stock, Shares Authorized Surety Bond Surety Bond [Member] Stock Issued During Period, Shares, New Issues Stock Issued During Period, Shares, New Issues State and Local Jurisdiction State and Local Jurisdiction [Member] Cover [Abstract] Cover [Abstract] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Maximum Maximum [Member] Exercisable, weighted average exercise price (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Weighted Average Exercise Price Net income attributable to noncontrolling interests Income from continuing operations attributable to noncontrolling interest Net Income (Loss) Attributable to Noncontrolling Interest Rate of compensation increase Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Surcharge Imposed Multiemployer Plan, Pension, Significant, Surcharge [Fixed List] Finite-Lived Intangible Assets, Net Finite-Lived Intangible Assets, Net Income Tax Contingency [Line Items] Income Tax Contingency [Line Items] Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Debt Instrument [Line Items] Debt Instrument [Line Items] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table] Summary of Restricted Stock Unit Activity Share-Based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity [Table Text Block] Adjustment to Non-PEO NEO Compensation Footnote Adjustment to Non-PEO NEO Compensation Footnote [Text Block] Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment Business Combination, Pro Forma Information, Nonrecurring Adjustment, Inventory, Fair Value Adjustment [Member] Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Intangibles Retirement Benefits [Abstract] Retirement Benefits [Abstract] Series B Preferred Stock Series B Preferred Stock [Member] Forgone Recovery due to Disqualification of Tax Benefits, Amount Forgone Recovery due to Disqualification of Tax Benefits, Amount Debt Instrument, Redemption, Period One Debt Instrument, Redemption, Period One [Member] Non-Rule 10b5-1 Arrangement Terminated Non-Rule 10b5-1 Arrangement Terminated [Flag] Income (loss) from continuing operations before income taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Entity Shell Company Entity Shell Company Summary of Amounts Relating to Geographic Location Schedule of Revenue from External Customers and Long-Lived Assets, by Geographical Areas [Table Text Block] Debt Instrument, Redemption, Period One, Upon Equity Issuance Debt Instrument, Redemption, Period One, Upon Equity Issuance [Member] Debt Instrument, Redemption, Period One, Upon Equity Issuance [Member] Summary of Carrying Value and Fair Value of Other Financial Instruments Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block] Schedule of Long-term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Statement of Stockholders' Equity [Abstract] Statement of Stockholders' Equity [Abstract] Total Segment Assets Operating Segments [Member] Summary of Performance Award Activity Share-Based Payment Arrangement, Performance Shares, Activity [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures Components of Cash Payments in Excess of Refunds Received Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Comprehensive income attributable to noncontrolling interests Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Line of Credit Facility [Table] Line of Credit Facility [Table] Asset Retirement Obligations Asset Retirement Obligation [Policy Text Block] Prior service cost (credit) Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax Net income (loss) from continuing operations attributable to Cliffs shareholders Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent Restructuring Reserve Restructuring Reserve Letter of Credit Letter of Credit [Member] United States Income (Loss) from Continuing Operations before Income Taxes, Domestic Operating Operating Lease, Liability, Current Finance leases, Interest on lease liabilities Finance Lease, Interest Expense Document Fiscal Period Focus Document Fiscal Period Focus Compensation rate increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase 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] Actuarial loss (gain) on benefit obligation Changes in Assumptions for Defined Benefit Plans, Actuarial (Gain) Loss [Member] Changes in Assumptions for Defined Benefit Plans, Actuarial (Gain) Loss [Member] Award Timing Method Award Timing Method [Text Block] Continuing operations (in dollars per share) Income (Loss) from Continuing Operations, Per Basic Share EBITDA Calculation [Domain] EBITDA Calculation [Domain] EBITDA Calculation [Domain] Expected reclassification of net losses related to hedge contracts Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred Award Type [Axis] Award Type [Axis] Adjusted EBITDA Calculation Adjusted EBITDA Calculation [Member] Adjusted EBITDA Calculation Summary of Unaudited Pro Forma Financial Information Business Combination, Pro Forma Information [Table Text Block] Trading Symbol Trading Symbol Business Combinations Business Combination [Policy Text Block] Valuation Allowance, percent Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent Stock and other incentive plans (in shares) Shares Issued, Shares, Share-Based Payment Arrangement, after Forfeiture Environmental Remediation Obligations [Abstract] Environmental Remediation Obligations [Abstract] Entity Address, City or Town Entity Address, City or Town Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Five Lessee, Operating Lease, Liability, to be Paid, Rolling Year Five Finance Lease, Weighted Average Discount Rate, Percent Finance Lease, Weighted Average Discount Rate, Percent FPT Florida FPT Florida [Member] FPT Florida Effect of exchange rate changes on cash Effect of Exchange Rate on Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Asset Retirement Obligation [Roll Forward] Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] Net deferred tax assets Deferred Tax Assets, Net of Valuation Allowance Non-PEO NEO Average Compensation Actually Paid Amount Non-PEO NEO Average Compensation Actually Paid Amount Restricted Stock Units (RSUs) Restricted Stock Units (RSUs) [Member] Statement of Income Location, Balance [Domain] Statement of Income Location, Balance [Domain] Severance Costs Severance Costs Mineral rights: Mineral Properties, Net [Abstract] Trade Accounts Receivable and Allowance for Credit Loss Credit Loss, Financial Instrument [Policy Text Block] Compensation Actually Paid vs. Other Measure Compensation Actually Paid vs. Other Measure [Text Block] Acquired through business combinations Defined Benefit Plan, Plan Assets, Business Combination Total purchase consideration Business Combination, Consideration Transferred Entity Emerging Growth Company Entity Emerging Growth Company 2028 Long-Term Debt, Maturity, Year Three Increase from Stelco Acquisition Asset Retirement Obligation, Liabilities Assumed, Acquisition Asset Retirement Obligation, Liabilities Assumed, Acquisition Net amount recognized Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax Finance Lease, Weighted Average Remaining Lease Term Finance Lease, Weighted Average Remaining Lease Term Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Outstanding at beginning of year (in shares) Outstanding at end of year (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year Change in Fair Value as of Vesting Date of Prior Year Equity Awards Vested in Covered Year [Member] Loss on derivative Derivative, Gain (Loss) on Derivative, Net 2026 Defined Benefit Plan, Expected Future Benefit Payment, Year One Payments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases Finance Lease, Liability, Undiscounted Excess Amount Finance Lease, Liability, Undiscounted Excess Amount Common shares, outstanding (in shares) Balance, beginning of period (in shares) Balance, end of period (in shares) Common Stock, Shares, Outstanding Gain (Loss) on Disposition of Other Assets Gain (Loss) on Disposition of Other Assets Summary of Revenue by market and product line Disaggregation of Revenue [Table Text Block] VEBA Veba Trust [Member] Relating to the Voluntary Employees' Beneficiary Association (VEBA) Trust Plans. Proceeds from issuance of debt Proceeds from Issuance of Senior Long-Term Debt Pension and OPEB assets Assets for Plan Benefits, Defined Benefit Plan Change in valuation allowance Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount Expenses and Losses, Reporting Currency Denominated, Value Expenses and Losses, Reporting Currency Denominated, Value Expenses and Losses, Reporting Currency Denominated, Value Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year Dividends or Other Earnings Paid on Equity Awards not Otherwise Reflected in Total Compensation for Covered Year [Member] Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivative Instruments and Hedging Activities Disclosure [Abstract] Change in benefit obligations: Defined Benefit Plan, Change in Benefit Obligation [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Annual Effective Interest Rate Debt Instrument, Interest Rate, Effective Percentage Entity Address, Postal Zip Code Entity Address, Postal Zip Code Restatement Determination Date Restatement Determination Date Income tax benefit (expense) Income tax benefit (expense) Income Tax Expense (Benefit) Income Tax Expense (Benefit) Income Tax Expense (Benefit) Direct Payments Direct Payments [Member] Direct payments made into the other postretirement benefit plans. Amortization: Disclosure Pensions And Other Postretirement Benefits Components Of Net Periodic Benefit Cost [Abstract] Disclosure - Pensions And Other Postretirement Benefits (Components Of Net Periodic Benefit Cost) [Abstract] Discontinued operations (in dollars per share) Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share Other comprehensive income (loss), Derivative financial instruments Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification, Tax Depletion Mineral Properties, Accumulated Depletion Preferred stock issued (in shares) Preferred Stock, Shares Issued Statement of Financial Position Location, Activity, Capitalization [Axis] Statement of Financial Position Location, Activity, Capitalization [Axis] Pension Adjustments Service Cost Pension Adjustments Service Cost [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Changes in derivative instruments, net of tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax Property, plant and equipment, gross Property, Plant and Equipment, Gross Other Significant Noncash Transaction, Value of Consideration Given, Common Shares Other Significant Noncash Transaction, Value of Consideration Given, Common Shares Other Significant Noncash Transaction, Value of Consideration Given, Common Shares Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Non-Current Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Non-Current Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Non-Current Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Expenses Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Expenses Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Expenses Number of employees represented by a union Entity Number of Employees, Amount Of Workforce Represented By A Union, Amount Entity Number of Employees, Amount Of Workforce Represented By A Union, Amount Compensation Actually Paid vs. Company Selected Measure Compensation Actually Paid vs. Company Selected Measure [Text Block] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Stelco Acquisition Goodwill, Acquired During Period Acquisition of Stelco, net of cash acquired Payments to Acquire Businesses, Net of Cash Acquired U.S. Federal Income Tax Paid, Federal, after Refund Received All Trading Arrangements All Trading Arrangements [Member] Equity Awards Adjustments, Footnote Equity Awards Adjustments, Footnote [Text Block] Total Shareholder Return Vs Peer Group Total Shareholder Return Vs Peer Group [Text Block] Finance Lease, Liability, Payments, Due in Rolling Year Three Finance Lease, Liability, to be Paid, Rolling Year Three Redemption price, percentage Debt Instrument, Redemption Price, Percentage Accrued expenses Accrued Liabilities, Current Domestic operating