Annual report pursuant to Section 13 and 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)
 12 Months Ended
Dec. 31, 2017
Related Party Transaction [Line Items]  
Noncontrolling Interest, Ownership Percentage by Parent 100.00%
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%
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%
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