Goodwill and intangible assets - Schedule of intangible assets subject to amortization (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance			$ 899,057
Intangible assets recognized during the period			7
Currency translation during the period			(18,416)
Intangible assets subject to amortization, ending balance	$ 880,648		880,648
Accumulated amortization, beginning balance			651,498
Amortization during the period	7,483	$ 9,322	15,050	$ 19,381
Currency translation during the period			(14,067)
Accumulated amortization, ending balance	652,481		652,481
Intangible assets subject to amortization, net	228,167		228,167		$ 247,559
Customer relationships
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance			625,263
Intangible assets recognized during the period			0
Currency translation during the period			(11,108)
Intangible assets subject to amortization, ending balance	614,155		614,155
Accumulated amortization, beginning balance			390,428
Amortization during the period			13,875
Currency translation during the period			(7,051)
Accumulated amortization, ending balance	397,252		397,252
Intangible assets subject to amortization, net	216,903		216,903
Patents
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance			266,969
Intangible assets recognized during the period			0
Currency translation during the period			(7,185)
Intangible assets subject to amortization, ending balance	259,784		259,784
Accumulated amortization, beginning balance			259,908
Amortization during the period			1,155
Currency translation during the period			(7,006)
Accumulated amortization, ending balance	254,057		254,057
Intangible assets subject to amortization, net	5,727		5,727
Other
Finite-lived Intangible Assets [Roll Forward]
Intangible assets subject to amortization, beginning balance			6,825
Intangible assets recognized during the period			7
Currency translation during the period			(123)
Intangible assets subject to amortization, ending balance	6,709		6,709
Accumulated amortization, beginning balance			1,162
Amortization during the period			20
Currency translation during the period			(10)
Accumulated amortization, ending balance	1,172		1,172
Intangible assets subject to amortization, net	$ 5,537		$ 5,537