Quarterly report pursuant to Section 13 or 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details)

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3 Months Ended
Mar. 31, 2015
Northshore [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Minnesota
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_NorthshoreMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Iron Ore
United Taconite [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Minnesota
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_UnitedTaconiteMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Iron Ore
Wabush [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Newfoundland and Labrador/ Quebec, Canada
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_WabushMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Iron Ore
Bloom Lake [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Quebec, Canada
Noncontrolling Interest, Ownership Percentage by Parent 82.80%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BloomLakeMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Iron Ore
Tilden [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Michigan
Noncontrolling Interest, Ownership Percentage by Parent 85.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_TildenMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Iron Ore
Empire [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Michigan
Noncontrolling Interest, Ownership Percentage by Parent 79.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_EmpireMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Iron Ore
Koolyanobbing [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Western Australia
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_KoolyanobbingMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Iron Ore
Pinnacle [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province West Virginia
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_PinnacleMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Coal
Oak Grove [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Alabama
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_OakGroveMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Coal
Black Label Deposit [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Ontario, Canada
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BlackLabelDepositMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Chromite
Black Thor Deposit [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Ontario, Canada
Noncontrolling Interest, Ownership Percentage by Parent 100.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BlackThorDepositMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Chromite
Big Daddy Chromite Deposit [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Ontario, Canada
Noncontrolling Interest, Ownership Percentage by Parent 70.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ clf_NameofOperationAxis
= clf_BigDaddyChromiteDepositMember
Segment Reporting Information, betfred reviewsDescription of Products and Services Chromite