Quarterly report pursuant to Section 13 or 15(d)

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

v3.10.0.1
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Schedule Of Subsidiaries) (Details)
6 Months Ended
Jun. 30, 2018
Northshore  
Related Party Transaction [Line Items]  
Entity Address, State or Province Minnesota
United Taconite  
Related Party Transaction [Line Items]  
Entity Address, State or Province Minnesota
Tilden  
Related Party Transaction [Line Items]  
Entity Address, State or Province Michigan
Empire  
Related Party Transaction [Line Items]  
Entity Address, State or Province Michigan
Koolyanobbing  
Related Party Transaction [Line Items]  
Entity Address, State or Province Western Australia