Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 01, 2018	Jul. 02, 2017	Jul. 01, 2018	Jul. 02, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 265,088	$ 238,329	$ 521,048	$ 470,650
Total before tax	(7,024)	(90,458)	(68,197)	(128,138)
Taxes on income from continuing operations	9,576	12,095	15,818	9,426
Net of tax	2,552	(78,363)	(52,379)	(118,712)
Total reclassifications, net of tax	1,258	2,231	1,897	4,740
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,734	1,727	3,480	3,453
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	23	30	47	59
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,757	1,757	3,527	3,512
Tax benefit	(408)	(625)	(819)	(1,249)
Total reclassifications, net of tax	1,349	1,132	2,708	2,263
Reclassification out of Accumulated Other Comprehensive Income [Member] | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	(118)	1,303	(951)	2,948
Total before tax	(118)	1,303	(951)	2,948
Taxes on income from continuing operations	27	(204)	140	(471)
Net of tax	$ (91)	$ 1,099	$ (811)	$ 2,477