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DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES (Narrative) (Details) (USD $) | 12 Months Ended | 9 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2013
Derivative [Line Items] |  |  |  |
Current Fiscal Year End Date | --12-31 |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | $ (28,200,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet | $ (51,700,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet | $ 25,600,000 us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet |
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net | (31,000,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet | (22,100,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet | 18,100,000 us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 159,100,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | 141,400,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet | 163,200,000 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet |
Accounts receivable, net | 205,600,000 us-gaap_AccountsReceivableNetCurrent | 270,000,000 us-gaap_AccountsReceivableNetCurrent |  |
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 | (14,300,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember | (4,100,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember | 0 us-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 | (17,700,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember | (1,900,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember | 0 us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember / 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 |  |  |
Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [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 | 0 us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months / us-gaap_BalanceSheetLocationAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  |  |
Foreign Exchange Contract [Member] | Cost of Sales [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | (16,900,000)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember |  | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember |
Australian Dollar Foreign Exchange Contract Hedge Designation [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | (13,900,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_AustralianDollarForeignExchangeContractHedgeDesignationMember | (34,700,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_AustralianDollarForeignExchangeContractHedgeDesignationMember | 20,200,000 us-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 | (13,200,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_AustralianDollarForeignExchangeContractHedgeDesignationMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember | (11,900,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_AustralianDollarForeignExchangeContractHedgeDesignationMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember | 14,800,000 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 | 220,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = clf_AustralianDollarForeignExchangeContractHedgeDesignationMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember | 323,000,000 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 | (15,100,000)us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months / us-gaap_BalanceSheetLocationAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember / us-gaap_DerivativeInstrumentRiskAxis = clf_AustralianDollarForeignExchangeContractHedgeDesignationMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  |  |
Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net | 0 us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember | (12,900,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember | 6,700,000 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 | 0 us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember | (8,200,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember | 3,300,000 us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember |
Canadian Dollar Foreign Exchange Contracts Hedge Designation [Member] | Designated as Hedging Instrument [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount | 0 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember | 285,900,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDesignationMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  |
Interest Rate Swap [Member] | Other Nonoperating Income (Expense) [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net | 300,000 us-gaap_GainLossOnFairValueHedgeIneffectivenessNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember |  |  | 100,000 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 | 250,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  |  |
Treasury Lock [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  |  | (1,300,000)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 | (100,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember | (100,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember | 0 us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember | 0 us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember | (400,000)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember |
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months | (100,000)us-gaap_CashFlowHedgeGainLossToBeReclassifiedWithinTwelveMonths / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_IncomeStatementLocationAxis = us-gaap_OtherNonoperatingIncomeExpenseMember |  |  |
Treasury Lock [Member] | Designated as Hedging Instrument [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount |  |  | 200,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |
Treasury Lock [Member] | Designated as Hedging Instrument [Member] | Accumulated Other Comprehensive Income (Loss) [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net |  | (1,300,000)us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet / us-gaap_BalanceSheetLocationAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_TreasuryLockMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  |
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 | 187,800,000 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CustomerSupplyAgreementMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember | 149,200,000 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CustomerSupplyAgreementMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember | 171,400,000 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] | Other Current Assets [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative assets | 63,200,000 us-gaap_DerivativeAssetsCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = clf_CustomerSupplyAgreementMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 55,800,000 us-gaap_DerivativeAssetsCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / 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 | (11,800,000)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember | (7,200,000)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember | (7,800,000)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, Eastern Canadian Iron Ore And Asia Pacific Iron Ore [Member] | Product Revenues [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Amount of gain/(loss) recognized in income on derivative | (11,800,000)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreEasternCanadianIronOreAndAsiaPacificIronOreMember | (7,200,000)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreEasternCanadianIronOreAndAsiaPacificIronOreMember |  |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U.S. Iron Ore, Eastern Canadian Iron Ore And Asia Pacific Iron Ore [Member] | Other Current Assets [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative assets | 0 us-gaap_DerivativeAssetsCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreEasternCanadianIronOreAndAsiaPacificIronOreMember | 3,100,000 us-gaap_DerivativeAssetsCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreEasternCanadianIronOreAndAsiaPacificIronOreMember |  |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U.S. Iron Ore, Eastern Canadian Iron Ore And Asia Pacific Iron Ore [Member] | Other Current Liabilities [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative liabilities | 11,800,000 us-gaap_DerivativeLiabilitiesCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreEasternCanadianIronOreAndAsiaPacificIronOreMember | 10,300,000 us-gaap_DerivativeLiabilitiesCurrent / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherCurrentLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreEasternCanadianIronOreAndAsiaPacificIronOreMember |  |
Provisional Pricing Arrangements [Member] | Not Designated as Hedging Instrument [Member] | U.S. Iron Ore and Eastern Canadian Iron Ore [Member] | Product Revenues [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Amount of gain/(loss) recognized in income on derivative |  |  | (7,800,000)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = clf_ProvisionalPricingArrangementsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / us-gaap_IncomeStatementLocationAxis = us-gaap_SalesRevenueGoodsNetMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreAndEasternCanadianIronOreMember |
$500 Million 3.95% 2018 Senior Notes [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Debt Instrument, Face Amount | 500,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember | 500,000,000.0 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = clf_FiveHundredMillionThreePointNineFiveTwoThousandEighteenSeniorNoteMember |  |
Bloom Lake [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 | (9,900,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember / clf_ReportingUnitAxis = clf_BloomLakeMember |  |  |
Bloom Lake [Member] | Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount | 183,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / clf_ReportingUnitAxis = clf_BloomLakeMember |  |  |
Bloom Lake [Member] | Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [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 | 0 us-gaap_ForeignCurrencyCashFlowHedgeGainLossToBeReclassifiedDuringNext12Months / us-gaap_BalanceSheetLocationAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgeDeDesignatedMemberMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember / clf_ReportingUnitAxis = clf_BloomLakeMember |  |  |
Bloom Lake [Member] | Foreign Exchange Contract [Member] | Cost of Sales [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net | (7,300,000)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / us-gaap_IncomeStatementLocationAxis = us-gaap_CostOfSalesMember / clf_ReportingUnitAxis = clf_BloomLakeMember |  |  |
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 | (500,000)us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet / us-gaap_DerivativeInstrumentRiskAxis = clf_CanadianDollarForeignExchangeContractsHedgePriortoDeDesignationMember / clf_ReportingUnitAxis = clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember |  |  |
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Canadian Dollar Foreign Exchange Contracts Hedge De Designated [Member] [Member] | Not Designated as Hedging Instrument [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
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