loss carryforward Deferred Tax Assets, Operating Loss Carryforwards, Domestic Income not subject to tax, Percent Effective Income Tax Rate Reconciliation, Tax Exempt Income, Percent Other items, net Effective Income Tax Rate Reconciliation, Other Adjustments, Amount Fair Value of Pension Assets by Asset Category Defined Benefit Plan, Plan Assets, Category [Table Text Block] Outstanding at beginning of year (in shares) Outstanding at end of year (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Nonvested, Number Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] Foreign operating loss carryforward Deferred Tax Assets, Operating Loss Carryforwards, Foreign Net cash provided (used) by operating activities Cash Provided by (Used in) Operating Activity, Including Discontinued Operation Amounts recognized in Statements of Financial Position: Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract] Sale of business Goodwill, Period Increase (Decrease) Effective Income Tax Rate Reconciliation [Table] Effective Income Tax Rate Reconciliation [Table] Useful life Property, Plant and Equipment, Useful Life Multiemployer Plan [Table] Multiemployer Plan [Table] Revenues from External Customers and Long-Lived Assets [Line Items] Revenues from External Customers and Long-Lived Assets [Line Items] Other current assets Business Combination, Recognized Asset Acquired, Other Asset, Current Equity Valuation Assumption Difference, Footnote Equity Valuation Assumption Difference, Footnote [Text Block] Demographic updates Change in Assumptions for Defined Benefit Plans, Demographic [Member] Change in Assumptions for Defined Benefit Plans, Demographic [Member] PEO Total Compensation Amount PEO Total Compensation Amount Long-term Debt, Type [Axis] Long-Term Debt, Type [Axis] Depreciation Depreciation Debt Instrument, Redemption, Period Three Debt Instrument, Redemption, Period Three [Member] Derivative liabilities Derivative Liability, Subject to Master Netting Arrangement, before Offset Defined Benefit Plan, Roll Forwards [Abstract] Defined Benefit Plan, Roll Forwards [Abstract] Amortization of intangible liability Amortization Of Intangible Liability Amortization Of Intangible Liability Other countries Other Countries [Member] Other Countries [Member] Eliminations Eliminations [Member] Eliminations Equity Components [Axis] Equity Components [Axis] Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] State and local Deferred Tax Assets, State Taxes Non-Rule 10b5-1 Arrangement Adopted Non-Rule 10b5-1 Arrangement Adopted [Flag] Number of reportable segments Number of Reportable Segments Deferred tax assets: Components of Deferred Tax Assets [Abstract] Variable Interest Entities Variable Interest Entity Disclosure [Text Block] Entity Address, State or Province Entity Address, State or Province Finance leases within cash flows from financing activities Finance Lease, Principal Payments Finite-Lived Intangible Liability, Accumulated Amortization Finite-Lived Intangible Liability, Accumulated Amortization Finite-Lived Intangible Liability, Accumulated Amortization Antidilutive Security, Excluded EPS Calculation [Table] Antidilutive Security, Excluded EPS Calculation [Table] 2029 Defined Benefit Plan, Expected Future Benefit Payment, Year Four Supplier Finance Program [Line Items] Supplier Finance Program [Line Items] Total current liabilities Liabilities, Current Product inventories Deferred Tax Assets, Inventory Individual: Individual [Axis] Defined contribution pension plans Defined Contribution Plan, Cost Disclosure Text Block [Abstract] Disclosure Text Block [Abstract] Income Tax Jurisdiction [Domain] Income Tax Jurisdiction [Domain] Schedule of Asset Retirement Obligations Schedule of Asset Retirement Obligations [Table Text Block] Right-of-use assets - finance leases Right-of-use assets obtained in exchange for new finance lease liabilities Right-of-Use Asset Obtained in Exchange for Finance Lease Liability Document Annual Report Document Annual Report Measurement Period Adjustments, Property, plant and equipment Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Property, Plant, and Equipment Effect of settlement Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement Forgone Recovery, Explanation of Impracticability Forgone Recovery, Explanation of Impracticability [Text Block] Repayments of senior notes Repayments of Long-Term Debt Entity Interactive Data Current Entity Interactive Data Current 6.750% 2030 Senior Notes 6.750% 2030 Senior Notes [Member] 6.750% 2030 Senior Notes AK Steel AK Steel [Member] AK Steel Interest crediting rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Weighted-Average Interest Crediting Rate Summary of Future Minimum Lease Payments under Noncancelable Finance and Operating Leases Lessee, Operating and Finance Lease, Liability, Maturity [Table Text Block] Lessee, Operating and Finance Lease, Liability, Maturity Funded status Defined Benefit Plan, Funded (Unfunded) Status of Plan Revenue Recognition Revenue from Contract with Customer [Policy Text Block] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Operating loss and other carryforwards Deferred Tax Assets, Operating Loss Carryforwards Relating to assets still held at the reporting date Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held Property, Plant and Equipment, Type [Axis] Long-Lived Tangible Asset [Axis] Settlements and special termination benefits Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement and Curtailment Relating to assets sold during the period Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold Pension and OPEB credits Pension and Other Postretirement Benefits Expense (Reversal of Expense), Noncash Total product inventories Inventory, Finished Goods and Work in Process, Net of Reserves 2027 Defined Benefit Plan, Expected Future Benefit Payment, Year Two Foreign currency translation Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Foreign Currency Translation Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Foreign Currency Translation Other non-current assets Other Assets, Noncurrent Customer [Domain] Customer [Domain] Entity Central Index Key Entity Central Index Key PEO Name PEO Name Asset Retirement Obligation Asset Retirement Obligation at Beginning of Year Asset Retirement Obligation at End of Year Asset Retirement Obligation Summary of Debt Instrument Maturities Schedule of Maturities of Long-Term Debt [Table Text Block] Revenues Revenues Revenues Changes in fair value of derivatives, net Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year Prior Year End Fair Value of Equity Awards Granted in Any Prior Year that Fail to Meet Applicable Vesting Conditions During Covered Year [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Outstanding Aggregate Erroneous Compensation Amount Outstanding Aggregate Erroneous Compensation Amount Beginning balance as of January 1 Ending balance as of December 31 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Defined Benefit Plan [Table] Defined Benefit Plan [Table] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Supplier Finance Program, Obligation, Addition Supplier Finance Program, Obligation, Addition Non-taxable or nondeductible items Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount [Abstract] Measurement Period Adjustments, Cash and cash equivalents Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Cash and Cash Equivalents Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Cash and Cash Equivalents Schedule Of Derivative Instruments In Statement Of Financial Position, Fair Value Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Non-current liabilities Non-Current Liabilities, Lessee [Abstract] Non-Current Liabilities, Lessee Exercise Price Award Exercise Price Entity Filer Category Entity Filer Category Class of Stock [Domain] Class of Stock [Domain] Employer contributions Defined Benefit Plan, Contributions by Employer, Net of Reimbursements Defined Benefit Plan, Contributions by Employer, Net of Reimbursements Valuation Allowance by Deferred Tax Asset [Axis] Valuation Allowance by Deferred Tax Asset [Axis] Benefits paid Defined Benefit Plan, Benefit Obligation, Benefits Paid Tax at U.S. statutory rate, percent Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Obligation discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Obligation Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Obligation Discount Rate Defined Benefit Plan, Expected Amounts [Abstract] Defined Benefit Plan, Expected Amounts [Abstract] Long-term debt Long-Term Debt, Excluding Current Maturities Expected asset amortization, year three Finite-Lived Intangible Asset, Expected Amortization, Year Three Statement of Financial Position Location, Balance [Axis] Statement of Financial Position Location, Balance [Axis] Finite-Lived Intangible Assets, Gross Finite-Lived Intangible Assets, Gross Basis Of Presentation And Significant Accounting Policies [Table] Basis Of Presentation And Significant Accounting Policies [Table] Basis Of Presentation And Significant Accounting Policies [Table] Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year Vesting Date Fair Value of Equity Awards Granted and Vested in Covered Year [Member] Entity Address, Address Line One Entity Address, Address Line One Measurement Period Adjustments, Accounts receivable, net Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Receivable Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Receivable AOCI (Loss) AOCI Attributable to Parent [Member] Depletion Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depletion, Amount Business Combination, Asset Acquisition, Transaction between Entities under Common Control, and Joint Venture Formation [Abstract] Business Combination, Pro Forma Information, Nonrecurring Adjustment [Domain] Business Combination, Pro Forma Information, Nonrecurring Adjustment [Domain] Derivative Instruments and Hedging Derivative Instruments and Hedging Activities Disclosure [Text Block] Prior service costs (credits) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Industry Sector [Axis] Industry Sector [Axis] Restructuring, Settlement and Impairment Provisions Restructuring, Settlement and Impairment Provisions Pension and OPEB Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest [Member] Other Liabilities Other Liabilities [Member] Fair Value as of Grant Date Award Grant Date Fair Value Other Businesses Other businesses [Member] Other businesses U.S. equities Defined Benefit Plan, Equity Securities, US [Member] Unrecognized tax benefits Effective Income Tax Rate Reconciliation, Tax Contingency, Amount Schedule Of Long-Term Debt Schedule of Long-Term Debt Instruments [Table Text Block] Derivative Instruments and Hedging Activities Disclosures [Table] Derivative Instruments and Hedging Activities Disclosures [Table] Interest limitation Deferred Tax Asset, Interest Limitation Deferred Tax Asset, Interest Limitation Employee stock plans (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-Based Payment Arrangements Granted (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period Pension Plan EIN Multiemployer Plan, Pension, Significant, Employer Identification Number Title and Position [Domain] Title and Position [Domain] Schedule of Noncash or Part Noncash Acquisitions Schedule of Noncash or Part Noncash Acquisitions [Table Text Block] Total operating costs Costs and Expenses Forfeited/canceled (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeited in Period Compensation cost related to outstanding awards not yet recognized Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount Fair value of long-term debt Long-Term Debt, Fair Value Changed Peer Group, Footnote Changed Peer Group, Footnote [Text Block] Adjustment To PEO Compensation, Footnote Adjustment To PEO Compensation, Footnote [Text Block] Weighted-average remaining contractual life of options outstanding Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Finance Lease, Liability, Statement of Financial Position [Extensible List] Finance Lease, Liability, Statement of Financial Position [Extensible Enumeration] Title