Annual report pursuant to Section 13 and 15(d)

FAIR VALUE OF FINANCIAL INSTRUMENTS FAIR VALUE OF FINANCIAL INSTRUMENTS (Impairment Charges on Financial and Nonfinancial Assets) (Details)

Betfred-casino review_Equipment, Fair Value v2.4.1.9
betfred sportsbook
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Impairment Loss $ 73.5us-gaap_GoodwillImpairmentLoss $ 80.9us-gaap_GoodwillImpairmentLoss  
Tangible Asset Impairment Charges 8,839.0us-gaap_TangibleAssetImpairmentCharges    
Finite-lived Intangible Assets, Fair Value Disclosure   1.6us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure  
Impairment of Intangible Assets (Excluding Goodwill) 15.5us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill 14.5us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill  
Other Asset Impairment Charges 8,956.4us-gaap_OtherAssetImpairmentCharges    
Equity Method Investment, Other than Temporary Impairment 0us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment 0us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment (365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Assets, Fair Value Disclosure, Nonrecurring 438.9us-gaap_AssetsFairValueDisclosureNonrecurring 47.9us-gaap_AssetsFairValueDisclosureNonrecurring  
Impairment Charges 9,039.1clf_ImpairmentCharges 318.4clf_ImpairmentCharges  
Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets, Fair Value Disclosure, Nonrecurring 0us-gaap_AssetsFairValueDisclosureNonrecurring
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0us-gaap_AssetsFairValueDisclosureNonrecurring
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Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Assets, Fair Value Disclosure, Nonrecurring 0us-gaap_AssetsFairValueDisclosureNonrecurring
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0us-gaap_AssetsFairValueDisclosureNonrecurring
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Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Finite-lived Intangible Assets, Fair Value Disclosure   1.6us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
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Assets, Fair Value Disclosure, Nonrecurring 438.9us-gaap_AssetsFairValueDisclosureNonrecurring
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= us-gaap_FairValueInputsLevel3Member
47.9us-gaap_AssetsFairValueDisclosureNonrecurring
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
 
Global Exploration Investment in Venture [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure 0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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= clf_GlobalExplorationInvestmentinVentureMember
   
Equity Method Investment, Other than Temporary Impairment 9.2us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
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= clf_GlobalExplorationInvestmentinVentureMember
   
Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure 0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure 0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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= clf_GlobalExplorationInvestmentinVentureMember
   
Global Exploration Investment in Venture [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure 0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_GlobalExplorationInvestmentinVentureMember
   
Amapa [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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= clf_AmapaMember
 
Equity Method Investment, Other than Temporary Impairment   67.6us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_AmapaMember
(365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_AmapaMember
Amapa [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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Amapa [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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Amapa [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Equity Method Investments, Fair Value Disclosure   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
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= clf_AmapaMember
 
Asia Pacific Iron Ore [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure 0us-gaap_GoodwillFairValueDisclosure
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Goodwill, Impairment Loss 73.5us-gaap_GoodwillImpairmentLoss
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Asia Pacific Iron BetfredOre [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure 0us-gaap_GoodwillFairValueDisclosure
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/ clf_ReportingUnitAxis
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Asia Pacific Iron BetfredOre [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure 0us-gaap_GoodwillFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Asia Pacific Iron BetfredOre [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure 0us-gaap_GoodwillFairValueDisclosure
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= us-gaap_FairValueInputsLevel3Member
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure   0us-gaap_GoodwillFairValueDisclosure
/ clf_ReportingUnitAxis
= clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
Goodwill, Impairment Loss   80.9us-gaap_GoodwillImpairmentLoss
/ clf_ReportingUnitAxis
= clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure   0us-gaap_GoodwillFairValueDisclosure
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/ clf_ReportingUnitAxis
= clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure   0us-gaap_GoodwillFairValueDisclosure
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/ clf_ReportingUnitAxis
= clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Fair Value Disclosure   0us-gaap_GoodwillFairValueDisclosure
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= us-gaap_FairValueInputsLevel3Member
/ clf_ReportingUnitAxis
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Wabush [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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
 
Impairment of Intangible Assets (Excluding Goodwill)   9.5us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
/ clf_ReportingUnitAxis
= clf_WabushMember
 
Wabush [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Finite-lived Intangible Assets, Fair Value Disclosure   0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
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Wabush [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Finite-lived Intangible Assets, Fair Value Disclosure   0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
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/ clf_ReportingUnitAxis
= clf_WabushMember
 
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure   46.3us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_ReportingUnitAxis
= clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
Tangible Asset Impairment Charges   155.4us-gaap_TangibleAssetImpairmentCharges
/ clf_ReportingUnitAxis
= clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure   0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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= clf_WabushandCliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure   0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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/ clf_ReportingUnitAxis
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Wabush and Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure   46.3us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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CQIM Reporting Unit [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Goodwill, Impairment Loss   997.3us-gaap_GoodwillImpairmentLoss
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Other Reporting Units [Member] | Other Reporting Units [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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Tangible Asset Impairment Charges 19.2us-gaap_TangibleAssetImpairmentCharges
/ clf_AssetgroupAxis
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/ clf_ReportingUnitAxis
= clf_OtherReportingUnitsMember
   
