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EARNINGS PER SHARE EARNINGS PER SHARE (Narrative Details) (Details)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Earnings Per Share [Abstract]          
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount 25.2us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 12.9us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 25.2us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount    
Antidilutive Incremental Common Shares Excluded and Attributable to Share-based Payment Arrangements     0.7clf_AntidilutiveIncrementalCommonSharesExcludedandAttributabletoSharebasedPaymentArrangements 0clf_AntidilutiveIncrementalCommonSharesExcludedandAttributabletoSharebasedPaymentArrangements 0.5clf_AntidilutiveIncrementalCommonSharesExcludedandAttributabletoSharebasedPaymentArrangements