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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details)
9 Months Ended
Sep. 30, 2015
Northshore [Member]  
Related Party Transaction [Line Items]  
Entity Address, State or Province Minnesota
Noncontrolling Interest, Ownership Percentage by Parent 100.00%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description 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%
Segment Reporting Information, Description of Products and Services Coal