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DEBT AND CREDIT FACILITIES - Schedule of Extinguishment of Debt (Details) - USD ($) $ in Millions |  |  |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | --- | --- | ---
Apr. 24, 2020 | Mar. 27, 2020 | Mar. 13, 2020 | Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount | $ 736.4 |  |  | $ 747.6 | $ 714.0 | $ 1,487.5 | $ 724.0
Reduction of Principal Long-Term Debt | $ 181.2 |  |  |  |  |  |
Gain (loss) on extinguishment of debt |  |  |  | 129.4 | (17.9) | 132.6 | (18.2)
Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 11.2 |  |  |
7.625% 2021 AK Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  | $ 8.5 | $ 364.2 | 0.0 |  | 372.7 |
Gain (loss) on extinguishment of debt |  |  |  | 0.0 |  | 0.4 |
7.50% 2023 AK Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  | $ 56.5 | $ 310.7 | 0.0 |  | 367.2 |
Gain (loss) on extinguishment of debt |  |  |  | 0.0 |  | 2.8 |
4.875% 2024 Senior Secured Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 5.5 |  | 5.5 |
Gain (loss) on extinguishment of debt |  |  |  | 0.5 |  | 0.5 |
6.375% 2025 Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 167.5 |  | 167.5 |
Gain (loss) on extinguishment of debt |  |  |  | 21.3 |  | 21.3 |
1.50% 2025 Convertible Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 20.0 |  | 20.0 |
Gain (loss) on extinguishment of debt |  |  |  | 1.3 |  | 1.3 |
4.875% 2021 Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  |  | 114.0 |  | 124.0
Gain (loss) on extinguishment of debt |  |  |  |  | (5.0) |  | (5.3)
5.75% 2025 Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 77.1 | 600.0 | 77.1 | 600.0
Gain (loss) on extinguishment of debt |  |  |  | 16.3 | $ (12.9) | 16.3 | $ (12.9)
7.00% 2027 Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 247.3 |  | 247.3 |
Gain (loss) on extinguishment of debt |  |  |  | 28.4 |  | 28.4 |
5.875% 2027 Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 194.5 |  | 194.5 |
Gain (loss) on extinguishment of debt |  |  |  | 48.7 |  | 48.7 |
6.25% 2040 Senior Notes [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | 35.7 |  | 35.7 |
Gain (loss) on extinguishment of debt |  |  |  | 12.9 |  | $ 12.9 |
AK Steel Holding Corporation [Member] | Unsecured Industrial Revenue Bonds [Member] |  |  |  |  |  |  |
Extinguishment of Debt [Line Items] |  |  |  |  |  |  |
Extinguishment of Debt, Amount |  |  |  | $ 7.3 |  |  |

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