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

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- Definition Amount of accumulated depreciation and depletion for physical assets used in the normal conduct of business to produce goods and services. + References No definition available. + Details
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Namespace Prefix: | clf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
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- Details
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Balance Type: | na
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- Definition Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 13 -Subparagraph a -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 7 + Details
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- Details
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- Details
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Balance Type: |
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- Details
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- Details
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- Details
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