Annual report pursuant to Section 13 and 15(d)

PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Assumed Health Care Cost Trend Rates) (Details)

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12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Ultimate health care cost trend rate 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate 5.00%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate
United States [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Health care cost trend rate assumed for next year 7.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ clf_RegionReportingInformationByRegionAxis
= clf_UnitedStatesMember
7.25%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ clf_RegionReportingInformationByRegionAxis
= clf_UnitedStatesMember
Canada [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Health care cost trend rate assumed for next year 4.25%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ clf_RegionReportingInformationByRegionAxis
= clf_CanadaMember
4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ clf_RegionReportingInformationByRegionAxis
= clf_CanadaMember