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SEGMENT REPORTING - Summary of Assets by Segment (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Segment Reporting Information [Line Items] |  |
Assets | $ 16,771 | $ 3,504
Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 16,088 | 2,557
Steelmaking [Member] | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 15,849 | 2,557
Other businesses | Operating Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | 239 | 0
Corporate and Other Segments [Member] |  |
Segment Reporting Information [Line Items] |  |
Assets | $ 683 | $ 947

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