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SEGMENT REPORTING (Schedule Of Segment Reporting Information, By Segment) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) |  |  |  |  |  |  |  |  | $ (8,311.6)us-gaap_ProfitLoss |  | $ 361.8 us-gaap_ProfitLoss |  | $ (1,126.6)us-gaap_ProfitLoss |
Revenues from product sales and services | 1,284.7 us-gaap_Revenues | 1,298.2 us-gaap_Revenues | 1,100.8 us-gaap_Revenues | 940.0 us-gaap_Revenues | 1,515.8 us-gaap_Revenues | 1,546.6 us-gaap_Revenues | 1,488.5 us-gaap_Revenues | 1,140.5 us-gaap_Revenues | 4,623.7 us-gaap_Revenues |  | 5,691.4 us-gaap_Revenues |  | 5,872.7 us-gaap_Revenues |
Revenues from producet sales and services, percent |  |  |  |  |  |  |  |  | 100.00%clf_PercentageOfSegmentRevenue |  | 100.00%clf_PercentageOfSegmentRevenue |  | 100.00%clf_PercentageOfSegmentRevenue |
SALES MARGIN | 168.7 us-gaap_GrossProfit | 127.5 us-gaap_GrossProfit | 92.0 us-gaap_GrossProfit | 63.2 us-gaap_GrossProfit | 294.5 us-gaap_GrossProfit | 348.7 us-gaap_GrossProfit | 268.2 us-gaap_GrossProfit | 237.9 us-gaap_GrossProfit | 451.4 us-gaap_GrossProfit |  | 1,149.3 us-gaap_GrossProfit |  | 1,172.1 us-gaap_GrossProfit |
Other operating expense |  |  |  |  |  |  |  |  | (9,896.7)us-gaap_OperatingExpenses |  | (478.3)us-gaap_OperatingExpenses |  | (1,480.9)us-gaap_OperatingExpenses |
Other expense |  |  |  |  |  |  |  |  | (158.4)us-gaap_NonoperatingIncomeExpense |  | (181.7)us-gaap_NonoperatingIncomeExpense |  | (193.0)us-gaap_NonoperatingIncomeExpense |
Income (loss) from continuing operations before income taxes and equity income (loss) from ventures |  |  |  |  |  |  |  |  | (9,603.7)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments |  | 489.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments |  | (501.8)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 504.0 us-gaap_DepreciationDepletionAndAmortization |  | 593.3 us-gaap_DepreciationDepletionAndAmortization |  | 525.8 us-gaap_DepreciationDepletionAndAmortization |
Capital Additions |  |  |  |  |  |  |  |  | 235.5 us-gaap_PropertyPlantAndEquipmentAdditions |  | 752.3 us-gaap_PropertyPlantAndEquipmentAdditions |  | 1,335.3 us-gaap_PropertyPlantAndEquipmentAdditions |
Interest expense, net |  |  |  |  |  |  |  |  | (185.2)us-gaap_InterestIncomeExpenseNet |  | (179.1)us-gaap_InterestIncomeExpenseNet |  | (195.6)us-gaap_InterestIncomeExpenseNet |
Income Tax Expense (Benefit) | (289.7)us-gaap_IncomeTaxExpenseBenefit |  |  |  |  |  |  |  | (1,302.0)us-gaap_IncomeTaxExpenseBenefit |  | 55.1 us-gaap_IncomeTaxExpenseBenefit |  | 255.9 us-gaap_IncomeTaxExpenseBenefit |
EBITDA |  |  |  |  |  |  |  |  | (8,924.4)clf_EBITDA |  | 1,189.3 clf_EBITDA |  | (149.3)clf_EBITDA |
Equity Method Investment, Other than Temporary Impairment |  |  |  |  |  |  |  |  | 0 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment |  | 0 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment |  | (365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment |
Loss on sale of Cliffs Logan County Coal |  |  |  |  |  |  |  |  | 419.6 clf_LossonsaleofCliffsLoganCountyCoal |  | 0 clf_LossonsaleofCliffsLoganCountyCoal |  | 0 clf_LossonsaleofCliffsLoganCountyCoal |
Adjusted EBITDA |  |  |  |  |  |  |  |  | 929.7 clf_AdjustedEBITDA |  | 1,428.3 clf_AdjustedEBITDA |  | 1,292.9 clf_AdjustedEBITDA |
U.S. Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues from product sales and services |  |  |  |  |  |  |  |  | 2,506.5 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 2,667.9 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 2,723.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Revenues from producet sales and services, percent |  |  |  |  |  |  |  |  | 54.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 47.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 46.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
SALES MARGIN |  |  |  |  |  |  |  |  | 710.4 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 901.9 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 976.2 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 107.4 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 120.3 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 100.9 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Capital Additions |  |  |  |  |  |  |  |  | 48.4 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | [1] | 53.3 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | [1] | 168.8 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | [1]
EBITDA |  |  |  |  |  |  |  |  | 805.6 clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 1,000.1 clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 1,045.3 clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Adjusted EBITDA |  |  |  |  |  |  |  |  | 831.2 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 1,030.8 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |  | 1,085.6 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember |
