Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 29, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 297,018	$ 289,614
Total before tax	(142,226)	(52,890)
Taxes on income from continuing operations	11,074	10,972
Net of tax	(131,152)	(41,918)
Total reclassifications, net of tax	1,369	1,188
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications, net of tax	(57)	(163)
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,852	1,740
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	8	22
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,860	1,762
Tax benefit	(434)	(411)
Total reclassifications, net of tax	1,426	1,351
Reclassification out of Accumulated Other Comprehensive Income | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	(66)	(186)
Total before tax	(66)	(186)
Taxes on income from continuing operations	9	23
Net of tax	$ (57)	$ (163)