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PROPERTY, PLANT AND EQUIPMENT (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Line Items]      
Depletion $ 173.0us-gaap_Depletion $ 206.5us-gaap_Depletion $ 209.8us-gaap_Depletion
Depreciation 320.6us-gaap_Depreciation 366.9us-gaap_Depreciation 293.5us-gaap_Depreciation
Document Fiscal Year Focus 2014    
Tangible Asset Impairment Charges 8,839.0us-gaap_TangibleAssetImpairmentCharges    
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Accumulated amount of capitalized interest included within construction in progress   31.4us-gaap_AccumulatedCapitalizedInterestCosts
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember		  
Interest costs capitalized during the period   17.4us-gaap_InterestCostsCapitalized
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember		  
Wabush [Member]      
Property, Plant and Equipment [Line Items]      
Tangible Asset Impairment Charges   145.1us-gaap_TangibleAssetImpairmentCharges
/ clf_ReportingUnitAxis
= clf_WabushMember		  
Impairment of Long-Lived Assets Held-for-use 140.1us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
/ clf_ReportingUnitAxis
= clf_WabushMember		 49.9us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
/ clf_ReportingUnitAxis
= clf_WabushMember		  
Land Rights And Mineral Rights [Member]      
Property, Plant and Equipment [Line Items]      
Tangible Asset Impairment Charges $ 5,772.7us-gaap_TangibleAssetImpairmentCharges
/ us-gaap_ImpairedLongLivedAssetsHeldAndUsedByTypeAxis
= us-gaap_MiningPropertiesAndMineralRightsMember