Annual report pursuant to Section 13 and 15(d)

INCOME TAXES INCOME TAXES (Schedule of Unrecognized Tax Benefits Rollforward) (Details)

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12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2010
Income Tax Contingency [Line Items]        
Unrecognized Tax Benefits $ 74.7us-gaap_UnrecognizedTaxBenefits $ 74.4us-gaap_UnrecognizedTaxBenefits $ 55.5us-gaap_UnrecognizedTaxBenefits $ 102.1us-gaap_UnrecognizedTaxBenefits
Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions 3.4us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 13.6us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions 2.7us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions  
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions 2.5us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 5.3us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions 11.1us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions  
Unrecognized Tax Benefits Increases Decreases Resulting From Currency Translation (0.2)clf_UnrecognizedTaxBenefitsIncreasesDecreasesResultingFromCurrencyTranslation 0clf_UnrecognizedTaxBenefitsIncreasesDecreasesResultingFromCurrencyTranslation 0clf_UnrecognizedTaxBenefitsIncreasesDecreasesResultingFromCurrencyTranslation  
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities (0.5)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities 0us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities (60.4)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities  
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations (3.7)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations 0us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations 0us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations  
Other Increases decreases to unrecognized tax benefits $ (1.2)clf_OtherIncreasesDecreasesToUnrecognizedTaxBenefits $ 0clf_OtherIncreasesDecreasesToUnrecognizedTaxBenefits $ 0clf_OtherIncreasesDecreasesToUnrecognizedTaxBenefits