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RELATED PARTIES (Summary Of Other Ownership Interests) (Details) - Hibbing [Member]
Sep. 30, 2017
Dec. 31, 2016
Arcelor Mittal [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 62.30%  
U. S. Steel Canada [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 14.70%  
Other Noncurrent Assets [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 23.00% 23.00%