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FAIR VALUE OF FINANCIAL INSTRUMENTS FAIR VALUE OF FINANCIAL INSTRUMENTS (Impairment Charges on Financial and Nonfinancial Assets) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Impairment Loss | $ 73.5 us-gaap_GoodwillImpairmentLoss | $ 80.9 us-gaap_GoodwillImpairmentLoss |
Tangible Asset Impairment Charges | 8,839.0 us-gaap_TangibleAssetImpairmentCharges |  |
Finite-lived Intangible Assets, Fair Value Disclosure |  | 1.6 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure |
Impairment of Intangible Assets (Excluding Goodwill) | 15.5 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill | 14.5 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill |
Other Asset Impairment Charges | 8,956.4 us-gaap_OtherAssetImpairmentCharges |  |
Equity Method Investment, Other than Temporary Impairment | 0 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment | 0 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment | (365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Assets, Fair Value Disclosure, Nonrecurring | 438.9 us-gaap_AssetsFairValueDisclosureNonrecurring | 47.9 us-gaap_AssetsFairValueDisclosureNonrecurring |
Impairment Charges | 9,039.1 clf_ImpairmentCharges | 318.4 clf_ImpairmentCharges |
Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets, Fair Value Disclosure, Nonrecurring | 0 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 0 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Assets, Fair Value Disclosure, Nonrecurring | 0 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 0 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Finite-lived Intangible Assets, Fair Value Disclosure |  | 1.6 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Assets, Fair Value Disclosure, Nonrecurring | 438.9 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 47.9 us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member |
Global Exploration Investment in Venture [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember |  |
Equity Method Investment, Other than Temporary Impairment | 9.2 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember |  |
Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember |  |
Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember |  |
Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember |  |
Amapa [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember |
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
Amapa [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember |
Amapa [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember |
Amapa [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember |
Asia Pacific Iron Ore [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure | 0 us-gaap_GoodwillFairValueDisclosure / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Goodwill, Impairment Loss | 73.5 us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure | 0 us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure | 0 us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure | 0 us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure |  | 0 us-gaap_GoodwillFairValueDisclosure / clf_ReportingUnitAxis = clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Goodwill, Impairment Loss |  | 80.9 us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure |  | 0 us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure |  | 0 us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Fair Value Disclosure |  | 0 us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Wabush [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Tangible Asset Impairment Charges |  | 145.1 us-gaap_TangibleAssetImpairmentCharges / clf_ReportingUnitAxis = clf_WabushMember |
Impairment of Long-Lived Assets Held-for-use | 140.1 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / clf_ReportingUnitAxis = clf_WabushMember | 49.9 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / clf_ReportingUnitAxis = clf_WabushMember |
Impairment of Intangible Assets (Excluding Goodwill) |  | 9.5 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / clf_ReportingUnitAxis = clf_WabushMember |
Wabush [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Finite-lived Intangible Assets, Fair Value Disclosure |  | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_WabushMember |
Wabush [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Finite-lived Intangible Assets, Fair Value Disclosure |  | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_WabushMember |
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure |  | 46.3 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_ReportingUnitAxis = clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Tangible Asset Impairment Charges |  | 155.4 us-gaap_TangibleAssetImpairmentCharges / clf_ReportingUnitAxis = clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure |  | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure |  | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember |
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure |  | 46.3 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember |
CQIM Reporting Unit [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Goodwill, Impairment Loss |  | 997.3 us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_CqimReportingUnitMember |
Other Reporting Units [Member] | Other Reporting Units [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_OtherReportingUnitsMember / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember |  |
Tangible Asset Impairment Charges | 19.2 us-gaap_TangibleAssetImpairmentCharges / clf_AssetgroupAxis = clf_OtherReportingUnitsMember / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember |  |
Other Reporting Units [Member] | Other Reporting Units [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_OtherReportingUnitsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember |  |
Other Reporting Units [Member] | Other Reporting Units [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_OtherReportingUnitsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember |  |
Other Reporting Units [Member] | Other Reporting Units [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_OtherReportingUnitsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember |  |
Bloom Lake [Member] | Eastern Canadian Iron Ore [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 187.9 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Tangible Asset Impairment Charges | 7,043.7 us-gaap_TangibleAssetImpairmentCharges / clf_AssetgroupAxis = clf_BloomLakeMember / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Impairment of Intangible Assets (Excluding Goodwill) | 56.2 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / clf_AssetgroupAxis = clf_BloomLakeMember / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Bloom Lake [Member] | Eastern Canadian Iron Ore [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Bloom Lake [Member] | Eastern Canadian Iron Ore [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Bloom Lake [Member] | Eastern Canadian Iron Ore [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 187.9 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_BloomLakeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
CLCC [Member] | North American Coal [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 62.6 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_ClccMember / clf_ReportingUnitAxis = clf_NorthAmericanCoalMember |  |
Tangible Asset Impairment Charges | 195.5 us-gaap_TangibleAssetImpairmentCharges / clf_AssetgroupAxis = clf_ClccMember / clf_ReportingUnitAxis = clf_NorthAmericanCoalMember |  |
CLCC [Member] | North American Coal [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_ClccMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_NorthAmericanCoalMember |  |
CLCC [Member] | North American Coal [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_ClccMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_NorthAmericanCoalMember |  |
CLCC [Member] | North American Coal [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 62.6 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_ClccMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_NorthAmericanCoalMember |  |
Asia Pacific [Member] | Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = us-gaap_AsiaPacificMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = us-gaap_AsiaPacificMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 72.4 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Tangible Asset Impairment Charges | 526.5 us-gaap_TangibleAssetImpairmentCharges / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 7.0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Impairment of Intangible Assets (Excluding Goodwill) | 24.2 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 72.4 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 7.0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Ferroalloys [Member] | Ferroalloys [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 12.2 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Tangible Asset Impairment Charges | 259.5 us-gaap_TangibleAssetImpairmentCharges / clf_AssetgroupAxis = clf_FerroalloysMember / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Impairment of Intangible Assets (Excluding Goodwill) | 0.3 us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / clf_AssetgroupAxis = clf_FerroalloysMember / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Ferroalloys [Member] | Ferroalloys [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Ferroalloys [Member] | Ferroalloys [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Ferroalloys [Member] | Ferroalloys [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 12.2 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_FerroalloysMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_FerroalloysMember |  |
Wabush [Member] | Eastern Canadian Iron Ore [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 42.7 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_WabushMember / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 0 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_WabushMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
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Wabush [Member] | Eastern Canadian Iron Ore [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 42.7 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_WabushMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Finite-lived Intangible Assets, Fair Value Disclosure | 0 us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_WabushMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_EasternCanadianIronOreMember |  |
Oak Grove [Member] | North American Coal [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Oak Grove [Member] | North American Coal [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Property, Plant, and Equipment, Fair Value Disclosure | 23.4 us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_OakGroveMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_NorthAmericanCoalMember |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
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Data Type: | na
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