Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 267,099	$ 239,476	$ 788,147	$ 710,126
Total before tax	(55,254)	(89,376)	(123,451)	(217,514)
(Benefit) taxes on income from continuing operations	(1,286)	9,978	14,532	19,404
Net of tax	(56,540)	(79,398)	(108,919)	(198,110)
Total reclassifications, net of tax	1,214	182	3,111	4,922
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,640	1,723	5,120	5,176
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	24	20	71	79
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,664	1,743	5,191	5,255
Tax benefit	(388)	(619)	(1,207)	(1,868)
Total reclassifications, net of tax	1,276	1,124	3,984	3,387
Reclassification out of Accumulated Other Comprehensive Income [Member] | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	(87)	(1,179)	(1,038)	1,769
Total before tax	(87)	(1,179)	(1,038)	1,769
(Benefit) taxes on income from continuing operations	25	237	165	(234)
Net of tax	$ (62)	$ (942)	$ (873)	$ 1,535