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DEBT AND CREDIT FACILITIES - Schedule of Extinguishment of Debt (Details) - USD ($) $ in Millions |  |  |  |  |  |  | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 12, 2021 | Mar. 11, 2021 | Jun. 01, 2020 | Apr. 24, 2020 | Mar. 27, 2020 | Mar. 13, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Feb. 11, 2021 | Dec. 31, 2020
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  | $ 736 |  |  | $ 421 | $ 748 | $ 1,278 | $ 1,488 |  |
Common shares, issued (in shares) |  |  |  |  |  |  | 506,832,537 |  | 506,832,537 |  | 20,000,000 | 506,832,537
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | $ (22) | 130 | $ (88) | 133 |  |
Reduction of Principal Long-Term Debt |  |  |  | $ 181 |  |  |  |  |  |  |  |
Industrial Revenue Bonds |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  | $ 7 |  |  |  |  |  |  |  |  |
9.875% 2025 Senior Secured Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  | $ 322 |  |  |  |  | 25 |  | 347 |  |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | (5) |  | (47) |  |  |
4.875% 2024 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 0 | 6 | 395 | 6 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | 0 | 1 | (14) | 1 |  |
7.625% 2021 AK Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  | $ 9 | $ 364 | 0 | 0 | 34 | 373 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | 0 | 0 | 0 | 0 |  |
7.50% Senior Secured Notes due July 2023 |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  | $ 57 | $ 311 | 0 | 0 | 13 | 367 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | 0 | 0 | 0 | 3 |  |
6.375% 2025 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 0 | 168 | 64 | 168 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | 0 | 21 | (7) | 21 |  |
6.375% 2025 AK Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 0 |  | 29 |  |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | 0 |  | (3) |  |  |
5.75% 2025 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  | 396 | 77 | 396 | 77 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  | $ (17) | 16 | $ (17) | 16 |  |
Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished | $ 535 |  |  |  |  |  |  | 12 |  |  |  |
1.50% 2025 Convertible Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | 20 |  | 20 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  |  | 1 |  | 1 |  |
7.00% 2027 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | 247 |  | 247 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  |  | 29 |  | 29 |  |
5.875% 2027 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | 194 |  | 194 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  |  | 49 |  | 49 |  |
6.25% 2040 Senior Notes |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt Extinguished |  |  |  |  |  |  |  | 36 |  | 36 |  |
Gain (Loss) on Extinguishment of Debt |  |  |  |  |  |  |  | $ 13 |  | $ 13 |  |

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