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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 $ 0us-gaap_GoodwillImpairmentLoss $ 73,500,000us-gaap_GoodwillImpairmentLoss
Other Asset Impairment Charges   562,000,000us-gaap_OtherAssetImpairmentCharges
Global Exploration Investment in Venture [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Equity Method Investments, Fair Value Disclosure   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_GlobalExplorationInvestmentinVentureMember
Equity Method Investment, Other than Temporary Impairment   9,200,000us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_GlobalExplorationInvestmentinVentureMember
Asia Pacific Iron Ore [Member]    
Fair Value, Assets And Liabilities Components [Line Items]    
Goodwill, Impairment Loss   73,500,000us-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 544,200,000clf_DebtInstrumentParValue
/ us-gaap_DebtInstrumentAxis
= clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 
Debt Instrument, Fair Value Disclosure 397,200,000us-gaap_DebtInstrumentFairValue
/ us-gaap_DebtInstrumentAxis
= clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 
Stated interest rate 7.75%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 
Debt discount 0.27clf_Debtdiscount
/ us-gaap_DebtInstrumentAxis
= clf_FivehundredFortyfourMillionSevenPointSevenFiveTwentyTwentySecondLienNotesMember
 
Debt Instrument, Unamortized Discount 147,000,000us-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   0us-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 $ 397,200,000us-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