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Summary of Significant Accounting Policies - Additional Information (Detail) - EUR (€)
€ in Millions
6 Months Ended
Jan. 01, 2019
Jun. 30, 2019
Dec. 31, 2018
Summary of significant accounting policies [line items]      
Short-term lease description   A lease term of 12 months or less from the commencement date  
Low value lease assets description   Leases of assets with a value below €5,000  
Right of use assets € 102    
Operating lease liability € 102    
Average incremental borrowing rate 7.60%    
Operating leases     € 133
IFRIC 23 [member]      
Summary of significant accounting policies [line items]      
Cumulative effect adjustment in retained earnings € 0    
IFRIC 23 [member] | Reclassifications [member]      
Summary of significant accounting policies [line items]      
Reclassification in Income tax payable and reduction of Deferred income tax assets and current tax receivables € 20