Trading Arrangement, Individual Title Peer Group Total Shareholder Return Amount Peer Group Total Shareholder Return Amount 2028 Defined Benefit Plan, Expected Future Benefit Payment, Year Three Pension Protection Act Zone Status Multiemployer Plan, Pension, Significant, Certified Zone Status [Fixed List] Total cash consideration Payments to Acquire Businesses, Gross Non-PEO NEO Non-PEO NEO [Member] Less: current portion Asset Retirement Obligation, Current Lease assets Deferred Tax Liabilities, Leasing Arrangements Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Four Lessee, Operating Lease, Liability, to be Paid, Rolling Year Four Business Combination, Pro Forma Information [Table] Business Combination, Pro Forma Information [Table] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Award Type [Domain] Award Type [Domain] Noncontrolling interests Noncontrolling interests Equity, Attributable to Noncontrolling Interest Equity Awards Adjustments Equity Awards Adjustments [Member] Other Other Noncash Income (Expense) 4.625% 2029 Senior Notes 4.625% 2029 Senior Notes [Member] 4.625% 2029 Senior Notes Pension Benefits Adjustments, Footnote Pension Benefits Adjustments, Footnote [Text Block] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Canada CANADA Compensation Amount Outstanding Recovery Compensation Amount Thereafter Unrecorded Unconditional Purchase Obligation, to be Paid, after Year Five Other foreign jurisdiction, percent Effective Income Tax Rate Reconciliation, Other Reconciling Items, Percent Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Recovery of Erroneously Awarded Compensation Disclosure [Line Items] Fair Value Estimate of Fair Value Measurement [Member] Retirement Plan Type [Domain] Retirement Plan Type [Domain] MNPI Disclosure Timed for Compensation Value MNPI Disclosure Timed for Compensation Value [Flag] AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward] Statutory rate difference, percent Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent Expected liability amortization, year four Finite-Lived Intangible Liabilities, Amortization Credit, Year Four Finite-Lived Intangible Liabilities, Amortization Credit, Year Four Amortization of prior service credits (costs) Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Amortization Of Prior Service Credit (Cost), Arising During Period, Before Tax Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Amortization Of Prior Service Credit (Cost), Arising During Period, Before Tax Cost Mineral Properties, Gross Components of Accumulated Other Comprehensive Income (Loss) Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Table Text Block] Basis of Presentation and Significant Accounting Policies [Line Items] Basis of Presentation and Significant Accounting Policies [Line Items] Basis of Presentation and Significant Accounting Policies Cash-Settled Sales Swaps Cash-Settled Sales Swaps [Member] Cash-Settled Sales Swaps Documents Incorporated by Reference [Text Block] Documents Incorporated by Reference [Text Block] Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Accumulated other comprehensive income Accumulated Other Comprehensive Income (Loss), Net of Tax 2026 Long-Term Debt, Maturity, Year One Total deferred tax liabilities Deferred Tax Liabilities, Gross 2030 Long-Term Debt, Maturity, Year Five Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Pension and OPEB Liability Non-current Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Pension and OPEB Liability Non-current Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Pension and OPEB Liability Non-current Performance Shares Performance Shares [Member] Aggregate Pension Adjustments Service Cost Aggregate Pension Adjustments Service Cost [Member] Postemployment Benefits [Abstract] Postemployment Benefits [Abstract] Amortization of net actuarial gain (loss) Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Amortization Of Net Actuarial Loss, Arising During Period Before Tax Amount arising during period, before tax, of accumulated other comprehensive income (loss) for amortization of net actuarial loss Statement of Financial Position Location, Activity, Capitalization [Domain] Statement of Financial Position Location, Activity, Capitalization [Domain] Inventories Inventories Inventory, Net Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Minimum Minimum [Member] Year that the ultimate rate is reached Defined Benefit Plan, Year Health Care Cost Trend Rate Reaches Ultimate Trend Rate Measurement Period Adjustments, Goodwill Goodwill, Measurement Period Adjustment Company Selected Measure Name Company Selected Measure Name Accrual for Environmental Loss Contingencies, Total Accrual for Environmental Loss Contingencies Fair Value Measurement [Domain] Fair Value Measurement [Domain] Business Combination, Pro Forma Information, Pro Forma Income (Loss), after Tax Business Combination, Pro Forma Information, Pro Forma Income (Loss), after Tax Asset Retirement Obligations Asset Retirement Obligation Disclosure [Text Block] Debt and Credit Facilities Debt Disclosure [Text Block] Aggregate Available Trading Arrangement, Securities Aggregate Available Amount Accounts payable Accounts payable Accounts Payable, Current 7.000% 2027 Senior Notes 7.000% 2027 Senior Notes [Member] 7.00% 2027 Senior Notes [Member] Asset Impairments Asset Impairments [Member] Asset Impairments Operating leases within cash flows from operating activities Operating Lease, Payments Stock Appreciation Rights (SARs) Stock Appreciation Rights (SARs) [Member] Current Foreign Tax Expense (Benefit) Current Foreign Tax Expense (Benefit) Goodwill [Roll Forward] Goodwill [Roll Forward] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Non-GAAP Measure Description Non-GAAP Measure Description [Text Block] Reconciliation of Assets from Segment to Consolidated [Table] Reconciliation of Assets from Segment to Consolidated [Table] Natural Gas Natural Gas and Natural Gas Liquids (NGL) [Member] Entity Small Business Entity Small Business Income Tax Disclosure [Abstract] Income Tax Disclosure [Abstract] Interest and penalties related to unrecognized tax benefits Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense Preferred stock outstanding (in shares) Preferred Stock, Shares Outstanding Current State and Local Tax Expense (Benefit) Current State and Local Tax Expense (Benefit) Document Transition Report Document Transition Report Interest cost discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration] Other non-current assets Business Combination, Recognized Asset Acquired, Other Asset, Noncurrent Supplier Finance Program, Obligation [Roll Forward] Supplier Finance Program, Obligation [Roll Forward] Finance Lease, Liability, Payment, Due Finance Lease, Liability, to be Paid Accrued employment costs Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Employment Costs Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Employment Costs Underlying Securities Award Underlying Securities Amount Other comprehensive income (loss) before reclassifications Other Comprehensive Income (Loss), before Reclassifications, before Tax Equity Component [Domain] Equity Component [Domain] Finance Lease, Liability, Payments, Due in Rolling Year Two Finance Lease, Liability, to be Paid, Rolling Year Two Document Period End Date Document Period End Date Discount rates Change in Assumptions for Defined Benefit Plans, Discount Rates [Member] Change in Assumptions for Defined Benefit Plans, Discount Rates [Member] Income Taxes Income Tax Disclosure [Text Block] Environmental Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Asset Retirement Obligations Awards Close in Time to MNPI Disclosures, Table Awards Close in Time to MNPI Disclosures [Table Text Block] Short-term leases Short-Term Lease, Cost EBITDA of noncontrolling interests EBITDA Of Noncontrolling Interests EBITDA Of Noncontrolling Interests Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible Enumeration] TOTAL EQUITY Balance, beginning of period Balance, end of period Equity, Including Portion Attributable to Noncontrolling Interest Net mineral rights Mineral Properties, Net Document Type Document Type Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Earnings Per Share Earnings Per Share [Text Block] Name Outstanding Recovery, Individual Name Capital in excess of par value of shares Additional Paid in Capital Product and Service [Axis] Product and Service [Axis] Schedule of Defined Benefit Plans Disclosures Schedule of Defined Benefit Plans Disclosures [Table Text Block] Effect of Change in Accounting Estimate, Type [Domain] Effect of Change in Accounting Estimate, Type [Domain] Total current assets Assets, Current Actual returns on assets under (over) expected Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Actual Returns On Assets (Over) Under Estimate, Arising During Period, Before Tax Amount arising during period, before tax, of accumulated other comprehensive income (loss) for actual returns on assets (over) under estimate Finite-lived intangible assets acquired Finite-Lived Intangible Assets Acquired Statistical Measurement [Axis] Statistical Measurement [Axis] Manufacturing supplies and critical spares Inventory, Supplies, Net of Reserves Authorized amount of stock repurchase Share Repurchase Program, Authorized, Amount Forfeitured/canceled, weighted average exercise price (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price Employee-based compensation Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Compensation Aggregate Erroneous Compensation Amount Aggregate Erroneous Compensation Amount Intrinsic value of options exercised Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period, Intrinsic Value Peer Group Issuers, Footnote Peer Group Issuers, Footnote [Text Block] Net income (loss) attributable to Cliffs shareholders Net loss attributable to acquisition Net Income (Loss) Available to Common Stockholders, Basic Current liabilities: Liabilities, Current [Abstract] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] FIP/RP Status Pending/Implemented Multiemployer Plan, Pension, Significant, Funding Improvement or Rehabilitation Plan, Implementation Status [Fixed List] Share price (in dollars per share) Business Combination, Price Per Share Quoted Prices in Active Markets for Identical Assets (Level 1) Fair Value, Inputs, Level 1 [Member] Credit facility, amount outstanding Borrowings and Letter of credit obligations Long-Term Line of Credit Gain on sale of business Gain (Loss) on Disposition of Assets Accounts payable Business Combination, Recognized Liability Assumed, Accounts Payable, Current Earnings (loss) per common share attributable to Cliffs common shareholders - diluted (in dollars per share) Decrease in diluted earnings per common share (in dollars per share) Earnings Per Share, Diluted Minimum fixed charge coverage ratio Debt Instrument, Covenant, Fixed Charge Coverage Ratio, Minimum Debt Instrument, Covenant, Fixed Charge Coverage Ratio, Minimum Accounts receivable, net Accounts Receivable, after Allowance for Credit Loss, Current Accelerated depreciation Restructuring and Related Cost, Accelerated Depreciation Defined Benefit Plan, Assumptions Defined Benefit Plan, Assumptions [Table Text Block] Shares sold (in shares) Sale of Stock, Number of Shares Issued in Transaction Common shares in treasury (in shares) Treasury Stock, Common, Shares State Income Tax Paid, State and Local, after Refund Received Consolidation Items [Domain] Consolidation Items [Domain] Auditor Location Auditor Location Pension and Other Post Retirement Benefits Defined Benefit Plan [Text Block] Included in income tax expense (benefit) Included in income tax expense (benefit) [Member] Included in income tax expense (benefit) [Member] Adj EBITDA Calculation [Axis] Adj EBITDA Calculation [Axis] Adj EBITDA Calculation Actuarial loss (gain) Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) Domestic Tax Jurisdiction Domestic Tax Jurisdiction [Member] Participant contributions Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant Plan amendments Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment