Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Annual Contributions To The Pension Plans) (Details)

v2.4.1.9
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Annual Contributions To The Pension Plans) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Company contributions $ 7.3us-gaap_DefinedBenefitPlanContributionsByEmployer $ 25.5us-gaap_DefinedBenefitPlanContributionsByEmployer
Expected company contributions, next fiscal year 6.8us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear  
Funded percentage 70.00%us-gaap_DefinedBenefitPlanFundedPercentage  
Value of VEBA Trust Assets as a Percentage of the Funding Obligation 0.9clf_ValueOfVebaTrustAssetsAsPercentageOfFundingObligation  
Pension Benefits [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Company contributions 60.5us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
53.7us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
Expected company contributions, next fiscal year 36.8us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
 
Veba Trust [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Company contributions 0us-gaap_DefinedBenefitPlanContributionsByEmployer
/ clf_OtherBenefitsAxis
= clf_VebaTrustMember
14.6us-gaap_DefinedBenefitPlanContributionsByEmployer
/ clf_OtherBenefitsAxis
= clf_VebaTrustMember
Expected company contributions, next fiscal year 0us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ clf_OtherBenefitsAxis
= clf_VebaTrustMember
 
Direct Payments [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Company contributions 7.3us-gaap_DefinedBenefitPlanContributionsByEmployer
/ clf_OtherBenefitsAxis
= clf_DirectPaymentsMember
10.9us-gaap_DefinedBenefitPlanContributionsByEmployer
/ clf_OtherBenefitsAxis
= clf_DirectPaymentsMember
Expected company contributions, next fiscal year $ 6.8us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ clf_OtherBenefitsAxis
= clf_DirectPaymentsMember