Other Reporting Units [Member] | Other Reporting Units [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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/ clf_ReportingUnitAxis
= clf_OtherReportingUnitsMember
   
Other Reporting Units [Member] | Other Reporting Units [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ clf_ReportingUnitAxis
= clf_OtherReportingUnitsMember
   
Other Reporting Units [Member] | Other Reporting Units [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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/ clf_ReportingUnitAxis
= clf_OtherReportingUnitsMember
   
Bloom Lake [Member] | Eastern Canadian Iron Ore [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 187.9us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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Tangible Asset Impairment Charges 7,043.7us-gaap_TangibleAssetImpairmentCharges
/ clf_AssetgroupAxis
= clf_BloomLakeMember
/ clf_ReportingUnitAxis
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
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/ clf_ReportingUnitAxis
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Impairment of Intangible Assets (Excluding Goodwill) 56.2us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
/ clf_AssetgroupAxis
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/ clf_ReportingUnitAxis
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Bloom Lake [Member] | Eastern Canadian Iron BetfredOre [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
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/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Bloom Lake [Member] | Eastern Canadian Iron BetfredOre [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
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= us-gaap_FairValueInputsLevel2Member
/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Bloom Lake [Member] | Eastern Canadian Iron BetfredOre [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 187.9us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
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/ clf_ReportingUnitAxis
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CLCC [Member] | North American Coal [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 62.6us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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Tangible Asset Impairment Charges 195.5us-gaap_TangibleAssetImpairmentCharges
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CLCC [Member] | North American Coal [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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CLCC [Member] | North American Coal [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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CLCC [Member] | North American Coal [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 62.6us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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/ clf_ReportingUnitAxis
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Asia Pacific [Member] | Asia Pacific Iron BetfredOre [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Asia Pacific Iron Ore [Member] | Asia Pacific Iron Ore [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 72.4us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Tangible Asset Impairment Charges 526.5us-gaap_TangibleAssetImpairmentCharges
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 7.0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Impairment of Intangible Assets (Excluding Goodwill) 24.2us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Asia Pacific Iron Ore [Member] | Asia Pacific Iron BetfredOre [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Asia Pacific Iron Ore [Member] | Asia Pacific Iron BetfredOre [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 72.4us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 7.0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_AsiaPacificIronOreMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Ferroalloys [Member] | Ferroalloys [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 12.2us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Tangible Asset Impairment Charges 259.5us-gaap_TangibleAssetImpairmentCharges
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Impairment of Intangible Assets (Excluding Goodwill) 0.3us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Ferroalloys [Member] | Ferroalloys [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Ferroalloys [Member] | Ferroalloys [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Ferroalloys [Member] | Ferroalloys [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 12.2us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_FerroalloysMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ clf_ReportingUnitAxis
= clf_FerroalloysMember
   
Wabush [Member] | Eastern Canadian Iron Ore [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 42.7us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_WabushMember
/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Tangible Asset Impairment Charges 132.6us-gaap_TangibleAssetImpairmentCharges
/ clf_AssetgroupAxis
= clf_WabushMember
/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
= clf_WabushMember
/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Impairment of Intangible Assets (Excluding Goodwill) 36.7us-gaap_ImpairmentOfIntangibleAssetsExcludingGoodwill
/ clf_AssetgroupAxis
= clf_WabushMember
/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Wabush [Member] | Eastern Canadian Iron BetfredOre [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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Wabush [Member] | Eastern Canadian Iron BetfredOre [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Wabush [Member] | Eastern Canadian Iron BetfredOre [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 42.7us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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Finite-lived Intangible Assets, Fair Value Disclosure 0us-gaap_FiniteLivedIntangibleAssetsFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
= clf_EasternCanadianIronOreMember
   
Oak Grove [Member] | North American Coal [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 23.4us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
= clf_OakGroveMember
/ clf_ReportingUnitAxis
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Tangible Asset Impairment Charges 267.5us-gaap_TangibleAssetImpairmentCharges
/ clf_AssetgroupAxis
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/ clf_ReportingUnitAxis
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Oak Grove [Member] | North American Coal [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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Oak Grove [Member] | North American Coal [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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Oak Grove [Member] | North American Coal [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 23.4us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ clf_ReportingUnitAxis
= clf_NorthAmericanCoalMember
   
Pinnacle [Member] | North American Coal [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 30.7us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ clf_ReportingUnitAxis
= clf_NorthAmericanCoalMember
   
Tangible Asset Impairment Charges 394.5us-gaap_TangibleAssetImpairmentCharges
/ clf_AssetgroupAxis
= clf_PinnacleMember
/ clf_ReportingUnitAxis
= clf_NorthAmericanCoalMember
   
Pinnacle [Member] | North American Coal [Member] | Fair Value, Inputs, Level 1 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ clf_ReportingUnitAxis
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Pinnacle [Member] | North American Coal [Member] | Fair Value, Inputs, Level 2 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure 0us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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Pinnacle [Member] | North American Coal [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Property, Plant, and Equipment, Fair Value Disclosure $ 30.7us-gaap_PropertyPlantAndEquipmentFairValueDisclosure
/ clf_AssetgroupAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ clf_ReportingUnitAxis
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