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Awards Close in Time to MNPI Disclosures [Table Text Block] Restructuring Reserve, Beginning Balance Restructuring Reserve, Ending Balance Restructuring Reserve Equity Component [Domain] Equity Component [Domain] PEO Actually Paid Compensation Amount PEO Actually Paid Compensation Amount Total expense included in earnings Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings Market Stock Units Market Stock Units [Member] Market Stock Units Total operating costs Costs and Expenses Adoption Date Trading Arrangement Adoption Date Stated interest rate Debt Instrument, Interest Rate, Stated Percentage Total Cliffs shareholders' equity Equity, Attributable to Parent Total other expense Nonoperating Income (Expense) Restructuring Type [Axis] Restructuring Type [Axis] Pension and OPEB assets Assets for Plan Benefits, Defined Benefit Plan ASSETS Assets [Abstract] Title Trading Arrangement, Individual Title 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Reporting [Table] Segment Reporting [Table] Schedule of Maturities of Long-term Debt Schedule of Maturities of Long-Term Debt [Table Text Block] Other current assets Other Assets, Current Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Disclosure [Abstract] All Trading Arrangements All Trading Arrangements [Member] Beginning balance as of January 1 Ending balance as of September 30 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value Other Performance Measure, Amount Other Performance Measure, Amount Award Timing MNPI Considered Award Timing MNPI Considered [Flag] Operating income (loss) Operating Income (Loss) Current Fiscal Year End Date Current Fiscal Year End Date Non-PEO NEO Average Total Compensation Amount Non-PEO NEO Average Total Compensation Amount Plate steel Plate steel [Member] Plate steel Common stock repurchases (shares) Stock Repurchased During Period, Shares Change in Fair Value as of 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Schedule of Variable Interest Entities Schedule of Variable Interest Entities [Table Text Block] Common shares in treasury (in shares) Treasury Stock, Common, Shares Accrued Environmental Loss Contingencies, Noncurrent Accrued Environmental Loss Contingencies, Noncurrent Long-term asset retirement obligations Asset Retirement Obligations, Noncurrent ACCUMULATED OTHER COMPREHENSIVE INCOME Comprehensive Income (Loss) Note [Text Block] Equity Components [Axis] Equity Components [Axis] 4.875% 2031 Senior Notes 4.875% 2031 Senior Notes [Member] 4.875% 2031 Senior Notes Product and Service [Domain] Product and Service [Domain] Class of Stock [Line Items] Class of Stock [Line Items] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] 7.500% 2031 Senior Notes 7.500% 2031 Senior Notes [Member] 7.500% 2031 Senior Notes Entity Interactive Data Current Entity Interactive Data Current Net income attributable to noncontrolling interests Net income (loss) attributable to 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