Annual report pursuant to Section 13 and 15(d)

FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details)

v2.4.1.9
FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Fair Value, Assets And Liabilities Components [Line Items]      
Management Estimate of 62% Fe 62.00%clf_ManagementEstimateOf62Fe    
Equity Method Investment, Other than Temporary Impairment $ 0us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment $ 0us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment $ (365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Goodwill, Impairment Loss 73.5us-gaap_GoodwillImpairmentLoss 80.9us-gaap_GoodwillImpairmentLoss  
Other Asset Impairment Charges 8,956.4us-gaap_OtherAssetImpairmentCharges    
Tangible Asset Impairment Charges 8,839.0us-gaap_TangibleAssetImpairmentCharges    
Global Exploration Investment in Venture [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Equity Method Investments, Fair Value Disclosure 0us-gaap_EquityMethodInvestmentsFairValueDisclosure
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_GlobalExplorationInvestmentinVentureMember
   
Equity Method Investment, Other than Temporary Impairment 9.2us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_GlobalExplorationInvestmentinVentureMember
   
Amapa [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Equity Method Investments, Fair Value Disclosure   0us-gaap_EquityMethodInvestmentsFairValueDisclosure
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= 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
Asia Pacific Iron Ore [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Goodwill, Impairment Loss 73.5us-gaap_GoodwillImpairmentLoss
/ clf_ReportingUnitAxis
= clf_AsiaPacificIronOreMember
   
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Goodwill, Impairment Loss   80.9us-gaap_GoodwillImpairmentLoss
/ clf_ReportingUnitAxis
= clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember
 
CQIM Reporting Unit [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Goodwill, Impairment Loss   997.3us-gaap_GoodwillImpairmentLoss
/ clf_ReportingUnitAxis
= clf_CqimReportingUnitMember
 
Wabush [Member]      
Fair Value, Assets And Liabilities Components [Line Items]      
Impairment of Long-Lived Assets Held-for-use 140.1us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
/ clf_ReportingUnitAxis
= clf_WabushMember
49.9us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
/ clf_ReportingUnitAxis
= clf_WabushMember
 
Tangible Asset Impairment Charges   $ 145.1us-gaap_TangibleAssetImpairmentCharges
/ clf_ReportingUnitAxis
= clf_WabushMember