Annual report pursuant to Section 13 and 15(d)

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

v2.4.1.9
PENSIONS AND OTHER POSTRETIREMENT BENEFITS (Assumed Health Care Cost Trend Rates) (Details)
 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