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SHARE-BASED COMPENSATION - Number and Movement of Potential Shares (Details) | 12 Months Ended
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Dec. 31, 2020 EUR (€) shares | Dec. 31, 2019 EUR (€) shares
Performance-Based RSU |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance (in shares) | 2,519,294 | 3,085,164
Potential Shares, Granted (in shares) | 1,049,839 | 1,028,342
Potential Shares, Over-performance (in shares) | 677,944 | 248,230
Potential Shares, Vested (in shares) | (1,458,985) | (1,758,062)
Potential Shares, Forfeited (in shares) | (193,765) | (84,380)
Potential Shares, Ending balance (in shares) | 2,594,327 | 2,519,294
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 12.11 | € 10.45
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 6.65 | 10.44
Weighted- Average Grant-Date Fair Value per Share, Over-performance | € | 11.52 | 8.94
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 11.52 | 7.97
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 10.94 | 8.02
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 10.17 | € 12.11
Percentage of vesting multiplier | 100.00% |
Performance based shares forfeited upon departure of certain beneficiaries (in shares) | 193,765 |
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) | 2,066,503 | 1,312,524
Potential Shares, Granted (in shares) | 910,047 | 899,926
Potential Shares, Over-performance (in shares) | 0 | 0
Potential Shares, Vested (in shares) | (589,655) | (106,000)
Potential Shares, Forfeited (in shares) | (154,984) | (39,947)
Potential Shares, Ending balance (in shares) | 2,231,911 | 2,066,503
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 8.08 | € 8.47
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 4.64 | 7.10
Weighted- Average Grant-Date Fair Value per Share, Over-performance | € | 0 | 0
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 7.50 | 4.55
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 7.37 | 8.31
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 6.88 | € 8.08
Equity Award Plans |  |
Disclosure of movement of potential shares [line items] |  |
Potential Shares, Beginning balance (in shares) | 79,526 | 57,913
Potential Shares, Granted (in shares) | 0 | 73,799
Potential Shares, Over-performance (in shares) | 0 | 0
Potential Shares, Vested (in shares) | (46,614) | (42,559)
Potential Shares, Forfeited (in shares) | 0 | (9,627)
Potential Shares, Ending balance (in shares) | 32,912 | 79,526
Weighted- Average Grant-Date Fair Value per Share, Beginning balance | € | € 8.71 | € 8.31
Weighted- Average Grant-Date Fair Value per Share, Granted | € | 0 | 8.39
Weighted- Average Grant-Date Fair Value per Share, Over-performance | € | 0 | 0
Weighted- Average Grant-Date Fair Value per Share, Vested | € | 8.94 | 7.60
Weighted- Average Grant-Date Fair Value per Share, Forfeited | € | 0 | 8.71
Weighted- Average Grant-Date Fair Value per Share, Ending balance | € | € 8.39 | € 8.71

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