Goodwill and intangible assets - Schedule of intangible assets subject to amortization (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018	Dec. 31, 2018
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance	$ 912,030
Intangible assets recognized during the period	2,092
Currency translation during the period	(11,699)
Intangible assets subject to amortization, ending balance	902,423
Accumulated amortization, beginning balance	657,601
Amortization during the period	6,729	$ 7,567
Currency translation during the period	(9,697)
Accumulated amortization, ending balance	654,633
Intangible assets subject to amortization, net	247,790		$ 254,430
Customer relationships
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance	651,012
Intangible assets recognized during the period	2,092
Currency translation during the period	(6,758)
Intangible assets subject to amortization, ending balance	646,346
Accumulated amortization, beginning balance	406,386
Amortization during the period	6,194
Currency translation during the period	(4,772)
Accumulated amortization, ending balance	407,808
Intangible assets subject to amortization, net	238,538
Patents
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance	254,483
Intangible assets recognized during the period	0
Currency translation during the period	(5,015)
Intangible assets subject to amortization, ending balance	249,468
Accumulated amortization, beginning balance	249,988
Amortization during the period	533
Currency translation during the period	(4,920)
Accumulated amortization, ending balance	245,601
Intangible assets subject to amortization, net	3,867
Other
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance	6,535
Intangible assets recognized during the period	0
Currency translation during the period	74
Intangible assets subject to amortization, ending balance	6,609
Accumulated amortization, beginning balance	1,227
Amortization during the period	2
Currency translation during the period	(5)
Accumulated amortization, ending balance	1,224
Intangible assets subject to amortization, net	$ 5,385