Pension and other postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 02, 2017	Mar. 27, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 717	$ 652
Interest cost	3,785	3,920
Expected return on plan assets	(6,743)	(6,198)
Net amortization and deferral	1,690	1,579
Net benefits expense (income)	(551)	(47)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	74	111
Interest cost	378	406
Expected return on plan assets	0	0
Net amortization and deferral	65	57
Net benefits expense (income)	$ 517	$ 574