Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 02, 2017	Jun. 26, 2016	Jul. 02, 2017	Jun. 26, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ (238,329)	$ (217,154)	$ (470,650)	$ (416,900)
Total before tax	(90,458)	(67,402)	(128,138)	(121,195)
Taxes on income from continuing operations	12,095	8,007	9,426	10,620
Net of tax	(78,363)	(59,395)	(118,712)	(110,575)
Total reclassifications, net of tax	2,231	2,191	4,740	4,777
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,727	1,620	3,453	3,242
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	30	14	59	28
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,757	1,634	3,512	3,270
Tax benefit	(625)	(581)	(1,249)	(1,161)
Total reclassifications, net of tax	1,132	1,053	2,263	2,109
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	1,303	1,501	2,948	3,372
Total before tax	1,303	1,501	2,948	3,372
Taxes on income from continuing operations	(204)	(363)	(471)	(704)
Net of tax	$ 1,099	$ 1,138	$ 2,477	$ 2,668