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SHARE-BASED COMPENSATION - Number and Movement of Potential Shares (Details) | 12 Months Ended
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Dec. 31, 2021 EUR (€) shares | Dec. 31, 2020 EUR (€) shares
Performance-Based RSU |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance (in shares) | 2,594,327 | 2,519,294
Potential Shares, Granted (in shares) | 614,555 | 1,049,839
Potential Shares, Over-performance (in shares) | 526,551 | 677,944
Potential Shares, Vested (in shares) | (1,161,718) | (1,458,985)
Potential Shares, Forfeited (in shares) | (47,188) | (193,765)
Potential Shares, Ending balance (in shares) | 2,526,527 | 2,594,327
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 10.17 | € 12.11
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 21.84 | 6.65
Weighted- Average Grant-Date Fair Value per Share, Over-performance | € | 15.31 | 11.52
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 15.31 | 11.52
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 10.29 | 10.94
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 11.71 | € 10.17
Percentage of vesting multiplier | 100.00% |
Performance based shares forfeited upon departure of certain beneficiaries (in shares) | 47,188 |
Performance based shares forfeited upon non-fulfillment of performance conditions (in shares) | 0 |
Restricted Stock Units |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance (in shares) | 2,231,911 | 2,066,503
Potential Shares, Granted (in shares) | 534,499 | 910,047
Potential Shares, Over-performance (in shares) | 0 | 0
Potential Shares, Vested (in shares) | (520,064) | (589,655)
Potential Shares, Forfeited (in shares) | (97,347) | (154,984)
Potential Shares, Ending balance (in shares) | 2,148,999 | 2,231,911
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 6.88 | € 8.08
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 13.90 | 4.64
Weighted- Average Grant-Date Fair Value per Share, Over-performance | € | 0 | 0
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 10.27 | 7.50
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 7.17 | 7.37
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 7.79 | € 6.88
Equity Award Plans |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance (in shares) | 32,912 | 79,526
Potential Shares, Granted (in shares) | 0 |
Potential Shares, Over-performance (in shares) | 0 |
Potential Shares, Vested (in shares) | (32,912) | (46,614)
Potential Shares, Forfeited (in shares) | 0 |
Potential Shares, Ending balance (in shares) | 0 | 32,912
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 8.39 | € 8.71
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 0 | 0
Weighted- Average Grant-Date Fair Value per Share, Over-performance | € | 0 | 0
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 8.39 | 8.94
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 0 | 0
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 0 | € 8.39

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