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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 |  | $ 24.1 us-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 | 0 us-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 |  |  | 0.2 us-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 |  |  |  |  |  | 250.0 invest_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 | 20.0 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = clf_AustralianDollarForeignExchangeContractHedgeDesignationMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  |  |  | 220.0 invest_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 | 0 us-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 | 0 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  |  |  | 0 invest_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 | 10.1 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CustomerSupplyAgreementMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember |  | 27.7 us-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 | 34.5 us-gaap_DerivativeAssetsCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_DerivativeFinancialInstrumentsAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = clf_CustomerSupplyAgreementMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  |  |  | 63.2 us-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 | 0 us-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 | 16.2 us-gaap_DerivativeLiabilitiesCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_DerivativeFinancialInstrumentsLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreAndAsiaPacificIronOreMember |  |  |  | 9.5 us-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 |  |  | 0.3 us-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 | 41.4 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / clf_ReportingUnitAxis = us-gaap_CorporateMember |  |  |  | 183.0 invest_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 |  |  | $ 0.9 us-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.

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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Current Liabilities -URI http://asc.fasb.org/extlink&oid=6509677 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 + Details
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- Definition The estimated net amount of unrealized gains or losses on foreign currency cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994 + Details
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