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Summary of significant accounting policies - Foreign currencies (Details)
€ in Thousands
12 Months Ended
Apr. 07, 2017
EUR (€)
Dec. 31, 2019
$ / €
Dec. 31, 2019
$ / €
₨ / €
Dec. 31, 2019
$ / €
£ / €
Dec. 31, 2019
$ / €
¥ / €
Dec. 31, 2018
$ / €
Dec. 31, 2018
$ / €
₨ / €
Dec. 31, 2018
$ / €
£ / €
Dec. 31, 2018
$ / €
¥ / €
Dec. 31, 2017
$ / €
Dec. 31, 2017
₨ / €
Dec. 31, 2017
£ / €
Dec. 31, 2017
¥ / €
Dec. 31, 2019
₨ / €
Dec. 31, 2019
£ / €
Dec. 31, 2019
¥ / €
Dec. 31, 2018
₨ / €
Dec. 31, 2018
£ / €
Dec. 31, 2018
¥ / €
Summary of significant accounting policies                                      
Expected dividends € 0                                    
Foreign exchange rates                                      
Average foreign exchange rate   1.1195 78.8361 0.8778 7.7355 1.1810 80.7332 0.8847 7.8081 1.1297 73.5324 0.8767 7.6290            
Closing foreign exchange rate   1.1234 1.1234 1.1234 1.1234 1.1450 1.1450 1.1450 1.1450         80.1870 0.8508 7.8205 79.7298 0.8945 7.8751