Eastern Canadian Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues from product sales and services |  |  |  |  |  |  |  |  | 563.4 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 978.7 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 1,008.9 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Revenues from producet sales and services, percent |  |  |  |  |  |  |  |  | 12.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 17.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 17.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
SALES MARGIN |  |  |  |  |  |  |  |  | (244.9)us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | (103.3)us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | (121.4)us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 135.6 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 178.5 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 160.2 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Capital Additions |  |  |  |  |  |  |  |  | 141.2 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember | [1] | 625.5 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember | [1] | 865.2 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember | [1]
EBITDA |  |  |  |  |  |  |  |  | (7,673.9)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | (192.8)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | (1,103.3)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Adjusted EBITDA |  |  |  |  |  |  |  |  | 0 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 0 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |  | 0 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_EasternCanadianIronOreMember |
Asia Pacific Iron Ore [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues from product sales and services |  |  |  |  |  |  |  |  | 866.7 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 1,224.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 1,259.3 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |
Revenues from producet sales and services, percent |  |  |  |  |  |  |  |  | 19.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 22.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 22.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |
SALES MARGIN |  |  |  |  |  |  |  |  | 121.7 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 367.1 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 311.0 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 145.9 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 153.7 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 151.9 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |
Capital Additions |  |  |  |  |  |  |  |  | 10.8 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | [1] | 13.0 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | [1] | 87.7 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | [1]
EBITDA |  |  |  |  |  |  |  |  | (369.8)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 500.4 clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 387.3 clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |
Adjusted EBITDA |  |  |  |  |  |  |  |  | 264.6 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 525.7 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |  | 402.1 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember |
North American Coal [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues from product sales and services |  |  |  |  |  |  |  |  | 687.1 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 821.9 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 881.1 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |
Revenues from producet sales and services, percent |  |  |  |  |  |  |  |  | 15.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 14.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 15.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |
SALES MARGIN |  |  |  |  |  |  |  |  | (135.8)us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | (14.5)us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | (1.8)us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 106.9 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 128.9 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 98.2 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |
Capital Additions |  |  |  |  |  |  |  |  | 28.8 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | [1] | 55.0 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | [1] | 144.1 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember | [1]
EBITDA |  |  |  |  |  |  |  |  | (1,326.8)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 129.5 clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 74.0 clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |
Adjusted EBITDA |  |  |  |  |  |  |  |  | (28.5)clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 154.0 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |  | 106.7 clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_NorthAmericanCoalMember |
All Other Segments [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues from product sales and services |  |  |  |  |  |  |  |  | 0 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | (1.4)us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 0.1 us-gaap_Revenues / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |
