Commitments and contingencies (Details) € in Thousands, $ in Thousands		1 Months Ended				3 Months Ended		6 Months Ended
	Dec. 28, 2012 EUR (€)	Nov. 30, 2014 yr	Dec. 31, 2013 EUR (€)	Jan. 31, 2012 EUR (€)	Mar. 21, 2011 claim	Jun. 28, 2014 EUR (€)	Sep. 29, 2012 EUR (€)	Jul. 04, 2015 USD ($)
BELGIUM | Foreign Tax Authority [Member]
Commitments And Contingencies [Line Items]
Interest income earned on deposits related to tax assessment							€ 1,583
Number of years of assessments appealed | yr		4
BELGIUM | Foreign Tax Authority [Member] | 2008 [Member]
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)				€ 23,789		€ 30,131
BELGIUM | Foreign Tax Authority [Member] | 2005 [Member]
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	€ 46,135
BELGIUM | Foreign Tax Authority [Member] | 2009 [Member]
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)	€ 35,567
BELGIUM | Foreign Tax Authority [Member] | 2006 [Member]
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)			€ 38,817
BELGIUM | Foreign Tax Authority [Member] | 2007 [Member]
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)			39,635
BELGIUM | Foreign Tax Authority [Member] | 2010 [Member]
Commitments And Contingencies [Line Items]
Assessment received from Belgian tax authority (in euros)			€ 43,117
Selling, general and administrative expenses [Member]
Commitments And Contingencies [Line Items]
Amount of legal settlement paid and related expenses | $								$ 125,000
Polyurethane Foam Antitrust Litigation [Member]
Commitments And Contingencies [Line Items]
Number of class action complaints | claim					2