Commitments and contingencies (Narrative) (Details) In Thousands, unless otherwise specified	1 Months Ended		3 Months Ended	1 Months Ended	3 Months Ended	0 Months Ended		1 Months Ended			3 Months Ended
	Mar. 21, 2011 claim	Nov. 30, 2014 BELGIUM Foreign Tax Authority [Member] Y	Sep. 29, 2012 BELGIUM Foreign Tax Authority [Member] EUR (€)	Jan. 31, 2012 BELGIUM Foreign Tax Authority [Member] 2008 [Member] EUR (€)	Jun. 28, 2014 BELGIUM Foreign Tax Authority [Member] 2008 [Member] EUR (€)	Dec. 28, 2012 BELGIUM Foreign Tax Authority [Member] 2005 [Member] EUR (€)	Dec. 28, 2012 BELGIUM Foreign Tax Authority [Member] 2009 [Member] EUR (€)	Dec. 31, 2013 BELGIUM Foreign Tax Authority [Member] 2006 [Member] EUR (€)	Dec. 31, 2013 BELGIUM Foreign Tax Authority [Member] 2007 [Member] EUR (€)	Dec. 31, 2013 BELGIUM Foreign Tax Authority [Member] 2010 [Member] EUR (€)	Apr. 04, 2015 Selling, General and Administrative Expenses [Member] USD ($)	Apr. 04, 2015 Polyurethane Foam Antitrust Litigation [Member] Pending Litigation [Member] USD ($)
Commitments And Contingencies [Line Items]
Number of class action complaints	2
Damages claimed												$ 867,400
Amount of legal settlement paid and related expenses											125,000
Assessment received from Belgian tax authority (in euros)				23,789	30,131	46,135	35,567	38,817	39,635	43,117
Interest income earned on deposits related to tax assessment			€ 1,583
Number of years of assessments appealed		4