Annual report pursuant to Section 13 and 15(d)

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Statements Of Financial Performance Location Table) (Details)

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 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net $ (28.2)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet $ (51.7)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet $ 25.6us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (31.0)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet (22.1)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet 18.1us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
Australian Dollar Foreign Exchange Contract Hedge Designation [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net (13.9)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember 		(34.7)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember 		20.2us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
Australian Dollar Foreign Exchange Contract Hedge betfred reviewsDesignation [Member] | Product Revenues [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (13.2)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		(11.9)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		14.8us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember
Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net 0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember 		(12.9)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember 		6.7us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember
Canadian Dollar Foreign Exchange Contracts Hedge betfred reviewsDesignation [Member] | Cost of Sales [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember 		(8.2)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember 		3.3us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
Canadian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net (14.3)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		(4.1)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
Canadian Dollar Foreign Exchange Contracts Hedge Prior to De betfred reviewsDesignation [Member] | Cost of Sales [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (17.7)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember 		(1.9)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember 		0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
Treasury Lock [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net     (1.3)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_TreasuryLockMember
Treasury Lock [Member] | Other Nonoperating Income (Expense) [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (0.1)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_TreasuryLockMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember 		(0.1)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_TreasuryLockMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember 		0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_TreasuryLockMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Canadian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member]      
Derivative [Line Items]      
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net $ (0.5)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
/ clf_ReportingUnitAxis
= clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember