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FAIR VALUE OF FINANCIAL INSTRUMENTS (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Management Estimate of 62% Fe | 62.00%clf_ManagementEstimateOf62Fe |  |
Equity Method Investment, Other than Temporary Impairment | $ 0 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment | $ 0 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment | $ (365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment
Goodwill, Impairment Loss | 73.5 us-gaap_GoodwillImpairmentLoss | 80.9 us-gaap_GoodwillImpairmentLoss |
Other Asset Impairment Charges | 8,956.4 us-gaap_OtherAssetImpairmentCharges |  |
Tangible Asset Impairment Charges | 8,839.0 us-gaap_TangibleAssetImpairmentCharges |  |
Global Exploration Investment in Venture [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember |  |
Equity Method Investment, Other than Temporary Impairment | 9.2 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_GlobalExplorationInvestmentinVentureMember |  |
Amapa [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Equity Method Investments, Fair Value Disclosure |  | 0 us-gaap_EquityMethodInvestmentsFairValueDisclosure / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember |
Equity Method Investment, Other than Temporary Impairment |  | 67.6 us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember | (365.4)us-gaap_EquityMethodInvestmentOtherThanTemporaryImpairment / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = clf_AmapaMember
Asia Pacific Iron Ore [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Goodwill, Impairment Loss | 73.5 us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_AsiaPacificIronOreMember |  |
Cliffs Chromite Ontario and Cliffs Chromite Far North [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Goodwill, Impairment Loss |  | 80.9 us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_CliffsChromiteOntarioandCliffsChromiteFarNorthMember |
CQIM Reporting Unit [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Goodwill, Impairment Loss |  | 997.3 us-gaap_GoodwillImpairmentLoss / clf_ReportingUnitAxis = clf_CqimReportingUnitMember |
Wabush [Member] |  |  |
Fair Value, Assets And Liabilities Components [Line Items] |  |  |
Impairment of Long-Lived Assets Held-for-use | 140.1 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / clf_ReportingUnitAxis = clf_WabushMember | 49.9 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / clf_ReportingUnitAxis = clf_WabushMember |
Tangible Asset Impairment Charges |  | $ 145.1 us-gaap_TangibleAssetImpairmentCharges / clf_ReportingUnitAxis = clf_WabushMember |

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- Definition This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=35687916&loc=d3e32787-111569 + Details
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