Annual report pursuant to Section 13 and 15(d)

PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details)

v2.4.1.9
PROPERTY, PLANT AND EQUIPMENT (Value Of Each Of The Major Classes Of Consolidated Depreciable Assets) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 2,217.7us-gaap_PropertyPlantAndEquipmentGross $ 13,570.9us-gaap_PropertyPlantAndEquipmentGross  
Accumulated Depreciation and Depletion, Property, Plant and Equipment 802.8clf_AccumulatedDepreciationandDepletionPropertyPlantandEquipment 2,417.5clf_AccumulatedDepreciationandDepletionPropertyPlantandEquipment  
Property, plant and equipment, net 1,414.9us-gaap_PropertyPlantAndEquipmentNet 11,153.4us-gaap_PropertyPlantAndEquipmentNet 11,207.3us-gaap_PropertyPlantAndEquipmentNet
Land Rights And Mineral Rights [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 590.2us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_MiningPropertiesAndMineralRightsMember
7,819.6us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_MiningPropertiesAndMineralRightsMember
 
Office And Information Technology [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 75.5us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerEquipmentMember
125.7us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerEquipmentMember
 
Buildings [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 65.6us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingMember
255.2us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingMember
 
Mining Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 732.6us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_EquipmentMember
1,819.3us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_EquipmentMember
 
Processing Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 567.4us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherMachineryAndEquipmentMember
2,148.6us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherMachineryAndEquipmentMember
 
Electric Power Facilities [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 48.8us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember
114.3us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember
 
Land Improvements [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 25.5us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandImprovementsMember
69.3us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandImprovementsMember
 
Other Assets [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross 60.8us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherAssetsMember
227.6us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherAssetsMember
 
Construction in Progress [Member]      
Property, Plant and Equipment [Line Items]      
Property, plant and equipment, gross $ 51.3us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
$ 991.3us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember