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. 25, 2016	Sep. 27, 2015	Sep. 25, 2016	Sep. 27, 2015
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 656	$ 470	$ 1,963	$ 1,410
Interest cost	3,948	4,492	11,797	13,463
Expected return on plan assets	(6,209)	(6,606)	(18,606)	(19,457)
Net amortization and deferral	1,781	1,563	4,937	4,620
Net benefits expense (income)	176	(81)	91	36
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	44	157	266	371
Interest cost	383	452	1,196	1,476
Expected return on plan assets	0	0	0	0
Net amortization and deferral	63	8	177	162
Net benefits expense (income)	$ 490	$ 617	$ 1,639	$ 2,009