Annual report pursuant to Section 13 and 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
 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 Designation [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 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 		(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 Designation [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