Goodwill and intangible assets - Schedule of intangible assets subject to amortization (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 29, 2019	Jun. 30, 2018	Jun. 29, 2019	Jun. 30, 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			(4,097)
Intangible assets subject to amortization, ending balance	$ 910,025		910,025
Accumulated amortization, beginning balance			657,601
Amortization during the period	6,955	$ 7,483	13,684	$ 15,050
Currency translation during the period			(3,105)
Accumulated amortization, ending balance	668,180		668,180
Intangible assets subject to amortization, net	241,845		241,845		$ 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			(2,464)
Intangible assets subject to amortization, ending balance	650,640		650,640
Accumulated amortization, beginning balance			406,386
Amortization during the period			12,597
Currency translation during the period			(1,523)
Accumulated amortization, ending balance	417,460		417,460
Intangible assets subject to amortization, net	233,180		233,180
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			(1,624)
Intangible assets subject to amortization, ending balance	252,859		252,859
Accumulated amortization, beginning balance			249,988
Amortization during the period			1,072
Currency translation during the period			(1,580)
Accumulated amortization, ending balance	249,480		249,480
Intangible assets subject to amortization, net	3,379		3,379
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			(9)
Intangible assets subject to amortization, ending balance	6,526		6,526
Accumulated amortization, beginning balance			1,227
Amortization during the period			15
Currency translation during the period			(2)
Accumulated amortization, ending balance	1,240		1,240
Intangible assets subject to amortization, net	$ 5,286		$ 5,286