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FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Fair Value, Assets And Liabilities Components [Line Items] |  |
Management Estimate of 62% Fe | 62.00%clf_ManagementEstimateOf62Fe |
Goodwill, Impairment Loss | $ 0 us-gaap_GoodwillImpairmentLoss | $ 73,500,000 us-gaap_GoodwillImpairmentLoss
Other Asset Impairment Charges |  | 562,000,000 us-gaap_OtherAssetImpairmentCharges
Global Exploration Investment in Venture [Member] |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |
Equity Method Investments, Fair Value Disclosure |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
Equity Method Investment, Other than Temporary Impairment |  | 9,200,000 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
Asia Pacific Iron Ore [Member] |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |
Goodwill, Impairment Loss |  | 73,500,000 us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember
Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |
Debt Instrument, Par Value | 544,200,000 clf_DebtInstrumentParValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |
Debt Instrument, Fair Value Disclosure | 397,200,000 us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |
Stated interest rate | 7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |
Debt discount | 0.27 clf_Debtdiscount / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |
Debt Instrument, Unamortized Discount | 147,000,000 us-gaap_DebtInstrumentUnamortizedDiscount / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember |
Fair Value, Inputs, Level 2 [Member] | Global Exploration Investment in Venture [Member] |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |
Equity Method Investments, Fair Value Disclosure |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember
Fair Value, Inputs, Level 2 [Member] | Five hundred Forty-four Million Seven Point Seven Five Twenty Twenty Second Lien Notes [Member] |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |
Debt Instrument, Fair Value Disclosure | $ 397,200,000 us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | $ 0 us-gaap_DebtInstrumentFairValue / us-gaap_DebtInstrumentAxis = clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member

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