Pension and other postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2018	Apr. 02, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 378	$ 717
Interest cost	3,722	3,785
Expected return on plan assets	(7,421)	(6,743)
Net amortization and deferral	1,707	1,690
Net benefits expense (income)	(1,614)	(551)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	52	74
Interest cost	378	378
Expected return on plan assets	0	0
Net amortization and deferral	62	65
Net benefits expense (income)	$ 492	$ 517