Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2021	Jun. 28, 2020	Jun. 27, 2021	Jun. 28, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 315,917	$ 288,662	$ 605,315	$ 585,680
Total before tax	99,695	23,291	186,990	165,517
(Benefit) tax	(16,412)	(11,848)	(28,840)	(22,922)
Net of tax	83,283	11,443	158,150	142,595
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications, net of tax	1,610	759	3,898	2,128
Reclassification out of Accumulated Other Comprehensive Income | Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,893	1,858	3,785	3,718
(Benefit) tax	(443)	(433)	(885)	(867)
Net of tax	1,450	1,425	2,900	2,851
Reclassification out of Accumulated Other Comprehensive Income | Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	2,144	1,850	4,287	3,702
Reclassification out of Accumulated Other Comprehensive Income | Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	(251)	8	(502)	16
Reclassification out of Accumulated Other Comprehensive Income | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	133	(685)	979	(751)
Total before tax	133	(685)	979	(751)
(Benefit) tax	27	19	19	28
Net of tax	$ 160	$ (666)	$ 998	$ (723)