Annual report pursuant to Section 13 and 15(d)

RELATED PARTIES (Summary Of Other Ownership Interests) (Details)

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Dec. 31, 2014
Dec. 31, 2012
Hibbing [Member] | Arcelor Mittal [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 62.30%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_ArcelorMittalMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_HibbingMember 		 
Hibbing [Member] | WISCO [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 0.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_WiscoMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_HibbingMember 		 
Hibbing [Member] | U. S. Steel Canada [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 14.70%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_USSteelCanadaMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_HibbingMember 		 
Empire [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent 79.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_EmpireMember 		 
Empire [Member] | Arcelor Mittal [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 21.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_EmpireMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_ArcelorMittalMember 		 
Empire [Member] | WISCO [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_EmpireMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_WiscoMember 		 
Empire [Member] | U. S. Steel Canada [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_EmpireMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_USSteelCanadaMember 		 
Tilden [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent 85.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_TildenMember 		 
Tilden [Member] | Arcelor Mittal [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_TildenMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_ArcelorMittalMember 		 
Tilden [Member] | WISCO [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_TildenMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_WiscoMember 		 
Tilden [Member] | U. S. Steel Canada [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 15.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_TildenMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_USSteelCanadaMember 		 
Bloom Lake [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent 82.80%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BloomLakeMember 		75.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BloomLakeMember
Bloom Lake [Member] | Arcelor Mittal [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_BloomLakeMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_ArcelorMittalMember 		 
Bloom Lake [Member] | WISCO [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 17.20%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_BloomLakeMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_WiscoMember 		 
Bloom Lake [Member] | U. S. Steel Canada [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ clf_NameofOperationAxis
= clf_BloomLakeMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= clf_USSteelCanadaMember 		 
Investments in Ventures [Member] | Hibbing [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 23.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BalanceSheetLocationAxis
= clf_InvestmentsinVenturesMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= clf_HibbingMember 		[1]  
[1] 1 At December 31, 2013, the classification for Hibbing was Other liabilities.