FAIR VALUE OF FINANCIAL INSTRUMENTS FAIR VALUE OF FINANCIAL INSTRUMENTS (Fair Value Measurements, Nonrecurring) (Details) (USD $) In Millions, unless otherwise specified
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3 Months Ended |
12 Months Ended |
Mar. 31, 2015
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Dec. 31, 2014
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Goodwill, Impairment Loss |
$ 0us-gaap_GoodwillImpairmentLoss
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$ 73.5us-gaap_GoodwillImpairmentLoss
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Assets, Fair Value Disclosure, Nonrecurring |
397.2us-gaap_AssetsFairValueDisclosureNonrecurring
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79.4us-gaap_AssetsFairValueDisclosureNonrecurring
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Impairment of goodwill and other long-lived assets |
269.5clf_ImpairmentCharges
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644.7clf_ImpairmentCharges
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Fair Value, Inputs, Level 1 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Assets, Fair Value Disclosure, Nonrecurring |
0us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
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0us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
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Fair Value, Inputs, Level 2 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Assets, Fair Value Disclosure, Nonrecurring |
397.2us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
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0us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
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Fair Value, Inputs, Level 3 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Assets, Fair Value Disclosure, Nonrecurring |
0us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
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79.4us-gaap_AssetsFairValueDisclosureNonrecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
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Global Exploration Investment in Venture [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Equity Method Investments, Fair Value Disclosure |
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0us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
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Equity Method Investment, Other than Temporary Impairment |
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9.2us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
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Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 1 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Equity Method Investments, Fair Value Disclosure |
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0us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
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Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 2 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Equity Method Investments, Fair Value Disclosure |
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0us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
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Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 3 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Equity Method Investments, Fair Value Disclosure |
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0us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
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Asia Pacific Iron Ore [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Goodwill, Fair Value Disclosure |
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0us-gaap_GoodwillFairValueDisclosure / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Goodwill, Impairment Loss |
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73.5us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 1 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Goodwill, Fair Value Disclosure |
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0us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 2 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Goodwill, Fair Value Disclosure |
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0us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 3 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Goodwill, Fair Value Disclosure |
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0us-gaap_GoodwillFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Other Reporting Units [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember
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Tangible Asset Impairment Charges |
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11.3us-gaap_TangibleAssetImpairmentCharges / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember
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Other Reporting Units [Member] | Fair Value, Inputs, Level 1 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember
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Other Reporting Units [Member] | Fair Value, Inputs, Level 2 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember
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Other Reporting Units [Member] | Fair Value, Inputs, Level 3 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_OtherReportingUnitsMember
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Asia Pacific [Member] | Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 2 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = us-gaap_AsiaPacificMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Finite-lived Intangible Assets, Fair Value Disclosure |
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0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = us-gaap_AsiaPacificMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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72.4us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Tangible Asset Impairment Charges |
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526.5us-gaap_TangibleAssetImpairmentCharges / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Other Asset Impairment Charges |
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24.2us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Finite-lived Intangible Assets, Fair Value Disclosure |
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7.0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 1 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Finite-lived Intangible Assets, Fair Value Disclosure |
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0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member] | Fair Value, Inputs, Level 3 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Property, Plant, and Equipment, Fair Value Disclosure |
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72.4us-gaap_PropertyPlantAndEquipmentFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Finite-lived Intangible Assets, Fair Value Disclosure |
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7.0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure / clf_AssetgroupAxis = clf_AsiaPacificIronOreMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
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Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Debt Instrument, Fair Value Disclosure |
397.2us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
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Gain on new debt issued |
269.5clf_Gainonnewdebtissued / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
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Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] | Fair Value, Inputs, Level 1 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Debt Instrument, Fair Value Disclosure |
0us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
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Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] | Fair Value, Inputs, Level 2 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Debt Instrument, Fair Value Disclosure |
397.2us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
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0us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
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Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] | Fair Value, Inputs, Level 3 [Member] |
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
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Debt Instrument, Fair Value Disclosure |
$ 0us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
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