Annual report pursuant to Section 13 and 15(d)

INCOME TAXES (Narrative) (Details)

v2.4.1.9
INCOME TAXES (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2010
Income Tax Contingency [Line Items]        
Unrecognized Tax Benefits $ 74,700,000us-gaap_UnrecognizedTaxBenefits $ 74,400,000us-gaap_UnrecognizedTaxBenefits $ 55,500,000us-gaap_UnrecognizedTaxBenefits $ 102,100,000us-gaap_UnrecognizedTaxBenefits
Undistributed Earnings of Foreign Subsidiaries 0us-gaap_UndistributedEarningsOfForeignSubsidiaries 1,200,000,000us-gaap_UndistributedEarningsOfForeignSubsidiaries    
Deferred Tax Assets, Operating Loss Carryforwards, State and Local 1,900,000,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal 157,900,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal    
Deferred Tax Assets, Operating Loss Carryforwards, Foreign 6,000,000,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign 3,500,000,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign    
Future Economic Realizable Benefit 108,100,000clf_FutureEconomicRealizableBenefit      
Deferred Tax Assets, Tax Credit Carryforwards, Foreign 5,800,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign 5,800,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign    
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax 267,700,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax 299,200,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax    
Income Tax Effects Allocated Directly To Equity, Equity Transactions, Acquisition of Noncontrolling Interest 0clf_IncomeTaxEffectsAllocatedDirectlyToEquityEquityTransactionsAcquisitionofNoncontrollingInterest 102,100,000clf_IncomeTaxEffectsAllocatedDirectlyToEquityEquityTransactionsAcquisitionofNoncontrollingInterest 0clf_IncomeTaxEffectsAllocatedDirectlyToEquityEquityTransactionsAcquisitionofNoncontrollingInterest  
Valuation Allowance, Deferred Tax Asset, Change in Amount 1,361,000,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount      
Document Fiscal Year Focus 2014      
Effective Income Tax Rate, Continuing Operations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 35.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations  
Current Fiscal Year End Date --12-31      
Reversal of Valuation Allowance on MRRT credits [Member]        
Income Tax Contingency [Line Items]        
Valuation Allowance, Deferred Tax Asset, Change in Amount 291,000,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= clf_ReversalofValuationAllowanceonMRRTcreditsMember 		     
Deferred Tax Asset, Canadian Deferred Tax Asset that will not be realized [Member]        
Income Tax Contingency [Line Items]        
Valuation Allowance, Deferred Tax Asset, Change in Amount 1,273,200,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= clf_DeferredTaxAssetForeignDeferredTaxAssetthatwillnotberealizedMember 		     
Deferred Tax Asset, Alternative Minimum Tax Credit Not Utilized [Member]        
Income Tax Contingency [Line Items]        
Valuation Allowance, Deferred Tax Asset, Change in Amount 28,600,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= clf_DeferredTaxAssetAlternativeMinimumTaxCreditNotUtilizedMember 		     
Deferred Ordinary Losses [Member]        
Income Tax Contingency [Line Items]        
Valuation Allowance, Deferred Tax Asset, Change in Amount 18,200,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= clf_DeferredOrdinaryLossesMember 		     
Deferred Tax Asset, Domestic Deferred Tax Asset that will not be realized [Member]        
Income Tax Contingency [Line Items]        
Valuation Allowance, Deferred Tax Asset, Change in Amount 402,500,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= clf_DeferredTaxAssetDomesticDeferredTaxAssetthatwillnotberealizedMember 		     
Deferred Tax Assets [Member]        
Income Tax Contingency [Line Items]        
Unrecognized Tax Benefits 49,500,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_BalanceSheetLocationAxis
= clf_DeferredTaxAssetsMember 		48,500,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_BalanceSheetLocationAxis
= clf_DeferredTaxAssetsMember 		   
Other Liabilities [Member]        
Income Tax Contingency [Line Items]        
Unrecognized Tax Benefits 25,200,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember 		25,900,000us-gaap_UnrecognizedTaxBenefits
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember 		   
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued 2,100,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember 		1,200,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember 		   
Deferred Tax Asset, Alternative Minimum Tax Credit Not Utilized [Member]        
Income Tax Contingency [Line Items]        
Deferred Tax Assets, Operating Loss Carryforwards, Foreign $ 540,700,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign
/ clf_IncomeTaxDescriptionAxis
= clf_DeferredTaxAssetAlternativeMinimumTaxCreditNotUtilizedMember