| FAIR VALUE OF FINANCIAL INSTRUMENTS (Carrying Value And Fair Value Of Financial Instruments Disclosure) (Details) (USD $)
 | Mar. 31, 2015 | Dec. 31, 2014 | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Debt Instrument, Face Amount | $ 3,621,500,000us-gaap_DebtInstrumentFaceAmount |  | $ 3,980,000,000us-gaap_DebtInstrumentFaceAmount |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Long-term Debt | 2,880,900,000us-gaap_LongTermDebt |  | 2,843,300,000us-gaap_LongTermDebt |  | 
      
        | Total long-term debt, carrying value | 2,880,900,000us-gaap_LongTermDebtNoncurrent |  | 2,843,300,000us-gaap_LongTermDebtNoncurrent |  | 
      
        | $700 Million 4.875% 2021 Senior Note [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Debt Instrument, Face Amount | 700,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 |  | 700,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Long-term Debt | 422,900,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 | [1] | 689,500,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 | [1] | 
      
        | Senior Notes - $1.3 Billion [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Debt Instrument, Face Amount | 1,300,000,000.0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_SeniorNotesOnePointThreeBillionMember
 |  | 1,300,000,000.0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_SeniorNotesOnePointThreeBillionMember
 |  | 
      
        | $400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Debt Instrument, Face Amount | 400,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 |  | 400,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Long-term Debt | 325,700,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 | [2] | 393,700,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 | [2] | 
      
        | $500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Debt Instrument, Face Amount | 500,000,000.0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 |  | 500,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Long-term Debt | 433,800,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 | [3] | 477,400,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 | [3] | 
      
        | Five hundred forty million Eight point two five Twenty twenty First Lien Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Debt Instrument, Face Amount | 540,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember
 |  | 0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember
 |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Long-term Debt | 503,500,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember
 | [3] |  |  | 
      
        | Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Debt Instrument, Face Amount | 544,200,000us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 |  | 0us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Debt Instrument, Fair Value Disclosure | 397,200,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 |  |  |  | 
      
        | Long-term Debt | 397,200,000us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 | [3] |  |  | 
      
        | Revolving Credit Facility [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Revolving loan, carrying value | 0us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis
 = us-gaap_RevolvingCreditFacilityMember
 | [4] | 0us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis
 = us-gaap_RevolvingCreditFacilityMember
 | [5] | 
      
        | Interest Rate Swap [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Fair Value Adjustment to Interest Rate Hedge | 2,700,000clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis
 = us-gaap_InterestRateSwapMember
 |  | 2,800,000clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis
 = us-gaap_InterestRateSwapMember
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Total long-term debt, fair value | 2,081,700,000us-gaap_LongTermDebtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 1,614,200,000us-gaap_LongTermDebtFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | $700 Million 4.875% 2021 Senior Note [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Debt Instrument, Fair Value Disclosure | 220,000,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 367,300,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | Senior Notes - $1.3 Billion [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Debt Instrument, Fair Value Disclosure | 425,800,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_SeniorNotesOnePointThreeBillionMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 704,000,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_SeniorNotesOnePointThreeBillionMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | $400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Debt Instrument, Fair Value Disclosure | 190,500,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 228,100,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | $500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Debt Instrument, Fair Value Disclosure | 337,900,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 312,000,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | Five hundred forty million Eight point two five Twenty twenty First Lien Notes [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Debt Instrument, Fair Value Disclosure | 507,600,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 0us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Debt Instrument, Fair Value Disclosure | 397,200,000us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 0us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | Revolving Credit Facility [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Revolving loan, fair value | 0us-gaap_LinesOfCreditFairValueDisclosure / us-gaap_DebtInstrumentAxis
 = us-gaap_RevolvingCreditFacilityMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 0us-gaap_LinesOfCreditFairValueDisclosure / us-gaap_DebtInstrumentAxis
 = us-gaap_RevolvingCreditFacilityMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Fair Value Adjustment to Interest Rate Hedge | 2,700,000clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis
 = us-gaap_InterestRateSwapMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 2,800,000clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis
 = us-gaap_InterestRateSwapMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Total long-term debt, carrying value | 2,880,900,000us-gaap_LongTermDebtNoncurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 2,843,300,000us-gaap_LongTermDebtNoncurrent / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | $700 Million 4.875% 2021 Senior Note [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Senior Notes, Noncurrent | 422,900,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 689,500,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_SevenHundredMillionFourPointEightSevenFiveTwoThousandTwentyoneSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Senior Notes - $1.3 Billion [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Senior Notes, Noncurrent | 795,100,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_SeniorNotesOnePointThreeBillionMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 1,279,900,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_SeniorNotesOnePointThreeBillionMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | $400 Million 5.90% 2020 Senior Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Senior Notes, Noncurrent | 325,700,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 393,700,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FourHundredMillionFivePointNineZeroTwoThousandAndTwentySeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | $500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Senior Notes, Noncurrent | 433,800,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 477,400,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Five hundred forty million Eight point two five Twenty twenty First Lien Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Senior Notes, Noncurrent | 503,500,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 0us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FivehundredfortymillionEightpointtwofiveTwentytwentyFirstLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Senior Notes, Noncurrent | 397,200,000us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 0us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Revolving Credit Facility [Member] |  |  |  |  | 
      
        | Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  | 
      
        | Senior Notes, Noncurrent | 0us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = us-gaap_RevolvingCreditFacilityMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 0us-gaap_SeniorLongTermNotes / us-gaap_DebtInstrumentAxis
 = us-gaap_RevolvingCreditFacilityMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        | Reported Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member] |  |  |  |  | 
      
        | Long-term debt: |  |  |  |  | 
      
        | Fair Value Adjustment to Interest Rate Hedge | $ 2,700,000clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis
 = us-gaap_InterestRateSwapMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | $ 2,800,000clf_FairValueAdjustmenttoInterestRateHedge / us-gaap_DebtInstrumentAxis
 = us-gaap_InterestRateSwapMember
 / us-gaap_FairValueByFairValueHierarchyLevelAxis
 = us-gaap_FairValueInputsLevel2Member
 / us-gaap_FairValueByMeasurementBasisAxis
 = us-gaap_CarryingReportedAmountFairValueDisclosureMember
 |  | 
      
        |  | 
      
        |  |