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SHARE-BASED COMPENSATION - Narrative (Details) € in Millions | 12 Months Ended
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Dec. 31, 2020 EUR (€) shares | Dec. 31, 2019 EUR (€) shares | Dec. 31, 2018 EUR (€) shares | Dec. 31, 2017 shares | Dec. 31, 2016
Share Based Payment Plans [line items] |  |  |  |  |
Expense from share-based payment transactions with employees | € | € 15 | € 16 | € 12 |  |
PSUs |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Share based compensation, vesting period | 3 years |  |  |  |
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 2016 |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares to be vested (in shares) |  | 248,230 | 433,032 | 184,469 |
PSUs granted in 2016 | Bottom of range |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Percentage of vesting multiplier | 0.00% |  |  |  |
PSUs granted in 2016 | Top of range |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Percentage of vesting multiplier | 300.00% |  |  |  |
PSUs granted in March 2016 | First testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 115.90%
PSUs granted in March 2016 | Second testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 229.90%
PSUs granted in March 2016 | Third testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 108.40%
PSUs granted in May 2016 | First testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 98.10%
PSUs granted in May 2016 | Second testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 217.20%
PSUs granted in May 2016 | Third testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 125.40%
PSUs granted in August 2016 | First testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 191.60%
PSUs granted in August 2016 | Second testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 282.20%
PSUs granted in August 2016 | Third testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 230.40%
PSUs granted in November 2016 | First testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 223.80%
PSUs granted in November 2016 | Second testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 148.70%
PSUs granted in November 2016 | Third testing period |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  |  | 286.40%
PSUs vested in March 2019 |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares granted (in shares) |  | 684,329 |  |  |
PSUs vested in May 2019 |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares granted (in shares) |  | 123,336 |  |  |
PSUs vested in August 2019 |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares granted (in shares) |  | 434,256 |  |  |
PSUs vested in November 2019 |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares granted (in shares) |  | 516,141 |  |  |
PSUs granted in July 2017 |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Total shareholder return (TSR) percentage achieved |  |  |  | 186.80% |
PSUs vested in July 2017 |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares granted (in shares) | 1,458,985 |  |  |  |
RSUs |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares granted (in shares) | 910,047 | 899,926 |  |  |
Vesting period on the grant date | 3 years |  |  |  |
RSUs | Board Members |  |  |  |  |
Share Based Payment Plans [line items] |  |  |  |  |
Potential additional shares granted (in shares) | 0 | 2 |  |  |

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