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RELATED PARTIES - Narrative (Details) - Hibbing [Member] | Dec. 31, 2020 | Dec. 09, 2020 | Dec. 31, 2019
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Related Party Transaction [Line Items] |  |  |
Ownership interest, equity method investment | 85.30% | 62.30% | 23.00%
ArcelorMittal USA [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% |  |

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