Pension and other postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 380	$ 725	$ 1,135	$ 2,162
Interest cost	3,681	3,773	11,119	11,347
Expected return on plan assets	(7,423)	(6,607)	(22,260)	(20,100)
Net amortization and deferral	1,687	1,668	5,090	5,049
Net benefits expense (income)	(1,675)	(441)	(4,916)	(1,542)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	(66)	61	38	210
Interest cost	285	426	1,042	1,183
Expected return on plan assets	0	0	0	0
Net amortization and deferral	(23)	75	101	206
Net benefits expense (income)	$ 196	$ 562	$ 1,181	$ 1,599