Changes in shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 25, 2016	Sep. 27, 2015	Sep. 25, 2016	Sep. 27, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ (214,046)	$ (215,501)	$ (630,946)	$ (641,102)
Total reclassifications, net of tax	1,535	1,967	6,312	4,267
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	535	1,168	3,907	1,431
Total before tax	535	1,168	3,907	1,431
Tax benefit	(187)	(221)	(891)	(321)
Total reclassifications, net of tax	348	947	3,016	1,110
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,829	1,571	5,071	4,782
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	15	0	43	0
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,844	1,571	5,114	4,782
Tax benefit	(657)	(551)	(1,818)	(1,625)
Total reclassifications, net of tax	$ 1,187	$ 1,020	$ 3,296	$ 3,157