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SUBSEQUENT EVENTS SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Millions |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 12, 2021 | Mar. 11, 2021 | Dec. 09, 2020 | Apr. 24, 2020 | Mar. 27, 2020 | Mar. 13, 2020 | Mar. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Feb. 17, 2021 | Feb. 11, 2021 | Jun. 19, 2020 | Apr. 17, 2020 | Mar. 16, 2020
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, issued (in shares) |  |  |  |  |  |  |  | 506,832,537 | 301,886,794 |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | $ 5,595 |  |  |  |  |  |  |
Debt Extinguished |  |  |  | $ 736 |  |  |  | 1,513 | $ 724 | $ 227 |  |  |  |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares, issued (in shares) |  |  |  |  |  |  |  |  |  |  |  | 20,000,000 |  |  |
Common Stock, Shares, Option to Purchase Additional Shares |  |  |  |  |  |  |  |  |  |  |  | 9,000,000 |  |  |
Arcelor Mittal [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Shares, Sold |  |  |  |  |  |  |  |  |  |  |  | 40,000,000 |  |  |
AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | 132 |  |  |  |  |  |  |
9.875% 2025 Senior Secured Notes | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  | $ 334 |  |  |  |  | $ 334 |  |  |  |  |  |  |  |
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  | $ 555 |  |  |  | $ 955 |  |  |  |  |  | $ 400 |
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage |  | 109.875% |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  | $ 322 |  |  |  |  |  |  |  |  |  |  |  |  |
9.875% 2025 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage |  |  |  |  |  |  |  | 109.875% |  |  |  |  |  |  |
5.875% 2027 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | $ 195 |  |  |  |  |  |  |
5.875% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | $ 556 | 750 |  |  |  |  |  |
5.875% 2027 Senior Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage |  |  |  |  |  |  |  | 105.875% |  |  |  |  |  |  |
6.75% 2026 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  | $ 725 |  | $ 845 |  |  |  |  | $ 120 |  |
6.75% 2026 Senior Secured Notes | Debt Instrument, Redemption, Period One, Upon Equity Issuance | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage |  |  |  |  |  |  |  | 106.75% |  |  |  |  |  |  |
4.625% 2029 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  |  |  |  | $ 500 |  |  |  |
4.875% 2031 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  |  |  |  | $ 500 |  |  |  |
4.875% 2024 Senior Secured Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | $ 6 |  |  |  |  |  |  |
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | 395 | $ 400 |  |  |  |  |  |
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage | 102.438% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 395 |  |  |  |  |  |  |  |  |  |  |  |  |  |
6.375% 2025 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | 168 |  |  |  |  |  |  |
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | 64 |  |  |  |  |  |  | $ 232
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage | 103.188% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 64 |  |  |  |  |  |  |  |  |  |  |  |  |  |
7.625% 2021 AK Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  | $ 9 | 364 |  | 373 |  |  |  |  |  |  |
7.625% 2021 AK Senior Notes | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | 34 |  |  |  |  |  |  |
7.625% 2021 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 34 |  |  |  |  |  |  |  |  |  |  |  |  |  |
7.50% 2023 AK Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  | $ 56 | $ 311 |  | 367 |  |  |  |  |  |  |
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | 13 |  |  |  |  |  |  |
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage | 101.875% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 13 |  |  |  |  |  |  |  |  |  |  |  |  |  |
6.375% 2025 AK Senior Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | 9 |  |  |  |  |  |  |
6.375% 2025 AK Senior Notes | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  |  | $ 29 |  |  |  |  |  |  |
6.375% 2025 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Redemption Price, Percentage | 103.188% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 29 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, shares issued, price per share |  |  |  |  |  |  |  |  |  |  |  | $ 16.12 |  |  |
ArcelorMittal USA [Member] | Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, Acquisitions |  |  | 78,186,671 |  |  |  |  | 78,000,000 |  |  |  |  |  |  |

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