Quarterly report pursuant to Section 13 or 15(d)

DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details)

v2.4.1.9
DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Jun. 30, 2014
Derivative [Line Items]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net $ (15.6)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet $ 24.1us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet    
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net (6.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet (12.8)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet    
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 0us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		(0.3)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember 		   
Interest Rate Swap [Member] | Other Nonoperating Income (Expense) [Member]        
Derivative [Line Items]        
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net   0.2us-gaap_GainLossOnFairValueHedgeIneffectivenessNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember 		   
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member]        
Derivative [Line Items]        
Derivative, Notional Amount       250.0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember
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 Contract Hedge Designation [Member] | Designated as Hedging Instrument [Member]        
Derivative [Line Items]        
Derivative, Notional Amount 20.0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember 		  220.0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember 		 
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] | Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member]        
Derivative [Line Items]        
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts (2.2)us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractHedgeDesignationMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_DesignatedAsHedgingInstrumentMember 		     
Australian Dollar Foreign Exchange Contract Prior To De Designation [Member] | Not Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member]        
Derivative [Line Items]        
Amount that will be reclassified to product revenues in the next 12 months upon settlement of the related contracts (12.5)us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_AccumulatedOtherComprehensiveIncomeMember
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_AustralianDollarForeignExchangeContractsDedesignatedMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember 		     
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 De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member]        
Derivative [Line Items]        
Derivative, Notional Amount 0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember 		  0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember 		 
Foreign Exchange Contract [Member] | Other Nonoperating Income (Expense) [Member]        
Derivative [Line Items]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (5.9)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember 		[1] (0.9)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherNonoperatingIncomeExpenseMember 		[1]    
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Product Revenues [Member]        
Derivative [Line Items]        
Amount of gain/(loss) recognized in income on derivative 10.1us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CustomerSupplyAgreementMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		27.7us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CustomerSupplyAgreementMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		   
Customer Supply Agreement [Member] | Not Designated as Hedging Instrument [Member] | Derivative Financial Instruments, Assets [Member]        
Derivative [Line Items]        
Derivative assets 34.5us-gaap_DerivativeAssetsCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_DerivativeFinancialInstrumentsAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CustomerSupplyAgreementMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember 		  63.2us-gaap_DerivativeAssetsCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_DerivativeFinancialInstrumentsAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CustomerSupplyAgreementMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember 		 
Provisional Pricing Arrangements [Member] | Product Revenues [Member]        
Derivative [Line Items]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net (16.2)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_ProvisionalPricingArrangementsMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		(2.7)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_ProvisionalPricingArrangementsMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember 		   
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Product Revenues [Member]        
Derivative [Line Items]        
Amount of gain/(loss) recognized in income on derivative (16.2)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_ProvisionalPricingArrangementsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreAndAsiaPacificIronOreMember 		(2.7)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_ProvisionalPricingArrangementsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SalesRevenueGoodsNetMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreAndAsiaPacificIronOreMember 		   
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Assets [Member]        
Derivative [Line Items]        
Derivative assets 0us-gaap_DerivativeAssetsCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_DerivativeFinancialInstrumentsAssetsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_ProvisionalPricingArrangementsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreAndAsiaPacificIronOreMember 		     
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U S Iron Ore And Asia Pacific Iron Ore [Member] | Derivative Financial Instruments, Liabilities [Member]        
Derivative [Line Items]        
Derivative liabilities 16.2us-gaap_DerivativeLiabilitiesCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_DerivativeFinancialInstrumentsLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_ProvisionalPricingArrangementsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreAndAsiaPacificIronOreMember 		  9.5us-gaap_DerivativeLiabilitiesCurrent
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_DerivativeFinancialInstrumentsLiabilitiesMember
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_ProvisionalPricingArrangementsMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreAndAsiaPacificIronOreMember 		 
Corporate [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   0.3us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember
/ clf_ReportingUnitAxis
= us-gaap_CorporateMember 		   
Corporate [Member] | Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member]        
Derivative [Line Items]        
Derivative, Notional Amount 41.4invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ clf_ReportingUnitAxis
= us-gaap_CorporateMember 		  183.0invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ clf_ReportingUnitAxis
= us-gaap_CorporateMember 		 
Corporate [Member] | Foreign Exchange Contract [Member] | Cost of Sales [Member]        
Derivative [Line Items]        
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net   $ 0.9us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ clf_ReportingUnitAxis
= us-gaap_CorporateMember 		$ (7.3)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_ForeignExchangeContractMember
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
/ clf_ReportingUnitAxis
= us-gaap_CorporateMember 		 
[1] At March 31, 2014, the location of the Gain (Loss) Recognized in Income on Derivative for Foreign Exchange Contracts was Cost of goods sold and operating expenses.