Goodwill and intangible assets - Schedule of intangible assets subject to amortization (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Apr. 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	19,126
Intangible assets subject to amortization, ending balance	918,190
Accumulated amortization, beginning balance	651,498
Amortization during the period	7,567	$ 10,059
Currency translation during the period	13,485
Accumulated amortization, ending balance	672,550
Intangible assets subject to amortization, net	245,640		$ 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	11,865
Intangible assets subject to amortization, ending balance	637,128
Accumulated amortization, beginning balance	390,428
Amortization during the period	6,975
Currency translation during the period	6,577
Accumulated amortization, ending balance	403,980
Intangible assets subject to amortization, net	233,148
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	7,082
Intangible assets subject to amortization, ending balance	274,051
Accumulated amortization, beginning balance	259,908
Amortization during the period	583
Currency translation during the period	6,898
Accumulated amortization, ending balance	267,389
Intangible assets subject to amortization, net	6,662
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	179
Intangible assets subject to amortization, ending balance	7,011
Accumulated amortization, beginning balance	1,162
Amortization during the period	9
Currency translation during the period	10
Accumulated amortization, ending balance	1,181
Intangible assets subject to amortization, net	$ 5,830