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Quarterly report pursuant to Section 13 or 15(d)

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

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RELATED PARTIES (Summary Of Other Ownership Interests) (Details) - Hibbing [Member]
Jun. 30, 2018
Dec. 31, 2017
Arcelor Mittal [Member] Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý
Ownership interest, equity method investment 62.30% Ìý
U. S. Steel Canada [Member] Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý
Ownership interest, equity method investment 14.70% Ìý
Other Noncurrent Assets [Member] Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý
Ownership interest, equity method investment 23.00% 23.00%