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RELATED PARTIES (Summary Of Other Ownership Interests) (Details) - Hibbing [Member] | Jun. 30, 2018 | Dec. 31, 2017
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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%

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- Definition The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571 + Details
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