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GOODWILL AND INTANGIBLE ASSETS AND LIABILITIES - Goodwill (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 | Mar. 13, 2020 | Dec. 31, 2019
Goodwill [Line Items] |  |  |
Goodwill | $ 1,406 |  | $ 2
ArcelorMittal USA [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 1,230 |  |
AK Steel [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 174 | $ 141 |
Steelmaking [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 1,232 |  | 2
Steelmaking [Member] | ArcelorMittal USA [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill, Period Increase (Decrease) | 1,230 |  |
Other businesses |  |  |
Goodwill [Line Items] |  |  |
Goodwill | 174 |  | $ 0
Other businesses | AK Steel [Member] |  |  |
Goodwill [Line Items] |  |  |
Goodwill, Period Increase (Decrease) | $ 174 |  |

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