Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 01, 2017	Sep. 25, 2016	Oct. 01, 2017	Sep. 25, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 239,476	$ 214,046	$ 710,126	$ 630,946
Total before tax	(89,376)	(73,714)	(217,514)	(194,909)
Taxes on income from continuing operations	9,978	7,514	19,404	18,134
Net of tax	(79,398)	(66,200)	(198,110)	(176,775)
Total reclassifications, net of tax	182	1,535	4,922	6,312
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,723	1,829	5,176	5,071
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	20	15	79	43
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,743	1,844	5,255	5,114
Tax benefit	(619)	(657)	(1,868)	(1,818)
Total reclassifications, net of tax	1,124	1,187	3,387	3,296
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	(1,179)	535	1,769	3,907
Total before tax	(1,179)	535	1,769	3,907
Taxes on income from continuing operations	237	(187)	(234)	(891)
Net of tax	$ (942)	$ 348	$ 1,535	$ 3,016