Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details)
 9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
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 100.00% 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 100.00% 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