Pension and other postretirement benefits Net benefit cost of pension and postretirement benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2015	Jun. 29, 2014	Jun. 28, 2015	Jun. 29, 2014
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 469	$ 447	$ 940	$ 897
Interest cost	4,485	4,486	8,971	8,969
Expected return on plan assets	(6,427)	(6,264)	(12,852)	(12,524)
Net amortization and deferral	1,528	1,070	3,058	2,141
Net benefit expense (income)	55	(261)	117	(517)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	107	138	214	276
Interest cost	511	596	1,023	1,193
Expected return on plan assets	0	0	0	0
Net amortization and deferral	78	27	155	53
Net benefit expense (income)	$ 696	$ 761	$ 1,392	$ 1,522