Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2021	Sep. 27, 2020	Sep. 26, 2021	Sep. 27, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 312,464	$ 298,977	$ 917,779	$ 884,657
Total before tax	229,201	115,654	416,191	281,171
(Benefit) tax	(29,695)	951	(58,535)	(21,971)
Net of tax	199,506	116,605	357,656	259,200
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications, net of tax	1,597	3,073	5,495	5,201
Reclassification out of Accumulated Other Comprehensive Income | Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,870	1,740	5,655	5,458
(Benefit) tax	(438)	(411)	(1,323)	(1,278)
Net of tax	1,432	1,329	4,332	4,180
Reclassification out of Accumulated Other Comprehensive Income | Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	2,121	1,731	6,408	5,433
Reclassification out of Accumulated Other Comprehensive Income | Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	(251)	9	(753)	25
Reclassification out of Accumulated Other Comprehensive Income | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	164	1,899	1,143	1,148
Total before tax	164	1,899	1,143	1,148
(Benefit) tax	1	(155)	20	(127)
Net of tax	$ 165	$ 1,744	$ 1,163	$ 1,021