Pension and other postretirement benefits Net benefit cost of pension and postretirement benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 27, 2016	Mar. 29, 2015
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 652	$ 471
Interest cost	3,920	4,486
Expected return on plan assets	(6,198)	(6,425)
Net amortization and deferral	1,579	1,530
Net benefits expense (income)	(47)	62
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	111	107
Interest cost	406	512
Expected return on plan assets	0	0
Net amortization and deferral	57	77
Net benefits expense (income)	$ 574	$ 696