Quarterly report pursuant to Section 13 or 15(d)

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

v2.4.1.9
DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Statements Of Financial Performance Location Table) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Derivative [Line Items]    
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion) $ (7.1)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet $ (2.3)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (6.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet (12.8)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
Australian Dollar Foreign Exchange Contract Hedge Designation [Member]    
Derivative [Line Items]    
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion) (2.6)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember 		5.5us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Product Revenues [Member]    
Derivative [Line Items]    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (6.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		(9.1)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember
Australian Dollar Foreign Exchange Contracts Hedge Prior to De Designation [Member]    
Derivative [Line Items]    
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion) (4.5)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member]    
Derivative [Line Items]    
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion) 0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember 		(7.8)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember
Canadian Dollar Foreign Exchange Contracts Hedge Designation [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 		(3.4)us-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]    
Amount of Gain (Loss) Recognized in OCI on Derivative (Effective Portion) 0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		0us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net $ 0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		$ (0.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember