Commitments and contingencies (Details) - Belgium - Foreign tax authority - EUR (€) € in Thousands		36 Months Ended
	Mar. 10, 2020	Dec. 31, 2014
2005
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)		€ 46,135
2006
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)		38,817
2007
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)		39,635
2008
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)		30,131
2009
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)		35,567
2010
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)		€ 43,117
2016
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	€ 67,959