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Trade Receivables - Summary of Trade Receivables (Detail) - EUR (€)
€ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net € 29,187 € 40,967 € 37,549
Gross trade receivables      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net 37,886 50,594  
Allowance for doubtful accounts [member]      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net (8,699) (9,627)  
Domestic customers [member] | Gross trade receivables      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net 14,401 20,247  
European Customers [member] | Gross trade receivables      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net 4,348 7,815  
North American customers [member] | Gross trade receivables      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net 4,986 3,573  
South American customers [member] | Gross trade receivables      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net 5,703 3,531  
Chinese customers [member] | Gross trade receivables      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net 4,055 7,233  
Other foreign customers [member] | Gross trade receivables      
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]      
Total trade receivables, net € 4,393 € 8,195