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Quarterly report pursuant to Section 13 or 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
Mar. 31, 2015
Dec. 31, 2014
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 1,872.0us-gaap_PropertyPlantAndEquipmentGross $ 1,870.2us-gaap_PropertyPlantAndEquipmentGross
Accumulated Depreciation and Depletion, Property, Plant and Equipment (824.8)clf_AccumulatedDepreciationandDepletionPropertyPlantandEquipment (799.7)clf_AccumulatedDepreciationandDepletionPropertyPlantandEquipment
Property, plant and equipment, net 1,047.2us-gaap_PropertyPlantAndEquipmentNet 1,070.5us-gaap_PropertyPlantAndEquipmentNet
Land Rights And Mineral Rights [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 500.5us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_MiningPropertiesAndMineralRightsMember
500.5us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_MiningPropertiesAndMineralRightsMember
Office And Information Technology [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 69.7us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerEquipmentMember
73.7us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerEquipmentMember
Buildings [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 59.8us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingMember
59.8us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_BuildingMember
Mining Equipment [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 584.9us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_EquipmentMember
585.1us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_EquipmentMember
Processing Equipment [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 512.7us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherMachineryAndEquipmentMember
510.2us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherMachineryAndEquipmentMember
Electric Power Facilities [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 44.2us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember
46.8us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ElectricGenerationEquipmentMember
Land Improvements [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 24.8us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandImprovementsMember
24.7us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_LandImprovementsMember
Other [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross 54.7us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherAssetsMember
55.0us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_OtherAssetsMember
Construction in Progress [Member] Ìý Ìý
Property, Plant and Equipment [Line Items] Ìý Ìý
Property, plant and equipment, gross $ 20.7us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember
$ 14.4us-gaap_PropertyPlantAndEquipmentGross
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ConstructionInProgressMember