Total expense included in earnings Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings Lessee, Operating Lease, Liability, Undiscounted Excess Amount Lessee, Operating Lease, Liability, Undiscounted Excess Amount Total refunds in excess of payments made Income Taxes Paid, Net Non U.S. and other bonds Non-US and Other Bonds [Member] Non-US and Other Bonds 6.875% 2029 Senior Notes 6.875% 2029 Senior Notes [Member] 6.875% 2029 Senior Notes Expected asset amortization, year two Finite-Lived Intangible Asset, Expected Amortization, Year Two Total debt Long-term Debt Long-Term Debt, Gross Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Accrued employment costs Employee-related Liabilities, Current Property, Plant and Equipment Property, Plant and Equipment Disclosure [Text Block] Fair value of commitment assumed in connection with Stelco Acquisition Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Business Combination Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Business Combination Lessee, Operating Lease, Liability, to be Paid Lessee, Operating Lease, Liability, to be Paid Pension Plan Number Multiemployer Plan, Pension, Significant, Plan Number Redemption notice period Debt Instrument, Redemption, Notice Period Debt Instrument, Redemption, Notice Period Available borrowing base on ABL Facility Line of Credit Facility, Current Borrowing Capacity Net distributions to noncontrolling interests Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Trade names and trademarks Trade Names [Member] Expiration Date Trading Arrangement Expiration Date Noncontrolling interest, percent Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Percent Expected asset amortization, year five Finite-Lived Intangible Asset, Expected Amortization, Year Five Equity: Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Other liabilities Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Other Property, plant and equipment, net Property, plant and equipment, net Property, Plant and Equipment, Net Restructuring and other charges Restructuring Charges Restructuring Charges 2026 Unrecorded Unconditional Purchase Obligation, to be Paid, Year One Compensation Actually Paid vs. Net Income Compensation Actually Paid vs. Net Income [Text Block] 6.250% 2040 Senior Notes 6.250% 2040 Senior Notes [Member] 6.25% 2040 Senior Notes [Member] Accounts receivable, net Increase (Decrease) in Accounts Receivable Inventories Inventory, Policy [Policy Text Block] Restructuring Cost [Table] Restructuring Cost [Table] Series A Preferred Stock Series A Preferred Stock [Member] Commodity Option Commodity Option [Member] Entity Current Reporting Status Entity Current Reporting Status Awards Close in Time to MNPI Disclosures Awards Close in Time to MNPI Disclosures [Table] Consolidated Entities [Domain] Consolidated Entities [Domain] Tax at U.S. statutory rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Foreign Currency Translation Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest [Member] Total consideration shares Total consideration shares Total consideration shares Statement of Financial Position [Abstract] Statement of Financial Position [Abstract] Executive Category: Executive Category [Axis] Hedge funds Hedge Funds [Member] Long-term asset retirement obligations Asset Retirement Obligations, Noncurrent State taxes, net, percent Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent Miscellaneous – net Other Operating Income (Expense), Net Finance Finance Lease, Liability, Current Service cost discount rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] Accumulated benefit obligation Defined Benefit Plan, Accumulated Benefit Obligation Current Fiscal Year End Date Current Fiscal Year End Date Health care cost trend rate assumed for next year Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year Developed technology Technology-Based Intangible Assets [Member] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Other current liabilities Business Combination, Recognized Liability Assumed, Other Liability, Current Foreign Currency Exchange Rate, Translation Foreign Currency Exchange Rate, Translation Equity Awards Adjustments, Excluding Value Reported in Compensation Table Equity Awards Adjustments, Excluding Value Reported in the Compensation Table [Member] Finance Lease, Liability, Payments, Due in Rolling Year Five Finance Lease, Liability, to be Paid, Rolling Year Five Excess of current cost over LIFO cost of iron ore inventories Excess of Replacement or Current Costs over Stated LIFO Value Schedule of Environmental Obligations Schedule of Environmental Loss Contingencies by Site [Table Text Block] Stock Issued During Period, Value, Acquisitions Stock Issued During Period, Value, Acquisitions Unrecognized Tax Benefits, Beginning Balance Unrecognized Tax Benefits, Ending Balance Unrecognized Tax Benefits Net identifiable assets acquired Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less) Anti-dilutive shares excluded from diluted earnings per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Weighted average number of shares: Weighted Average Number of Shares Outstanding, Diluted [Abstract] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] 2027 Long-Term Debt, Maturity, Year Two Other Commitments [Table] Other Commitments [Table] Net cash provided (used) by financing activities Cash Provided by (Used in) Financing Activity, Including Discontinued Operation Measure: Measure [Axis] Measurement Period Adjustments, Accrued employment costs Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Employment Cost Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Employment Cost 2027 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Two Basis of Presentation and Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block] Amount of employees represented by a union, percent Entity Number of Employees, Amount Of Workforce Represented By A Union, Percent Entity Number of Employees, Amount Of Workforce Represented By A Union, Percent Settlements Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Current Year Prior Service Cost, Arising During Period, Before Tax Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Current Year Prior Service Cost, Arising During Period, Before Tax Property, Plant and Equipment, Type [Domain] Long-Lived Tangible Asset [Domain] Remaining authorized purchase amount Share Repurchase Program, Remaining Authorized, Amount Cost of goods sold Cost of goods sold Cost of Product and Service Sold Notional Amount Derivative, Nonmonetary Notional Amount 7.500% 2031 Senior Notes 7.500% 2031 Senior Notes [Member] 7.500% 2031 Senior Notes Pay vs Performance Disclosure, Table Pay vs Performance [Table Text Block] Supplier Finance Program, Obligation, Beginning Balance Supplier Finance Program, Obligation, Ending Balance Supplier Finance Program Supplier Finance Program, Obligation Forgone Recovery due to Violation of Home Country Law, Amount Forgone Recovery due to Violation of Home Country Law, Amount Entity Tax Identification Number Entity Tax Identification Number Grantee Status [Axis] Grantee Status [Axis] Tax Credits, percent Effective Income Tax Rate Reconciliation, Tax Credit, Percent Schedule of Variable Interest Entities Schedule of Variable Interest Entities [Table Text Block] SunCoke Middletown SunCoke Middletown [Member] SunCoke Middletown [Member] Outstanding at beginning of year, weighted average exercise price (in dollars per share) Outstanding at end of year, weighted average exercise price (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Weighted Average Exercise Price Derivative Instruments Accumulated Gain (Loss), Cash Flow Hedge, Including Noncontrolling Interest [Member] Option to acquire, amount of ownership interest Derivative Instrument, Percentage Of Interest In Commodity Option Derivative Instrument, Percentage Of Interest In Commodity Option Common shares, issued (in shares) Common Stock, Shares, Issued Operating Lease, Weighted Average Remaining Lease Term Operating Lease, Weighted Average Remaining Lease Term 2029 Long-Term Debt, Maturity, Year Four Steel producers Steel producers [Member] Steel producers Goodwill and Intangible Assets and Liabilities Goodwill and Intangible Assets Disclosure [Text Block] Asset retirement obligations related to active operations Asset Retirement Obligations, Related to Active Operations Asset Retirement Obligations, Related to Active Operations Entity Public Float Entity Public Float Supplier Finance Program, Obligation, Settlement Supplier Finance Program, Obligation, Settlement Schedule of Goodwill by Segment Schedule of Goodwill [Table Text Block] Trading Arrangement: Trading Arrangement [Axis] Total Shareholder Return Amount Total Shareholder Return Amount Total expense included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings Net actuarial gain Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Gain (Loss), before Tax Expected liability amortization, year five Finite-Lived Intangible Liabilities, Amortization Credit, Year Five Finite-Lived Intangible Liabilities, Amortization Credit, Year Five Restructuring Costs and Asset Impairment Charges Restructuring Costs and Asset Impairment Charges Security Exchange Name Security Exchange Name Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Identifiable Assets Acquired Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Identifiable Assets Acquired Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Identifiable Assets Acquired Foreign Income Tax Paid, Foreign, after Refund Received Credit facility, maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Stelco Holdings Inc Stelco Holdings Inc [Member] Stelco Holdings Inc Defined Benefit Plan, Plan Assets, Category [Domain] Defined Benefit Plan, Plan Assets, Category [Domain] Common stock repurchases (in shares) Common stock repurchases (in shares) Stock Repurchased During Period, Shares Other current liabilities Other Liabilities, Current Participant contributions Defined Benefit Plan, Plan Assets, Contributions by Plan Participant Pension and OPEB liabilities Non-current liabilities Liability, Defined Benefit Plan, Noncurrent Environmental Loss Contingency, Noncurrent, Statement of Financial Position [Extensible Enumeration] Environmental Loss Contingency, Noncurrent, Statement of Financial Position [Extensible Enumeration] Depreciation, Depletion And Amortization Attributable To Noncontrolling Interests Depreciation, Depletion And Amortization Attributable To Noncontrolling Interests Depreciation, Depletion And Amortization Attributable To Noncontrolling Interests Statement [Line Items] Statement [Line Items] Redemption of debt Extinguishment of Debt, Amount Schedule of Restructuring Reserve by Type of Cost Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Rule 10b5-1 Arrangement Adopted Rule 10b5-1 Arrangement Adopted [Flag] 7.000% 2032 Senior Notes 7.000% 2032 Senior Notes [Member] 7.000% 2032 Senior Notes Cash and cash equivalents Cash and Cash Equivalent Common Stock Common Stock [Member] Distributed (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period Funded ratio Multiemployer Plan, Pension, Significant, Funded Status Percentage Multiemployer Plan, Pension, Significant, Funded Status Percentage Cold-rolled steel Cold-rolled steel [Member] Cold-rolled steel Statistical Measurement [Domain] Statistical Measurement [Domain] Other current assets Other Current Assets [Member] Pension and OPEB liabilities Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Postretirement Benefits Property, plant and equipment Business Combination, Recognized Asset Acquired, Property, Plant, and Equipment Business Combination [Abstract] Summary of Notional Amount of Outstanding Hedge Contracts Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] 7.000% 2027 AK Senior Notes 7.000% 2027 AK Senior Notes [Member] 7.000% 2027 AK Senior Notes Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] Operating Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] U.S. corporate bonds Debt Security, Corporate, US [Member] Other non-current assets Other Noncurrent Assets [Member] Asset Impairment Impairment or Disposal of Long-Lived Assets, Policy [Policy Text Block] Leases [Abstract] Earnings (loss) per common share attributable to Cliffs shareholders - basic Earnings per common share attributable to Cliffs common shareholders - basic Earnings Per Share, Basic [Abstract] Intangible Asset, Acquired, Finite-Lived [Line Items] Intangible Asset, Acquired, Finite-Lived [Line Items] Noncash or Part Noncash Acquisitions [Table] Noncash or Part Noncash Acquisitions [Table] Common stock per share Business Combination, Consideration Transferred, Equity Interest, Share Issued, Number of Shares Type of Restructuring [Domain] Type of Restructuring [Domain] Interest expense, net Interest Income (Expense), Nonoperating Retirement Plan Name [Axis] Retirement Plan Name [Axis] Unrecorded Unconditional Purchase Obligations Disclosure Unrecorded Unconditional Purchase Obligations Disclosure [Table Text Block] Contributions Multiemployer Plan, Employer Contribution, Cost Trading Arrangements, by Individual Trading Arrangements, by Individual [Table] Earnings (loss) per common share attributable to Cliffs common shareholders - basic (in dollars per share) Decrease in basic earnings per common share (in dollars per share) Earnings Per Share, Basic Assumed Health Care Cost Trend Rates Schedule of Health Care Cost Trend Rates [Table Text Block] Summary of Borrowing Capacity Under the ABL Facility Schedule of Line of Credit Facilities [Table Text Block] Payables, accrued employment and accrued expenses Increase (Decrease) in Accounts Payable and Accrued Liabilities Remediation payments Asset Retirement Obligation, Liabilities Settled Unrecognized tax benefits, Percent Effective Income Tax Rate Reconciliation, Tax Contingency, Percent Restructuring Reserve, Settled without Cash Restructuring Reserve, Settled without Cash Total deferred tax assets before valuation allowance Deferred Tax Assets, Gross Discount rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Total other comprehensive loss Other Comprehensive Income (Loss), Net of Tax Summary of Results by Segment Schedule of Segment Reporting Information, by Segment [Table Text Block] Discontinued operations (in dollars per share) Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share Asset retirement and environmental obligations Asset retirement and environmental obligations Asset retirement and environmental obligations Debt Instrument, Redemption, Period [Axis] Debt Instrument, Redemption, Period [Axis] Special termination benefits Defined Benefit Plan, Other Cost (Credit) Revision in estimated cash flows Asset Retirement Obligation, Revision of Estimate Award Timing MNPI Disclosure Award Timing MNPI Disclosure [Text Block] Variable Interest Entity, Primary Beneficiary Variable Interest Entity, Primary Beneficiary [Member] Other comprehensive income (loss), Foreign currency translation Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax Line of Credit Facility, Maximum Borrowing Capacity, Tranche B Line of Credit Facility, Maximum Borrowing Capacity, Tranche B Line of Credit Facility, Maximum Borrowing Capacity, Tranche B Line of Credit Facility, Maximum Borrowing Capacity, Tranche A Line of Credit Facility, Maximum Borrowing Capacity, Tranche A Line of Credit Facility, Maximum Borrowing Capacity, Tranche A Costs and Expenses Share-Based Payment Arrangement, Expense Hedging Designation [Domain] Hedging Designation [Domain] Finance Lease, Liability, Current, Statement of Financial Position [Extensible List] Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration] Automotive Automotive [Member] Automotive [Member] Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets Disclosure [Abstract] Current assets: Assets, Current [Abstract] TOTAL LIABILITIES AND EQUITY Liabilities and Equity Foreign Currency Foreign Currency Transactions and Translations Policy [Policy Text Block] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Other Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Other Change Acquisition (in shares) Stock Issued During Period, Shares, Acquisitions Adj EBITDA Calculation [Domain] Adj EBITDA Calculation [Domain] Adj EBITDA Calculation [Domain] Net losses (gains) reclassified from AOCI to net income Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent Distributors and converters Distributors and converters [Member] Distributors and converters Stock based compensation expense, net of tax Share-Based Payment Arrangement, Expense, after Tax Expected liability amortization, year one Finite-Lived Intangible Liabilities, Amortization Credit, Next Twelve Months Finite-Lived Intangible Liabilities, Amortization Credit, Next Twelve Months Capital additions Segment, Expenditure, Addition to Long-Lived Assets Common Shares in Treasury Treasury Stock, Common [Member] Income (loss) from continuing operations Income (loss) from continuing operations Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest Pay vs Performance Disclosure [Line Items] Operating Lease, Liability Operating Lease, Liability U.S. government securities US Government Agencies Debt Securities [Member] 2021 Equity Plan 2021 Equity Plan [Member] 2021 Equity Plan Pension benefits, employer contributions and payments Defined Benefit Plan, Plan Assets, Payments and Contributions by Employer Defined Benefit Plan, Plan Assets, Payments and Contributions by Employer LIABILITIES AND EQUITY Liabilities [Abstract] Lessee, Operating Lease, Liability, Payments, Due after Rolling Year Five Lessee, Operating Lease, Liability, to be Paid, after Rolling Year Five Summary of Changes in the Valuation Allowance Summary of Valuation Allowance [Table Text Block] Acquisitions Business Combination [Text Block] Finance leases, Amortization of lease cost Finance Lease, Right-of-Use Asset, Amortization Restricted Amount (percent of original aggregate principal) Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed Other non-current liabilities Other Liabilities, Noncurrent Other, net Other adjustments, net Other adjustments, net Debt Instrument, Redemption, Period [Domain] Debt Instrument, Redemption, Period [Domain] 5.875% 2027 Senior Notes 5.875% 2027 Senior Notes [Member] 5.875% 2027 Senior Notes [Member] Goodwill impairment Goodwill impairment Goodwill, Impairment Loss Finite-Lived Intangible Liability, Net Finite-Lived Intangible Liability, Net Finite-Lived Intangible Liability, Net Coated steel Coated steel [Member] Coated steel Variable Interest Entity [Table] Variable Interest Entity [Table] Statement of Cash Flows [Abstract] Company Selected Measure Amount Company Selected Measure Amount Employer contributions Defined Benefit Plan, Plan Assets, Contributions by Employer Comprehensive income (loss) attributable to Cliffs shareholders Comprehensive Income (Loss), Net of Tax, Attributable to Parent Name Measure Name Schedule of Income (Loss) From Continuing Operations Before Income Taxes, Domestic and Foreign Schedule of Income before Income Tax, Domestic and Foreign [Table Text Block] Class of Stock [Line Items] Class of Stock [Line Items] Industry Sector [Domain] Industry Sector [Domain] Expected return on plan assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-Term Rate of Return on Plan Assets Income (loss) from discontinued operations, net of tax Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Two Lessee, Operating Lease, Liability, to be Paid, Rolling Year Two Unsecured Debt Unsecured Debt [Member] Revision of Prior Period, Adjustment Revision of Prior Period, Adjustment [Member] Income Tax Contingency [Table] Income Tax Contingency [Table] Total long-term debt Long-Term Debt Legal Entity [Axis] Legal Entity [Axis] TOTAL ASSETS Assets Assets Expected asset amortization, year one Finite-Lived Intangible Asset, Expected Amortization, Year One Distributed, weighted average grant date fair value (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Plan amendments Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Other, Arising During Period, Before Tax Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), Other, Arising During Period, Before Tax Geographical [Domain] Geographical [Domain] Income Taxes Income Tax, Policy [Policy Text Block] Repurchase price if triggering event occurs Repurchase Price If Triggering Event Occurs Repurchase Price If Triggering Event Occurs OPERATING ACTIVITIES Cash Provided by (Used in) Operating Activity, Including Discontinued Operation [Abstract] Business Combination, Pro Forma Information [Line Items] Business Combination, Pro Forma Information [Line Items] Effect of inventory build Inventory Adjustments Segment Reporting, Other Segment Item, Amount Segment Reporting, Other Segment Item, Amount Weighted average life, finite-lived intangible assets Acquired Finite-Lived Intangible Assets, Weighted Average Useful Life Private equity Private Equity Funds [Member] Other Defined Benefit Plan, Plan Assets, Increase (Decrease) for Other Change Defined Benefit Plan, Plan Assets, Increase (Decrease) for Other Change Amortization Of Inventory Step-Up Amortization Of Inventory Step-Up Amortization Of Inventory Step-Up Variable Interest Entity [Line Items] Variable Interest Entity [Line Items] Expected asset amortization, year four Finite-Lived Intangible Asset, Expected Amortization, Year Four Restructuring and other charges Restructuring Costs Lease liabilities Deferred Tax Assets, Lease Liabilities Deferred Tax Assets, Lease Liabilities Measurement Period Adjustments, Other current assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Current Assets Variable Interest Entities Consolidation, Variable Interest Entity, Policy [Policy Text Block] Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Right-of-use assets obtained in exchange for new operating lease liabilities Right-of-Use Asset Obtained in Exchange for Operating Lease Liability IAM National Pension Fund's National Pension Plan IAM National Pension Fund's National Pension Plan [Member] IAM National Pension Fund's National Pension Plan Significant  Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Repairs And Maintenance Repairs And Maintenance Policy Text Block [Policy Text Block] Repairs and Maintenance Policy Document Financial Statement Error Correction [Flag] Document Financial Statement Error Correction [Flag] State and local income taxes, net of federal income tax Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Insider Trading Policies and Procedures Not Adopted Insider Trading Policies and Procedures Not Adopted [Text Block] Business Combination [Domain] Business Combination [Domain] Change in plan assets: Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward] Components of the Income Tax Provision (Benefit) on Continuing Operations Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Plan Name [Axis] Plan Name [Axis] Property, Plant and Equipment Property, Plant and Equipment, Policy [Policy Text Block] PEO PEO [Member] Land, land improvements and mineral rights Land, land improvements and mineral rights [Member] Land, land improvements and mineral rights [Member] Employee Severance Employee Severance [Member] Audit Information [Abstract] Audit Information Intangible assets Deferred Tax Liabilities, Intangible Assets Performance adjustment (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Conditions Adjustment in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Conditions Adjustment in Period Income Tax Paid, by Individual Jurisdiction [Line Items] Income Tax Paid, by Individual Jurisdiction [Line Items] Number of employees Entity Number of Employees United States UNITED STATES Changes in operating assets and liabilities, net of business combination: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity, Increase (Decrease) in Operating Capital [Abstract] Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Income Taxes Land improvements Land Improvements [Member] Valuation allowance Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount Property, plant and equipment and mineral rights Deferred Tax Liabilities, Property, Plant and Equipment Retained Earnings (Deficit) Retained Earnings [Member] Other