Pension and other postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 01, 2018	Jul. 02, 2017	Jul. 01, 2018	Jul. 02, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 376	$ 720	$ 754	$ 1,437
Interest cost	3,718	3,789	7,439	7,574
Expected return on plan assets	(7,415)	(6,750)	(14,836)	(13,493)
Net amortization and deferral	1,695	1,691	3,403	3,381
Net benefits expense (income)	(1,626)	(550)	(3,240)	(1,101)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	52	75	104	149
Interest cost	378	379	756	757
Expected return on plan assets	0	0	0	0
Net amortization and deferral	62	66	124	131
Net benefits expense (income)	$ 492	$ 520	$ 984	$ 1,037