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SHARE-BASED COMPENSATION - Narrative (Details) € / shares in Units, € in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 shares | Jun. 30, 2021 EUR (€) shares € / shares | Jun. 30, 2020 EUR (€) shares | Jun. 30, 2018
Share Based Payment Plans [line items] |  |  |  |
Potential additional shares granted (in shares) |  | 1,149,054 |  |
Expense from share-based payment transactions with employees | € |  | € 7 | € 8 |
Antidilutive securities (in shares) | 4,880,758 |  | 4,878,787 |
PSUs |  |  |  |
Share Based Payment Plans [line items] |  |  |  |
Share based compensation, vesting period |  | 3 years |  |
Fair value at grant date (in euros) (in EUR per share) | € / shares |  | € 21.84 |  |
PSUs | Bottom of range |  |  |  |
Share Based Payment Plans [line items] |  |  |  |
Percentage of vesting multiplier |  | 0.00% |  |
PSUs | Top of range |  |  |  |
Share Based Payment Plans [line items] |  |  |  |
Percentage of vesting multiplier |  | 200.00% |  |
PSUs granted in May 2018 |  |  |  |
Share Based Payment Plans [line items] |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  | 182.90%
PSUs vested in May 2021 |  |  |  |
Share Based Payment Plans [line items] |  |  |  |
Potential additional shares granted (in shares) |  | 1,161,718 |  |
RSUs |  |  |  |
Share Based Payment Plans [line items] |  |  |  |
Potential additional shares granted (in shares) |  | 534,499 |  |
Vesting period on the grant date |  | 3 years |  |
Fair value at grant date (in euros) (in EUR per share) | € / shares |  | € 13.9 |  |

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