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ACQUISITIONS - ArcelorMittal USA Consideration (Details) - ArcelorMittal USA - USD ($) $ / shares in Units, $ in Millions |  | 3 Months Ended
--- | --- | ---
Dec. 09, 2020 | Mar. 31, 2021
Business Acquisition [Line Items] |  |
Fair value of settlement of a pre-existing relationship | $ 237 |
Cash consideration | 635 |
Consideration transferred | 2,600 |
Cash consideration pursuant to the AM USA Transaction Agreement | 505 |
Cash consideration for purchase of the remaining JV partner's interest of Kote and Tek | 182 |
Estimated total cash consideration receivable | (52) |
Accounts receivable | 97 |
Freestanding derivative asset from customer supply agreement | 140 |
Common Stock |  |
Business Acquisition [Line Items] |  |
Fair value of shares issued | $ 990 |
Stock issued during period, shares, acquisitions (in shares) | 78,186,671 |
Price per share of Cliffs common shares (in dollars per share) | $ 12.66 |
Series B Preferred Stock |  |
Business Acquisition [Line Items] |  |
Fair value of shares issued | $ 738 |
Stock issued during period, shares, acquisitions (in shares) | 583,273 | 583,273
Price per share of Cliffs common shares (in dollars per share) | $ 1,266 |

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