Commitments and contingencies (Details) - Belgium - Foreign tax authority € in Thousands		1 Months Ended			3 Months Ended
	Dec. 28, 2012 EUR (€)	Nov. 30, 2014 yr	Dec. 31, 2013 EUR (€)	Jan. 31, 2012 EUR (€)	Jun. 28, 2014 EUR (€)	Sep. 29, 2012 EUR (€)
Commitments And Contingencies [Line Items]
Interest income earned on deposits related to tax assessment (in euros)						€ 1,583
Number of years of assessments appealed | yr		4
2008
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)				€ 23,789	€ 30,131
2005
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	€ 46,135
2009
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	€ 35,567
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
2010
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)			€ 43,117