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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details)
6 Months Ended
Jun. 30, 2017
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
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