Annual report pursuant to Section 13 and 15(d)

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

v3.3.1.900
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2013
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  
Bloom Lake [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent   82.80%
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