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PROPERTY, PLANT AND EQUIPMENT (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Property, Plant and Equipment [Line Items] |  |  |
Depletion | $ 173.0 us-gaap_Depletion | $ 206.5 us-gaap_Depletion | $ 209.8 us-gaap_Depletion
Depreciation | 320.6 us-gaap_Depreciation | 366.9 us-gaap_Depreciation | 293.5 us-gaap_Depreciation
Document Fiscal Year Focus | 2014 |  |
Tangible Asset Impairment Charges | 8,839.0 us-gaap_TangibleAssetImpairmentCharges |  |
Construction in Progress [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Accumulated amount of capitalized interest included within construction in progress |  | 31.4 us-gaap_AccumulatedCapitalizedInterestCosts / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember |
Interest costs capitalized during the period |  | 17.4 us-gaap_InterestCostsCapitalized / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_ConstructionInProgressMember |
Wabush [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Tangible Asset Impairment Charges |  | 145.1 us-gaap_TangibleAssetImpairmentCharges / clf_ReportingUnitAxis = clf_WabushMember |
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 |
Land Rights And Mineral Rights [Member] |  |  |
Property, Plant and Equipment [Line Items] |  |  |
Tangible Asset Impairment Charges | $ 5,772.7 us-gaap_TangibleAssetImpairmentCharges / us-gaap_ImpairedLongLivedAssetsHeldAndUsedByTypeAxis = us-gaap_MiningPropertiesAndMineralRightsMember |  |

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- Definition This is focus fiscal year of the document report in CCYY format. For a 2006 annual report, which may also provide financial information from prior periods, fiscal 2006 should be given as the fiscal year focus. Example: 2006. + References No definition available. + Details
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- Definition Amount of accumulated interest costs capitalized as part of property, plant and equipment cost basis. + References No definition available. + Details
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- Definition The noncash expense charged against earnings to recognize the consumption of natural resources. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Definition The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 958 -SubTopic 225 -Section 45 -Paragraph 11 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28367520&loc=d3e92212-112881 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Impairment -URI http://asc.fasb.org/extlink&oid=6515133 + Details
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- Definition Amount of interest capitalized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Allowance for Funds Used during Construction -URI http://asc.fasb.org/extlink&oid=6504829 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 + Details
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