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Other Current Assets - Additional Information (Detail) - EUR (€)
€ in Thousands
 6 Months Ended
Jun. 25, 2018
Jul. 04, 2016
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Trade and other receivables [abstract]            
Research tax credit     € 16,802   € 10,829 € 9,200
Other income research tax credit     € 6,000 € 5,400    
Research tax credit current adjustment   € 900        
Research tax credit final reassessment received € 58          
Research tax credit current adjustment reversed € 900