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ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Loss Contingencies [Line Items] |  |
Environmental | $ 3.7 us-gaap_AccrualForEnvironmentalLossContingencies | $ 5.5 us-gaap_AccrualForEnvironmentalLossContingencies
Mine Reclamation and Closing Liability, current and noncurrent | 165.3 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent | 165.3 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
Total environmental and mine closure obligations | 169.0 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent | 170.8 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent
Less current portion | 3.4 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrent | 5.2 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrent
Long term environmental and mine closure obligations | 165.6 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityNoncurrent | 165.6 clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityNoncurrent
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member] |  |
Loss Contingencies [Line Items] |  |
Mine Reclamation and Closing Liability, current and noncurrent | 121.8 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent / us-gaap_LossContingenciesByNatureOfContingencyAxis = clf_OwnedOrOperatingFacilitiesMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember | 120.9 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent / us-gaap_LossContingenciesByNatureOfContingencyAxis = clf_OwnedOrOperatingFacilitiesMember / us-gaap_StatementBusinessSegmentsAxis = clf_USIronOreMember
Asia Pacific Iron Ore [Member] | Owned Or Operating Facilities [Member] |  |
Loss Contingencies [Line Items] |  |
Mine Reclamation and Closing Liability, current and noncurrent | 20.3 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent / us-gaap_LossContingenciesByNatureOfContingencyAxis = clf_OwnedOrOperatingFacilitiesMember / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember | 21.5 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent / us-gaap_LossContingenciesByNatureOfContingencyAxis = clf_OwnedOrOperatingFacilitiesMember / us-gaap_StatementBusinessSegmentsAxis = clf_AsiaPacificIronOreMember
LTV Steel Mining Company [Member] | Previously Owned Or Operating Facilities [Member] |  |
Loss Contingencies [Line Items] |  |
Mine Reclamation and Closing Liability, current and noncurrent | $ 23.2 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent / us-gaap_LossContingenciesByNatureOfContingencyAxis = clf_PreviouslyOwnedOrOperatingFacilitiesMember / clf_MineClosureAxis = clf_LTVSteelMiningCompanyMember | $ 22.9 clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent / us-gaap_LossContingenciesByNatureOfContingencyAxis = clf_PreviouslyOwnedOrOperatingFacilitiesMember / clf_MineClosureAxis = clf_LTVSteelMiningCompanyMember

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- Definition Amount of environmental loss contingencies, such as presence of hazardous waste, relevant information from reports issued by regulators, and estimated costs to achieve compliance with regulatory requirements. As well as, the amount of a reclamation and mine closing liability that is associated with a legal obligation for the closure and reclamation of a mine including the removal of buildings, equipment, machinery and other physical remnants of mining, closure of tailings impoundments, leach pads and other mine features, and contouring, covering and revegetation of waste rock piles and other disturbed areas. + References No definition available. + Details
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- Definition Total costs accrued as of the balance sheet date for environmental loss contingencies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346 + Details
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