Pension and other postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 01, 2017	Sep. 25, 2016	Oct. 01, 2017	Sep. 25, 2016
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 725	$ 656	$ 2,162	$ 1,963
Interest cost	3,773	3,948	11,347	11,797
Expected return on plan assets	(6,607)	(6,209)	(20,100)	(18,606)
Net amortization and deferral	1,668	1,781	5,049	4,937
Net benefits expense (income)	(441)	176	(1,542)	91
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	61	44	210	266
Interest cost	426	383	1,183	1,196
Expected return on plan assets	0	0	0	0
Net amortization and deferral	75	63	206	177
Net benefits expense (income)	$ 562	$ 490	$ 1,599	$ 1,639