Commitments and contingencies (Details) - Belgium - Foreign tax authority - EUR (€) € in Thousands		1 Months Ended
	Dec. 28, 2012	Mar. 30, 2019	Dec. 31, 2013	Jan. 31, 2012
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