Changes in Assumptions for Defined Benefit Plans, Other Actuarial (Gain) Loss [Member] Changes in Assumptions for Defined Benefit Plans, Other Actuarial (Gain) Loss [Member] Income Tax Jurisdiction [Axis] Income Tax Jurisdiction [Axis] Share-based Payment Arrangement [Abstract] Share-Based Payment Arrangement [Abstract] Cash, cash equivalents, and restricted cash at beginning of year Cash, cash equivalents, and restricted cash at end of year Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Continuing Operation Income Statement [Abstract] Income Statement [Abstract] Option to acquire, consideration amount Derivative Asset Effect of Fair Value Measurements Using Significant Unobservable Inputs on Changes in Plan Assets Schedule of Effect of Significant Unobservable Inputs, Changes in Plan Assets [Table Text Block] Intangible assets Intangible Assets, Net (Excluding Goodwill) Product and Service [Domain] Product and Service [Domain] Finite-Lived Intangible Liability, Gross Finite-Lived Intangible Liability, Gross Finite-Lived Intangible Liability, Gross Acquisition related costs Business Combination, Transaction Cost, Excluding Separately Recognized Transaction Continuing operations (in dollars per share) Income (Loss) from Continuing Operations, Per Diluted Share Net income (loss) Net income (loss) Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Restatement does not require Recovery Restatement Does Not Require Recovery [Text Block] Business Combination [Table] Business Combination [Table] Organization, Consolidation and Presentation of Financial Statements [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Income not subject to tax Effective Income Tax Rate Reconciliation, Tax Exempt Income, Amount Income Tax Paid, by Individual Jurisdiction [Table] Income Tax Paid, by Individual Jurisdiction [Table] Summary of the Purchase Price Allocation to Assets Acquired and Liabilities Assumed Business Combination, Recognized Asset Acquired and Liability Assumed [Table Text Block] City Area Code City Area Code Award Timing, How MNPI Considered Award Timing, How MNPI Considered [Text Block] Interest capitalized Interest Costs Capitalized Net deferred tax liabilities Deferred Tax Liabilities, Net INVESTING ACTIVITIES Cash Provided by (Used in) Investing Activity, Including Discontinued Operation [Abstract] Pay vs Performance Disclosure Pay vs Performance Disclosure [Table] Operating Lease, Liability, Statement of Financial Position [Extensible List] Operating Lease, Liability, Statement of Financial Position [Extensible Enumeration] Deferred Tax and Other Liabilities, Noncurrent Deferred Tax and Other Liabilities, Noncurrent [Member] Deferred Tax and Other Liabilities, Noncurrent Net actuarial loss (gain) Defined Benefit Plan, Amortization of Gain (Loss) Total Lease, Cost Effective Income Tax Rate Reconciliation [Line Items] Effective Income Tax Rate Reconciliation [Line Items] Selling, general and administrative expenses Selling, General and Administrative Expenses [Member] Cost of goods sold Cost of Sales [Member] Interest crediting rate Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Weighted-Average Interest Crediting Rate Goodwill Beginning balance Ending balance Goodwill Summary of Lease Costs Lease, Cost [Table Text Block] Environmental Exit Cost [Table] Environmental Exit Cost [Table] Asset impairment Asset impairment Tangible Asset Impairment Charges Interest expense limitation carryforward Deferred Tax Asset, Interest Carryforward Deferred Federal Income Tax Expense (Benefit) Deferred Federal Income Tax Expense (Benefit) Unrecognized Tax Benefits [Roll Forward] Unrecognized Tax Benefits [Roll Forward] Auditor Name Auditor Name Equipment Equipment [Member] Inventories Business Combination, Recognized Asset Acquired, Inventory, Current Beginning balance as of January 1 Ending balance as of December 31 Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs Accounts receivable Business Combination, Recognized Asset Acquired, Receivable, Current Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Other Performance Measure, Amount Other Performance Measure, Amount Debt issuance costs Payments of Debt Issuance Costs Pension and Other Postretirement Plans Pension and Other Postretirement Plans, Policy [Policy Text Block] Deferred Tax Asset [Domain] Deferred Tax Asset [Domain] Related Party [Domain] Related and Nonrelated Parties [Domain] Effective Income Tax Rate Reconciliation, Amount [Abstract] Income Tax Expense (Benefit), Effective Income Tax Rate Reconciliation, Amount [Abstract] Real estate Real Estate [Member] Other current liabilities Other Current Liabilities [Member] Net increase (decrease) in cash and cash equivalents Cash, Cash Equivalent, Restricted Cash, and Restricted Cash Equivalent, Period Increase (Decrease), Excluding Exchange Rate Effect, Including Discontinued Operation Income tax refunds Proceeds from Income Tax Refunds Derivative Instrument [Axis] Derivative Instrument [Axis] Benefits paid Defined Benefit Plan, Plan Assets, Benefits Paid Fair Value Measurements Fair Value of Financial Instruments, Policy [Policy Text Block] Restricted cash Restricted Cash Other current assets Other Assets, Current Effect of Change in Accounting Estimate, Type [Axis] Effect of Change in Accounting Estimate, Type [Axis] Entity [Domain] Entity [Domain] Effect of settlement Defined Benefit Plan, Plan Assets, Payment for Settlement Other Restructuring Other Restructuring [Member] Investment in affiliates Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures Net cash used by investing activities Cash Provided by (Used in) Investing Activity, Including Discontinued Operation Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table Aggregate Change in Present Value of Accumulated Benefit for All Pension Plans Reported in Summary Compensation Table [Member] Unrealized Gain (Loss) on Derivatives and Commodity Contracts Unrealized Gain (Loss) on Derivatives and Commodity Contracts Gain (Loss) on Disposition of Business Gain (Loss) on Disposition of Business Interest paid on debt obligations net of capitalized interest Interest Paid, Excluding Capitalized Interest, Operating Activity Document Fiscal Year Focus Document Fiscal Year Focus Operating lease, right-of-use asset Operating Lease, Right-of-Use Asset Provision for income tax expense (benefit) and effective income tax rate including discrete items (percent) Effective Income Tax Rate Reconciliation, Percent Operating leases Operating Lease, Cost Decrease due to tax positions in prior year Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Deferred income taxes Deferred Tax and Other Liabilities, Noncurrent Carrying Value Reported Value Measurement [Member] Plan Name [Domain] Plan Name [Domain] Total actuarial liability Multiemployer Plan, Pension, Significant, Accumulated Benefit Obligation Acquisition-related costs Business Combination, Acquisition-Related Cost, Expense Expected period of recognition for outstanding awards not yet recognized Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition Exercised (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period Deferred tax asset valuation allowance Deferred tax asset valuation allowance Balance at beginning of year Balance at end of year Deferred Tax Assets, Valuation Allowance Equity [Abstract] Equity [Abstract] Acquisition-related costs Acquisition-related expenses and adjustments Acquisition-related expenses and adjustments Goodwill and Other Intangible Assets and Liabilities Goodwill and Intangible Assets, Policy [Policy Text Block] Global equities Defined Benefit Plan, Equity Securities, Non-US [Member] Buildings Buildings Building [Member] Derivative Financial Instruments and Hedging Activities Derivatives, Policy [Policy Text Block] Segment Reporting Segment Reporting Disclosure [Text Block] Goodwill [Table] Goodwill [Table] 2028 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Three Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Plate Plate [Member] Plate Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Schedule of Acquired Finite-Lived Intangible Asset by Major Class [Table] Fair value of plan assets — beginning of year Fair value of plan assets — end of year Fair value of pension assets by asset category Defined Benefit Plan, Plan Assets, Amount Current Federal Tax Expense (Benefit) Current Federal Tax Expense (Benefit) Other Commitments [Line Items] Other Commitments [Line Items] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Other Other Capitalized Property Plant and Equipment [Member] Components Of Net Periodic Benefit Cost Schedule of Net Benefit Costs [Table Text Block] Other non-current liabilities Business Combination, Recognized Liability Assumed, Other Liability, Noncurrent Compensation Actually Paid vs. Total Shareholder Return Compensation Actually Paid vs. Total Shareholder Return [Text Block] Accumulated Other Comprehensive Income (Loss) Comprehensive Income (Loss) Note [Text Block] Amortization of intangible assets Amortization of Intangible Assets Selling, general and administrative expenses Selling, general and administrative expenses Selling, General and Administrative Expense Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Asset Retirement and Environmental Obligations Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Asset Retirement and Environmental Obligations Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Asset Retirement and Environmental Obligations Common stock repurchases, net of excise tax Stock Repurchased During Period, Value Components of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Depreciation, depletion and amortization Depreciation, depletion and amortization Depreciation, Depletion and Amortization Structured credit Structured Credit [Member] Structured Credit [Member] Fixed income Fixed Income Investments [Member] Schedule of Revenues from External Customers and Long-Lived Assets [Table] Schedule of Revenues from External Customers and Long-Lived Assets [Table] ABL Facility Revolving Credit Facility [Member] Statutory rate difference Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount Finance Lease, Liability Finance Lease, Liability Arrangement Duration Trading Arrangement Duration Hot-rolled steel Hot-rolled steel [Member] Hot-rolled steel Segments [Axis] Segments [Axis] Fair Value by Liability Class [Domain] Fair Value by Liability Class [Domain] Acquired through business combinations Defined Benefit Plan, Benefit Obligation, Business Combination Local Phone Number Local Phone Number Additional 402(v) Disclosure Additional 402(v) Disclosure [Text Block] Other investing activities Payment for (Proceeds from) Other Investing Activity Thereafter Long-Term Debt, Maturity, after Year Five Stock Compensation Plans Share-Based Payment Arrangement [Text Block] Goodwill impairment, percent Effective Income Tax Rate Reconciliation, Disposition of Business, Percent Non-cash accruals Capital Expenditures Incurred But Not Yet Paid, Increase (Decrease) In Period Increase (decrease) in future cash outflow to pay for purchases of fixed assets that have occurred during the period. Cash and cash equivalents Business Combination, Recognized Asset Acquired, Cash and Cash Equivalent Finance Lease, Liability, Payments, Due in Rolling Year Four Finance Lease, Liability, to be Paid, Rolling Year Four ASSETS Assets [Abstract] Underlying Security Market Price Change Underlying Security Market Price Change, Percent Supplier Finance Program Supplier Finance Program [Table Text Block] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Debt Instrument [Axis] Debt Instrument [Axis] Forfeited/canceled, weighted average grant date fair value (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Business Combination [Line Items] Business Combination [Line Items] Loss on extinguishment of debt Gain (Loss) on Extinguishment of Debt Business Combination, Pro Forma Information, Nonrecurring Adjustment [Axis] Business Combination, Pro Forma Information, Nonrecurring Adjustment [Axis] Percent of shares actually earned based on performance Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Rights, Percentage