Quarterly report pursuant to Section 13 or 15(d)

ENVIRONMENTAL AND MINE CLOSURE OBLIGATIONS (Summary Of Mine Closure Obligations) (Details)

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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
Loss Contingencies [Line Items]    
Environmental $ 3.7us-gaap_AccrualForEnvironmentalLossContingencies $ 5.5us-gaap_AccrualForEnvironmentalLossContingencies
Mine Reclamation and Closing Liability, current and noncurrent 165.3clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent 165.3clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
Total environmental and mine closure obligations 169.0clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent 170.8clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrentAndNoncurrent
Less current portion 3.4clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrent 5.2clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityCurrent
Long term environmental and mine closure obligations 165.6clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityNoncurrent 165.6clf_EnvironmentalLossContingencyAndMineReclamationAndClosingLiabilityNoncurrent
U.S. Iron Ore [Member] | Owned Or Operating Facilities [Member]    
Loss Contingencies [Line Items]    
Mine Reclamation and Closing Liability, current and noncurrent 121.8clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= clf_OwnedOrOperatingFacilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_USIronOreMember
120.9clf_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 20.3clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= clf_OwnedOrOperatingFacilitiesMember
/ us-gaap_StatementBusinessSegmentsAxis
= clf_AsiaPacificIronOreMember
21.5clf_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 $ 23.2clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= clf_PreviouslyOwnedOrOperatingFacilitiesMember
/ clf_MineClosureAxis
= clf_LTVSteelMiningCompanyMember
$ 22.9clf_MineReclamationAndClosingLiabilityCurrentAndNoncurrent
/ us-gaap_LossContingenciesByNatureOfContingencyAxis
= clf_PreviouslyOwnedOrOperatingFacilitiesMember
/ clf_MineClosureAxis
= clf_LTVSteelMiningCompanyMember