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DEBT AND CREDIT FACILITIES - Debt Extinguishments (Details) - USD ($) $ in Millions |  |  |  |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 12, 2021 | Mar. 11, 2021 | Apr. 24, 2020 | Mar. 27, 2020 | Mar. 13, 2020 | Mar. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Apr. 17, 2020 | Mar. 16, 2020
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  | $ 736 |  |  |  | $ 1,513 | $ 724 | $ 227 |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 130 | (18) | (7) |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 5,595 |  |  |  |
Reduction of Principal Long-Term Debt |  |  | 181 |  |  |  |  |  |  |  |
AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 132 |  |  |  |
Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 35 |  |  |  |
Unsecured Industrial Revenue Bonds | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 7 |  |  |  |
7.625% 2021 AK Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  | $ 9 | $ 364 |  | 373 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 0 |  |  |  |
7.625% 2021 AK Senior Notes | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 34 |  |  |  |
7.625% 2021 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 34 |  |  |  |  |  |  |  |  |  |
7.50% 2023 AK Senior Notes [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  | $ 56 | $ 311 |  | 367 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 3 |  |  |  |
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 13 |  |  |  |
7.50% 2023 AK Senior Notes [Member] | AK Steel [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | 13 |  |  |  |  |  |  |  |  |  |
4.875% 2024 Senior Secured Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 6 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 1 |  |  |  |
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 395 | 400 |  |  |
4.875% 2024 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | 395 |  |  |  |  |  |  |  |  |  |
6.375% 2025 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 168 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 21 |  |  |  |
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 64 |  |  |  | $ 232
6.375% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | 64 |  |  |  |  |  |  |  |  |  |
1.50% 2025 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 20 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 1 |  |  |  |
1.50% 2025 Convertible Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 296 | 316 |  |  |
5.75% 2025 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 77 | 600 | 2 |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 16 | (13) | 0 |  |
5.75% 2025 Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 396 | 473 |  |  |
7.00% 2027 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 262 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 27 |  |  |  |
7.00% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 73 |  |  |  | $ 335
5.875% 2027 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 195 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 49 |  |  |  |
5.875% 2027 Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 556 | 750 |  |  |
6.25% 2040 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 36 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | 13 |  |  |  |
6.25% 2040 Senior Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 263 | 298 |  |  |
6.375% 2025 AK Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 9 |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  | (1) |  |  |  |
6.375% 2025 AK Senior Notes | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | 29 |  |  |  |
6.375% 2025 AK Senior Notes | AK Steel [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 29 |  |  |  |  |  |  |  |  |  |
4.875% 2021 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | 124 | 14 |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  |  | $ (5) | 0 |  |
5.90% 2020 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  |  | 89 |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  |  |  | (3) |  |
4.80% 2020 Senior Notes |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  |  | 122 |  |
Gain (loss) on extinguishment of debt |  |  |  |  |  |  |  |  | $ (4) |  |
9.875% 2025 Senior Secured Notes | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  | $ 334 |  |  |  | $ 334 |  |  |  |  |
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  | $ 555 |  |  |  | 955 |  |  | $ 400 |
9.875% 2025 Senior Secured Notes | Cleveland-Cliffs Inc. [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  | $ 322 |  |  |  |  |  |  |  |  |
Unsecured Industrial Revenue Bonds | AK Steel [Member] |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Par Value |  |  |  |  |  |  | $ 66 |  |  |  |

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