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Investments - Additional Information (Detail) - EUR (€)
€ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Investment [Line Items]          
Non-current investments € 12,319   € 12,319   € 16,945
Purchases of investments     30,551 € 29,560  
Proceeds from investment maturities     45,114 18,931  
Foreign currency exchange gains   € 6,917 871 € 4,111  
Foreign currency exchange losses 1,083        
Mature in 12 months [member]          
Investment [Line Items]          
Non-current investments 6,500   6,500    
Finance Income Expense [Member]          
Investment [Line Items]          
Foreign currency exchange gains € 400        
Foreign currency exchange losses     € 700