Pension and other postretirement benefits Net benefit cost of pension and postretirement benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2015	Sep. 28, 2014	Sep. 27, 2015	Sep. 28, 2014
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 470	$ 453	$ 1,410	$ 1,350
Interest cost	4,492	4,548	13,463	13,517
Expected return on plan assets	(6,606)	(6,240)	(19,457)	(18,764)
Net amortization and deferral	1,563	1,143	4,620	3,284
Net benefit expense (income)	(81)	(96)	36	(613)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	157	42	371	318
Interest cost	452	434	1,476	1,627
Expected return on plan assets	0	0	0	0
Net amortization and deferral	8	(58)	162	(5)
Net benefit expense (income)	$ 617	$ 418	$ 2,009	$ 1,940