Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2021	Mar. 29, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 289,398	$ 297,018
Total before tax	87,295	142,226
(Benefit) tax	(12,428)	(11,074)
Net of tax	74,867	131,152
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications, net of tax	2,288	1,369
Reclassification out of Accumulated Other Comprehensive Income | Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,892	1,860
(Benefit) tax	(442)	(434)
Net of tax	1,450	1,426
Reclassification out of Accumulated Other Comprehensive Income | Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	2,143	1,852
Reclassification out of Accumulated Other Comprehensive Income | Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	(251)	8
Reclassification out of Accumulated Other Comprehensive Income | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	846	(66)
Total before tax	846	(66)
(Benefit) tax	(8)	9
Net of tax	$ 838	$ (57)