Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 02, 2017	Mar. 27, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ (232,321)	$ (199,746)
Total reclassifications, net of tax	2,509	2,586
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	1,645	1,871
Total before tax	1,645	1,871
Tax benefit	(267)	(341)
Total reclassifications, net of tax	1,378	1,530
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,726	1,622
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	29	14
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,755	1,636
Tax benefit	(624)	(580)
Total reclassifications, net of tax	$ 1,131	$ 1,056