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ACQUISITIONS - Narrative (Details) $ in Millions |  |  | 10 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 09, 2020 USD ($) | Mar. 13, 2020 | Dec. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |
Revenues |  |  |  | $ 5,354 | $ 1,990 | $ 2,332
Net income (loss) attributable to Cliffs shareholders |  |  |  | (122) | 293 | 1,128
Amortization of Inventory Step-up |  |  |  | 96 | 0 | 0
Severance Costs |  |  |  | 38 | 2 | 0
Business Combination, Acquisition Related Costs |  |  |  | 90 | $ 7 | $ 0
ArcelorMittal USA [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Revenues | $ 446 |  |  |  |  |
Net income (loss) attributable to Cliffs shareholders | (40) |  |  |  |  |
Amortization of Inventory Step-up | $ 21 |  |  |  |  |
Business Combination, Acquisition Related Costs |  |  |  | 26 |  |
AK Steel [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Revenues |  |  | $ 3,573 |  |  |
Net income (loss) attributable to Cliffs shareholders |  |  | (302) |  |  |
Amortization of Inventory Step-up |  |  | 74 |  |  |
Severance Costs |  |  | $ 35 |  |  |
Business Combination, Acquisition Related Costs |  |  |  | $ 26 |  |
Business Acquisition, Shares Exchange Ratio |  | 0.400 |  |  |  |
AK Steel [Member] | Common Stock [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business Acquisition, Shares Exchange Ratio |  |  |  | 0.400 |  |

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