Commitments and contingencies (Details) - Belgium - Foreign tax authority - EUR (€) € in Thousands	1 Months Ended	36 Months Ended
	Mar. 30, 2019	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
2011
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	€ 40,617
2012
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	39,732
2013
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	11,358
2014
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	23,919
2015
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	30,610
2016
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	93,145
2017
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	€ 79,933