Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 288,662	$ 300,269	$ 585,680	$ 589,883
Total before tax	(23,291)	(87,172)	(165,517)	(140,062)
Taxes on income from continuing operations	11,848	3,844	22,922	14,816
Net of tax	(11,443)	(83,328)	(142,595)	(125,246)
Total reclassifications, net of tax	759	1,242	2,128	2,430
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications, net of tax			(723)	(271)
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,850	1,738	3,702	3,478
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	8	22	16	44
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,858	1,760	3,718	3,522
Tax benefit	(433)	(410)	(867)	(821)
Total reclassifications, net of tax	1,425	1,350	2,851	2,701
Reclassification out of Accumulated Other Comprehensive Income | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	(685)	(179)	(751)	(365)
Total before tax	(685)	(179)	(751)	(365)
Taxes on income from continuing operations	19	71	28	94
Net of tax	$ (666)	$ (108)	$ (723)	$ (271)