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SUBSEQUENT EVENTS (Details) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2021 EUR (€) | Dec. 31, 2020 EUR (€) | Dec. 31, 2019 EUR (€) | Dec. 31, 2018 EUR (€) | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Disclosure of non-adjusting events after reporting period [line items] |  |  |  |  |  |
Nominal value |  | € 2,386,000,000 |  |  |  |
Repurchase of senior notes |  | 200,000,000 | € 100,000,000 | € 0 |  |
Senior Unsecured Notes due 2025 |  |  |  |  |  |
Disclosure of non-adjusting events after reporting period [line items] |  |  |  |  |  |
Nominal value |  | € 530,000,000 |  |  |  | $ 650,000,000
Nominal rate |  | 6.625% |  |  |  |
Offering of Senior Notes | Constellium Senior Sustainability-Linked Notes issued February 2021 due February 2029 |  |  |  |  |  |
Disclosure of non-adjusting events after reporting period [line items] |  |  |  |  |  |
Nominal value | $ |  |  |  |  | $ 500,000,000 |
Nominal rate | 3.75% |  |  |  |  |
Redemption of Senior Notes | Senior Unsecured Notes due 2025 |  |  |  |  |  |
Disclosure of non-adjusting events after reporting period [line items] |  |  |  |  |  |
Nominal rate | 6.625% |  |  |  |  |
Repurchase of senior notes | € 650,000,000 |  |  |  |  |

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