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%