RELATED PARTIES (Summary Of Other Ownership Interests) (Details)	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%