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SUPPLEMENTARY FINANCIAL STATEMENT INFORMATION - Non-cash Investing and Financing Activities (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
ArcelorMittal USA [Member] |  |  |
Noncash or Part Noncash Acquisitions [Line Items] |  |  |
Other Significant Noncash Transaction, Value of Consideration Given, Pre-existing Relationship | $ 237 | $ 0 | $ 0
ArcelorMittal USA [Member] | Common Stock [Member] |  |  |
Noncash or Part Noncash Acquisitions [Line Items] |  |  |
Other Significant Noncash Transaction, Value of Consideration Given | 990 | 0 | 0
ArcelorMittal USA [Member] | Series B Preferred Stock |  |  |
Noncash or Part Noncash Acquisitions [Line Items] |  |  |
Other Significant Noncash Transaction, Value of Consideration Given | 738 | 0 | 0
AK Steel [Member] | Common Stock [Member] |  |  |
Noncash or Part Noncash Acquisitions [Line Items] |  |  |
Other Significant Noncash Transaction, Value of Consideration Given | 618 | 0 | 0
Share-based Payment Arrangement [Member] | AK Steel [Member] |  |  |
Noncash or Part Noncash Acquisitions [Line Items] |  |  |
Other Significant Noncash Transaction, Value of Consideration Given | $ 4 | $ 0 | $ 0

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- Definition The value of the noncash (or part noncash) consideration given (for example, liability, equity) in a transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of a transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4313-108586 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4332-108586 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4304-108586 + Details
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