Environmental Loss Contingency, Current, Statement of Financial Position [Extensible Enumeration] Environmental Loss Contingency, Current, Statement of Financial Position [Extensible Enumeration] Stated interest rate Debt Instrument, Interest Rate, Stated Percentage Operating Operating Lease, Liability, Noncurrent Entity Voluntary Filers Entity Voluntary Filers Lessee, Operating Lease, Liability, Payments, Due in Rolling Year Three Lessee, Operating Lease, Liability, to be Paid, Rolling Year Three 4.875% 2031 Senior Notes 4.875% 2031 Senior Notes [Member] 4.875% 2031 Senior Notes Derivative assets Derivative Asset, Subject to Master Netting Arrangement, before Offset Land rights Land Property, Plant and Equipment [Abstract] Property, Plant and Equipment [Abstract] Entity Registrant Name Entity Registrant Name Retirement Plan Name [Domain] Retirement Plan Name [Domain] Accrued expenses Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Expenses Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Accrued Expenses Stock Price or TSR Estimation Method Stock Price or TSR Estimation Method [Text Block] Net periodic benefit credits other than service cost component Net periodic benefit credits other than service cost component Net periodic benefit cost (credit) Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Lessee, Operating Lease, Liability, Payments, Due Next Rolling Twelve Months Lessee, Operating Lease, Liability, to be Paid, Next Rolling 12 Months Debt Instrument, Redemption, Period Two Debt Instrument, Redemption, Period Two [Member] Supplier Finance Program, Obligation, Statement of Financial Position [Extensible Enumeration] Supplier Finance Program, Obligation, Statement of Financial Position [Extensible Enumeration] Income taxes Increase (Decrease) in Income Taxes Payable, Net of Income Taxes Receivable Revision of Prior Period [Domain] Revision of Prior Period [Domain] Other assets (liabilities), net Other Assets (Liabilities), Net Other Assets (Liabilities), Net Energy [Axis] Energy [Axis] Finance Lease, Right-of-Use Asset, before Accumulated Amortization Finance Lease, Right-of-Use Asset, before Accumulated Amortization Number of operating segments Number of Operating Segments Weighted-average remaining intrinsic value Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding Equipment financed with seller Property, Plant, And Equipment, Financed Purchase Property, Plant, And Equipment, Financed Purchase Structured credit Structured Finance [Member] Equipment Machinery and Equipment [Member] Granted, weighted average grant date fair value (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Related Party [Axis] Related and Nonrelated Parties [Axis] Interest cost Defined Benefit Plan, Interest Cost Amounts recognized in accumulated other comprehensive loss (income): Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract] Other non-current liabilities Other Noncurrent Liabilities [Member] Restatement Determination Date: Restatement Determination Date [Axis] Multiemployer Plan [Line Items] Multiemployer Plan [Line Items] Supplier Finance Program [Table] Supplier Finance Program [Table] Stock and other incentive plans Shares Issued, Value, Share-Based Payment Arrangement, after Forfeiture Deferred Foreign Income Tax Expense (Benefit) Deferred Foreign Income Tax Expense (Benefit) Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Other financing activities Proceeds from (Payment for) Other Financing Activity Name Trading Arrangement, Individual Name Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Nonvested, Number of Shares [Roll Forward] Alabama ALABAMA Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Non-Current Liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Non-Current Liabilities Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Non-Current Liabilities Other comprehensive income (loss), Pension and OPEB Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Statement of Comprehensive Income [Abstract] Statement of Comprehensive Income [Abstract] Deferred Tax Asset, Valuation Allowance [Table] Deferred Tax Asset, Valuation Allowance [Table] Name Awards Close in Time to MNPI Disclosures, Individual Name Energy [Domain] Energy [Domain] Outstanding at beginning of year, weighted average grant date fair value (in dollars per share) Outstanding at end of year, weighted average grant date fair value (in dollars per share) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Payments for Restructuring Payments for Restructuring Restructuring Type [Axis] Restructuring Type [Axis] Aggregate Erroneous Compensation Not Yet Determined Aggregate Erroneous Compensation Not Yet Determined [Text Block] ICFR Auditor Attestation Flag ICFR Auditor Attestation Flag Estimated net periodic benefit cost Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year Stock and other incentive plans, APIC APIC, Share-Based Payment Arrangement, Increase for Cost Recognition Income tax Other Comprehensive Income (Loss) before Reclassifications, Tax Supplier Relationships [Member] Supplier Relationships [Member] Supplier Relationships Liability Class [Axis] Liability Class [Axis] Foreign Tax Jurisdiction Foreign Tax Jurisdiction [Member] Auditor Firm ID Auditor Firm ID Segments [Domain] Segments [Domain] Statement of Income Location, Balance [Axis] Statement of Income Location, Balance [Axis] Foreign Income (Loss) from Continuing Operations before Income Taxes, Foreign Borrowing capacity available Line of Credit Facility, Remaining Borrowing Capacity Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Benefit obligations — beginning of year Benefit obligations — end of year Defined Benefit Plan, Benefit Obligation Investment in ventures Deferred Tax Liabilities, Investments Statement of Financial Position Location, Balance [Domain] Statement of Financial Position Location, Balance [Domain] Inventories Increase (Decrease) in Inventories Adjusted EBITDA Adjusted EBITDA, Earnings (Loss) Adjusted EBITDA, Earnings (Loss) Accrued Environmental Loss Contingencies, Current Accrued Environmental Loss Contingencies, Current Non-current assets: Assets, Noncurrent [Abstract] Allowance for depreciation and depletion Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Earnings Per Share Earnings Per Share, Policy [Policy Text Block] Accretion expense Asset Retirement Obligation, Accretion Expense Business Combination, Pro Forma Information, Nonrecurring Adjustment, Acquisition-Related Cost Business Combination, Pro Forma Information, Nonrecurring Adjustment, Acquisition-Related Cost [Member] Measurement Period Adjustments, Accounts payable Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Payable Senior notes Senior Notes [Member] All Executive Categories All Executive Categories [Member] Significant Components of Deferred Tax Assets and Liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Finished and semi-finished goods Inventory, Finished Goods, Net of Reserves Right-of-use assets related to finance leases Finance Lease, Right-of-Use Asset, after Accumulated Amortization Title and Position [Axis] Title and Position [Axis] Commitments and contingencies (See NOTE 20) Commitments and Contingencies Asset retirement and environmental obligations Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Non-Current Liabilities, Asset retirement and environmental obligations Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Non-Current Liabilities, Asset retirement and environmental obligations Intangible assets Business Combination, Recognized Asset Acquired, Identifiable Intangible Asset, Finite-Lived Investments Measured at Net Asset Value Fair Value Measured at Net Asset Value Per Share [Member] Business Combination [Axis] Business Combination [Axis] Comprehensive income (loss) Comprehensive income (loss) Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Lease Obligations Lessee, Financing and Operating Leases Disclosure [Text Block] Lessee, Financing and Operating Leases Disclosure Non- controlling Interests Noncontrolling Interest [Member] EBITDA Calculation [Axis] EBITDA Calculation [Axis] EBITDA Calculation Steelmaking Steelmaking [Member] Steelmaking Measurement Period Adjustments, Total net assets acquired Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Consideration Transferred Market value of assets Multiemployer Plan, Pension, Significant, Plan Asset Investments in Affiliates Equity Method Investments [Policy Text Block] Valuation Allowance [Line Items] Valuation Allowance [Line Items] Non-taxable income related to noncontrolling interests Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Amount 7.375% 2033 Senior Notes 7.375% 2033 Senior Notes [Member] 7.375% 2033 Senior Notes Adj EBITDA Calculation Adj EBITDA Calculation [Member] Adj EBITDA Calculation PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Percentage of Plan Assets Defined Benefit Plan, Plan Assets, Actual Allocation, Percentage Forfeited/canceled (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Forfeitures and Expirations in Period Proceeds from government grant reimbursements Proceeds from government grant reimbursements Proceeds from government grant reimbursements Measurement Period Adjustments, Inventories Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Inventory Earnings (loss) per common share attributable to Cliffs shareholders - diluted Earnings (loss) per common share attributable to Cliffs common shareholders - diluted: Earnings Per Share, Diluted [Abstract] Revenue from Contract with Customer [Abstract] Hedging Designation [Axis] Hedging Designation [Axis] Deferred income taxes Business Combination, Recognized Liability Assumed, Deferred Tax Liability Sales Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decrease for Sale Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits Schedule of Unrecognized Tax Benefits Roll Forward [Table Text Block] Slab and other steel products Other steel products [Member] Other steel products 7.625% 2034 Senior Notes 7.625% 2034 Senior Notes [Member] 7.625% 2034 Senior Notes Commitments Commitments [Member] Changes in pension and OPEB, net of tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax Derivative Contract [Domain] Derivative Contract [Domain] Proceeds from issuance of common shares Proceeds from Issuance of Common Stock 2030 Defined Benefit Plan, Expected Future Benefit Payment, Year Five All Individuals All Individuals [Member] Long-term Debt, Type [Domain] Long-Term Debt, Type [Domain] Noncash or Part Noncash Acquisitions [Line Items] Noncash or Part Noncash Acquisitions [Line Items] Fair Value Disclosures [Abstract] Fair Value Disclosures [Abstract] Purchase of property, plant and equipment Cash paid for capital expenditures including deposits Payments to Acquire Property, Plant, and Equipment Name Forgone Recovery, Individual Name Expiration period Share-Based Compensation Arrangement by Share-Based Payment Award, Expiration Period 2029 Unrecorded Unconditional Purchase Obligation, to be Paid, Year Four Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested Year-over-Year Change in Fair Value of Equity Awards Granted in Prior Years That are Outstanding and Unvested [Member] Total debt consideration Business Combination, Consideration Transferred, Debt Consideration Business Combination, Consideration Transferred, Debt Consideration Reserve for investment commitments Reserve For Investment Commitments Reserve for Investment Commitments Foreign currency translation Goodwill, Foreign Currency Translation, Gain (Loss) Directors Plan Directors Plan [Member] Directors Plan [Member] Performance adjustment, weighted average grant date fair value (in dollars per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Adjustment in Period, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Performance Adjustment in Period, Weighted Average Grant Date Fair Value Erroneous Compensation Analysis Erroneous Compensation Analysis [Text Block] Intangible Asset, Finite-Lived [Table] Intangible Asset, Finite-Lived [Table] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Geographical [Axis] Geographical [Axis] Other, net Increase (Decrease) in Other Operating Assets and Liabilities, Net Rule 10b5-1 Arrangement Terminated Rule 10b5-1 Arrangement Terminated [Flag] Revenues Business Combination, Pro Forma Information, Pro Forma Revenue Erroneously Awarded Compensation Recovery Erroneously Awarded Compensation Recovery [Table] Title of 12(b) Security Title of 12(b) Security Accrued Environmental Loss Contingencies, Noncurrent Accrued Environmental Loss Contingencies, Noncurrent Other foreign jurisdictions Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward] Infrastructure and manufacturing Infrastructure and manufacturing [Member] Infrastructure and manufacturing Earnings Per Share [Abstract] Earnings Per Share [Abstract] Total other expense Nonoperating Income (Expense) Schedule of Potentially Dilutive Shares Excluded from the Computation of Diluted Earnings Per Share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] HRC Hot-Rolled Coil Steel [Member] Hot-Rolled Coil Steel Finance Lease, Liability, Payments, Due in Rolling after Year Five Finance Lease, Liability, to be Paid, after Rolling Year Five Debt Security, Corporate, Non-US Debt Security, Corporate, Non-US [Member] Actuarial loss (gain) on benefit obligation Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax Exercised, weighted average exercise price (in dollars per share) Share-Based Compensation Arrangements by Share-Based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Award Timing Disclosures [Line Items] Total share exchange consideration Business Combination, Consideration Transferred, Equity Interest, Fair Value Method Roll-forward of Asset Retirement Obligation Liability Schedule of Change in Asset Retirement Obligation [Table Text Block] Schedule of Inventories Schedule of Inventory, Current [Table Text Block] Service cost Service cost Defined Benefit Plan, Service Cost Summary of Lease Amounts Included in Statements of Consolidated Financial Position Assets And Liabilities, Lessee [Table Text Block] Assets And Liabilities, Lessee Other comprehensive income (loss) before reclassifications, net of tax Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Capital Stock Equity [Text Block] Deferred State and Local Income Tax Expense (Benefit) Deferred State and Local Income Tax Expense (Benefit) Foreign currency translation Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Foreign Currency Translation Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Foreign Currency Translation Other Operating Income (Expense) Other Operating Income (Expense) [Member] Net income (loss) attributable to Cliffs shareholders Net Income (Loss) Attributable to Parent Illinois ILLINOIS Non-current liabilities: Liabilities, Noncurrent [Abstract] Stainless and electrical steel Stainless and electrical steel [Member] Stainless and electrical steel Construction in progress Construction in Progress [Member] Defined Benefit Plan, Plan Assets, Level 3 Reconciliation [Line Items] Defined Benefit Plan, Plan Assets, Level 3 Reconciliation [Line Items] Reconciliation of Income Tax Attributable to Continuing Operations Computed at Statutory Rate Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Performance/vesting period Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period Other items, net, percent Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Preferred stock authorized (in shares) Preferred Stock, Shares Authorized Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Repurchase of common shares Repurchase of common shares Payments for Repurchase of Common Stock Adoption Date Trading Arrangement Adoption Date Customer [Axis] Customer [Axis] Customer relationships Customer Relationships [Member] Operating income (loss) Operating Income (Loss) Line of Credit Line of Credit [Member] Retained earnings (deficit) Retained Earnings (Accumulated Deficit) Current liabilities Current Liabilities, Lessee [Abstract] Current Liabilities, Lessee Third Party Third Party [Member] Third Party Summary of Total Compensation Expense Recognized for Stock-Based Compensation Awards Share-Based Payment Arrangement, Cost by Plan [Table Text Block] Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested Year-end Fair Value of Equity Awards Granted in Covered Year that are Outstanding and Unvested [Member] Class of Stock [Axis] Class of Stock [Axis] Environmental Remediation Costs Environmental Costs, Policy [Policy Text Block] Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table Aggregate Grant Date Fair Value of Equity Award Amounts Reported in Summary Compensation Table [Member] Per capita healthcare costs Changes in Assumptions for Defined Benefit Plans, Per Capita Claims [Member] Changes in Assumptions for Defined Benefit Plans, Per Capita Claims [Member] Other assets Deferred Tax Liabilities, Other Retirement Plan Type [Axis] Retirement Plan Type [Axis] Statement [Table] Statement [Table] Other non-operating income Other Nonoperating Income (Expense) Actual return on plan assets Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) Schedule of Fair Value of Purchase Consideration Business Combination [Table Text Block] Deferred income taxes Deferred Income Tax Expense (Benefit) Adjustments to reconcile net income (loss) to net cash provided (used) by operating activities: Adjustment to Reconcile Net Income to Cash Provided by (Used in) Operating Activity [Abstract] Lapses in statutes of limitations Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations FINANCING ACTIVITIES Cash Provided by (Used in) Financing Activity, Including Discontinued Operation [Abstract] Other Foreign Tax Jurisdiction, Other [Member] Total net assets acquired Business Combination, Recognized Asset Acquired to Liability Assumed, Excess (Less), and Goodwill Michigan MICHIGAN Finance Finance Lease, Liability, Noncurrent Leases Lessor, Leases [Policy Text Block] Mining permits Mining Permits [Member] Mining Permits [Member] All Adjustments to Compensation All Adjustments to Compensation [Member] Electricity Electricity [Member] Amendment Flag Amendment Flag Price per share of shares sold (in dollars per share) Sale of Stock, Price Per Share Income tax benefit Share-Based Payment Arrangement, Expense, Tax Benefit Number of shares authorized (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized Summary of Intangible Assets and Liabilities Schedule of Acquired Finite-Lived Intangible Assets by Major Class [Table Text Block] OPEB Other Postretirement Benefits Plan [Member] Plan Assets and Asset Allocation Schedule of Allocation of Plan Assets [Table Text Block] Cost of 36,292,287 common shares in treasury (2024 - 37,102,625 shares) Treasury Stock, Common, Value Termination Date Trading Arrangement Termination Date Environmental Exit Cost [Line Items] Environmental Exit Cost [Line Items] Insider Trading Policies and Procedures Adopted Insider Trading Policies and Procedures Adopted [Flag] Raw materials Inventory, Raw Materials, Net of Reserves Operating Lease, Weighted Average Discount Rate, Percent Operating Lease, Weighted Average Discount Rate, Percent Summary of Stock Option Activity Share-Based Payment Arrangement, Option, Activity [Table Text Block] Basic (in shares) Weighted Average Number of Shares Outstanding, Basic Unrecognized tax benefits that would impact effective tax rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Earnings Per Share Computation Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Director Director [Member] Exercisable (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercisable, Number Subsequent Events [Abstract] Subsequent Events [Abstract] Segment Reporting [Abstract] Segment Reporting [Abstract] Special termination benefits Defined Benefit Plan, Cost of Providing Special and Contractual Termination Benefits Debt Disclosure [Abstract] Debt Disclosure [Abstract] Other income (expense): Nonoperating Income (Expense) [Abstract] Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Axis] Share-based Payment Arrangement, Nonemployee Share-Based Payment Arrangement, Nonemployee [Member] Operating costs: Costs and Expenses [Abstract] Income tax expense (benefit) Reclassification from AOCI, Current Period, Tax Depletion Depletion Losses (gains) reclassified from AOCI to net income Reclassification from AOCI, Current Period, before Tax, Attributable to Parent Borrowings (repayments) under credit facilities Proceeds from (Repayments of) Lines of Credit Document Financial Statement Restatement Recovery Analysis [Flag] Document Financial Statement Restatement Recovery Analysis [Flag] Expected liability amortization, year three Finite-Lived Intangible Liabilities, Amortization Credit, Year Three Finite-Lived Intangible Liabilities, Amortization Credit, Year Three Forgone Recovery due to Expense of Enforcement, Amount Forgone Recovery due to Expense of Enforcement, Amount Consolidated Entities [Axis] Consolidated Entities [Axis] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items] Other Plans Retirement Plan Name, Other [Member] Common shares, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Derivative Instruments and Hedging Activities Disclosures [Line Items] Derivative Instruments and Hedging Activities Disclosures [Line Items] Finance Lease, Liability, Payments, Due in Next Rolling Twelve Months Finance Lease, Liability, to be Paid, Next Rolling 12 Months Multiemployer Plan Multiemployer Plan [Table Text Block] 2031-2035 Defined Benefit Plan, Expected Future Benefit Payment, after Year Five for Next Five Years Use of Estimates Use of Estimates, Policy [Policy Text Block] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Finance leases within cash flows from operating activities Finance Lease, Interest Payment on Liability Current liabilities Liability, Defined Benefit Plan, Current Other comprehensive income (loss), Total Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent Litigation Settlement, Amount Awarded to Other Party Litigation Settlement, Amount Awarded to Other Party Business Acquisition, Shares Exchange Ratio Business Acquisition, Shares Exchange Ratio Business Acquisition, Shares Exchange Ratio Subsequent Events Subsequent Events [Text Block] Changes in foreign currency translation Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Insider Trading Arrangements [Line Items] Equity securities Equity Securities [Member] Federal & state provision to return, percent Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Percent Percentage depletion in excess of depletion, percent Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depletion, Percent TOTAL LIABILITIES Liabilities Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Defined Benefit Plan, Information about Plan Assets [Abstract] Defined Benefit Plan, Information about Plan Assets [Abstract] Expected liability amortization, year two Finite-Lived Intangible Liabilities, Amortization Credit, Year Two Finite-Lived Intangible Liabilities, Amortization Credit, Year Two Common Shares - par value $0.125 per share Common Stock, Value, Outstanding Pension Adjustments Prior Service Cost Pension Adjustments Prior Service Cost [Member] Increases for tax positions in current year Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Material Terms of Trading Arrangement Material Terms of Trading Arrangement [Text Block] Trade names and trademarks Trademarks and Trade Names [Member] Expected employer contributions, next fiscal year Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year Stock Issued During Period, Value, New Issues Stock Issued During Period, Value, New Issues Summary of Purchase Price Allocated to Identifiable Intangible Assets and Liabilities Acquired Business Combination, Intangible Asset, Acquired, Finite-Lived and Indefinite-Lived [Table Text Block] Consolidation Items [Axis] Consolidation Items [Axis] Deferred tax liabilities: Components of Deferred Tax Liabilities [Abstract] Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Non-NEOs Non-NEOs [Member] Total amount recognized Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position VEBA Assets Trust for Benefit of Employees [Member] Other Liabilities