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Property, plant and equipment - Summary of Depreciation expense (Detail) - EUR (€)
€ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about property, plant and equipment [line items]      
Depreciation, property, plant and equipment € 2,103 € 1,797 € 2,058
Research and development expenses [Member]      
Disclosure of detailed information about property, plant and equipment [line items]      
Depreciation, property, plant and equipment 1,503 1,315 1,757
General and administrative expenses [Member]      
Disclosure of detailed information about property, plant and equipment [line items]      
Depreciation, property, plant and equipment € 600 € 482 € 301