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RELATED PARTIES (Summary Of Other Ownership Interests) (Details)
Dec. 31, 2018
Dec. 31, 2017
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent 100.00%  
Hibbing [Member] | Arcelor Mittal [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 62.30%  
Hibbing [Member] | U. S. Steel Canada [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 14.70%  
Empire [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent   100.00%
Tilden [Member]    
Related Party Transaction [Line Items]    
Noncontrolling Interest, Ownership Percentage by Parent   100.00%
Investments in Ventures [Member] | Hibbing [Member]    
Related Party Transaction [Line Items]    
Ownership interest, equity method investment 23.00%