Revenues from producet sales and services, percent |  |  |  |  |  |  |  |  | 0.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 0.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 0.00%clf_PercentageOfSegmentRevenue / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |
SALES MARGIN |  |  |  |  |  |  |  |  | 0 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | (1.9)us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 8.1 us-gaap_GrossProfit / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 8.2 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 11.9 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |  | 14.6 us-gaap_DepreciationDepletionAndAmortization / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember |
Capital Additions |  |  |  |  |  |  |  |  | 6.3 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | [1] | 5.5 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | [1] | 69.5 us-gaap_PropertyPlantAndEquipmentAdditions / us-gaap_StatementBusinessSegmentsAxis = us-gaap_AllOtherSegmentsMember | [1]
Other Segment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
EBITDA |  |  |  |  |  |  |  |  | (359.5)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_OtherSegmentMember |  | (247.9)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_OtherSegmentMember |  | (552.6)clf_EBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_OtherSegmentMember |
Adjusted EBITDA |  |  |  |  |  |  |  |  | (137.6)clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_OtherSegmentMember |  | (282.2)clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_OtherSegmentMember |  | (301.5)clf_AdjustedEBITDA / us-gaap_StatementBusinessSegmentsAxis = clf_OtherSegmentMember |
EBITDA Calculation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | (504.0)us-gaap_DepreciationDepletionAndAmortization / clf_EBITDACalculationAxis = clf_EBITDACalculationMember |  | (593.3)us-gaap_DepreciationDepletionAndAmortization / clf_EBITDACalculationAxis = clf_EBITDACalculationMember |  | (525.8)us-gaap_DepreciationDepletionAndAmortization / clf_EBITDACalculationAxis = clf_EBITDACalculationMember |
Amapa [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Other than Temporary Impairment |  |  |  |  |  |  |  |  |  |  | 67.6 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember |  | (365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember |
Adjusted EBITDA Calculation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loss on sale of Cliffs Logan County Coal |  |  |  |  |  |  |  |  | (419.6)clf_LossonsaleofCliffsLoganCountyCoal / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 0 clf_LossonsaleofCliffsLoganCountyCoal / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 0 clf_LossonsaleofCliffsLoganCountyCoal / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |
Wabush mine related costs |  |  |  |  |  |  |  |  | (158.7)clf_Wabushminerelatedcosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | (72.7)clf_Wabushminerelatedcosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | (30.1)clf_Wabushminerelatedcosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |
Bloom Lake mine costs |  |  |  |  |  |  |  |  | (137.9)clf_BloomLakeminecosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 46.5 clf_BloomLakeminecosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 6.4 clf_BloomLakeminecosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |
Foreign Exchange Remeasurement |  |  |  |  |  |  |  |  | 30.7 clf_ForeignExchangeRemeasurement / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 64.0 clf_ForeignExchangeRemeasurement / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | (3.2)clf_ForeignExchangeRemeasurement / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |
Proxy contest and change in control costs in SG&A |  |  |  |  |  |  |  |  | (26.6)clf_ProxycontestandchangeincontrolcostsinSGA / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 0 clf_ProxycontestandchangeincontrolcostsinSGA / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 0 clf_ProxycontestandchangeincontrolcostsinSGA / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |
Litigation Judgment Costs |  |  |  |  |  |  |  |  | (96.3)clf_LitigationJudgmentCosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | (9.6)clf_LitigationJudgmentCosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | 0 clf_LitigationJudgmentCosts / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |
Severance Costs in SG&A |  |  |  |  |  |  |  |  | $ (15.8)clf_SeveranceCostsinSGA / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | $ (16.4)clf_SeveranceCostsinSGA / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |  | $ 0 clf_SeveranceCostsinSGA / clf_AdjustedEBITACalculationAxis = clf_AdjustedEBITDACalculationMember |
[1] | 1 Includes capital lease additions and non-cash accruals. Refer to NOTE 17 - CASH FLOW INFORMATION.

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