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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) employees | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |
Entity Number of Employees | employees | 25,000 |  |
Foreign Currency Transaction Gain (Loss), Realized | $ | $ 0 | $ 0 | $ 228
Asia Pacific Iron Ore [Member] |  |  |
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |
Foreign Currency Transaction Gain (Loss), Realized | $ |  |  | $ 228
Workforce Subject to Collective Bargaining Arrangements [Member] |  |  |
Basis of Presentation and Significant Accounting Policies [Line Items] |  |  |
Entity Number of Employees | employees | 18,500 |  |

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