INCOME TAX - Income Tax Reconciliation Using Composite Statutory Income Tax Rate Applicable by Tax Jurisdiction (Details) - EUR (€) € in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of average effective tax rate and applicable tax rate [abstract]
(Loss) / income before income tax	€ (34)	€ 82	€ 222
Statutory tax rate applicable to parent company	32.00%	34.40%	25.00%
Income tax benefit / (expense) calculated at statutory tax rate	€ 11	€ (28)	€ (55)
Effect of foreign tax rate	2	3	2
Changes in recognized and unrecognized deferred tax assets	15	(10)	30
Change in laws and tax rates	0	21	0
Other	(11)	(4)	(9)
Total income tax benefit / (expense)	€ 17	€ (18)	€ (32)
Effective income tax rate	49.00%	22.00%	14.00%

Adjustments for deferred tax expense to reconcile profit (loss) to net cash flow from (used in) operating activities. [Refer: Deferred tax expense (income); Profit (loss)]

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The applicable income tax rate.

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The tax expense (income) divided by the accounting profit. [Refer: Accounting profit]

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The aggregate amount included in the determination of profit (loss) for the period in respect of current tax and deferred tax. [Refer: Current tax expense (income); Deferred tax expense (income)]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that the entity does not separately disclose in the same statement or note. [Refer: Accounting profit; Applicable tax rate]

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The profit (loss) before tax expense or income. [Refer: Profit (loss)]

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The amount that represents the difference between the tax expense (income) and the product of the accounting profit multiplied by the applicable tax rate(s) that relates to changes in the tax rate. [Refer: Accounting profit]

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