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STOCK COMPENSATION PLANS STOCK COMPENSATION PLANS (Incentive Plan Details) (Details) | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of performance shares granted |  |  |  |  | 2,016,996 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod |  |
Performance Shares [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of performance shares granted | 400,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 283,530 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 124,630 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 385,585 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 1,233,685 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 806,271 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 501,346 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Estimated Forfeitures | 40,911 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardNonOptionEquityInstrumentsEstimatedForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 24,380 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardNonOptionEquityInstrumentsEstimatedForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 21,098 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardNonOptionEquityInstrumentsEstimatedForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 120,299 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardNonOptionEquityInstrumentsEstimatedForfeitures / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest | 359,089 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsExpectedtoVest / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 259,150 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsExpectedtoVest / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 103,532 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsExpectedtoVest / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | 265,286 clf_SharebasedCompensationArrangementbySharebasedPaymentAwardEquityInstrumentsOtherthanOptionsExpectedtoVest / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |  |

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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Expected to Vest + References No definition available. + Details
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- Definition Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Estimated Forfeitures + References No definition available. + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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