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Intangible assets - Summary of Changes to intangible assets (Detail) - EUR (€)
€ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance € 1,008  
Currency translation differences   € 31
Ending Balance 914 1,008
Patent And Licenses [Member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance 851  
Currency translation differences   31
Ending Balance 730 851
Software Licenses [Member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance 157  
Ending Balance 184 157
Gross carrying amount [member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance 1,863 1,752
Additions 104 91
Currency translation differences (97) 20
Ending Balance 1,870 1,863
Gross carrying amount [member] | Patent And Licenses [Member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance 1,220 1,201
Currency translation differences (88) 19
Ending Balance 1,132 1,220
Gross carrying amount [member] | Software Licenses [Member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance 643 551
Additions 104 91
Currency translation differences (9) 1
Ending Balance 738 643
Accumulated impairment [member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance (855) (713)
Additions 126 111
Currency translation differences (25)  
Ending Balance (956) (855)
Accumulated impairment [member] | Patent And Licenses [Member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance (369) (314)
Additions 56 24
Currency translation differences (22)  
Ending Balance (403) (369)
Accumulated impairment [member] | Software Licenses [Member]    
Disclosure of reconciliation of changes in intangible assets and goodwill [line items]    
Beginning Balance (486) (399)
Additions 71 87
Currency translation differences (3)  
Ending Balance € (554) € (486)