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Income Tax - Summary of Reconciliation Between Reported Tax Expense and Theoretical Tax Expense Loss Before Taxes (Details) - EUR (€)
€ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Reconciliation Of Average Effective Tax Rate And Applicable Tax Rate [Abstract]      
Loss before tax € (173) € (1,233) € (535)
Tax using the Luxembourg tax rate (45) (334) (156)
Effect of tax rates in foreign jurisdictions (11) (10) 15
Permanent differences (7) 15 12
Change in unrecognized deferred taxes (43) 329 132
Deferred tax on foreign exchange differences 8 0 0
Other 3 2 1
Income tax (benefit)/expense € (95) € 2 € 4