Retirement plans (Details) (Pension Plans, Defined Benefit [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 24, 2012	Jun. 30, 2013	Jun. 24, 2012
Pension Plans, Defined Benefit [Member]
Company's pension costs
Service cost-benefits earned during the period	$ 1,756	$ 1,747	$ 3,881	$ 3,779
Interest cost on benefit obligation	35,071	38,077	70,254	77,731
Expected return on plan assets	(49,299)	(47,145)	(98,842)	(94,794)
Amortization of prior service cost	1,898	1,917	3,776	3,845
Amortization of actuarial loss	16,190	12,919	31,550	26,424
Net periodic cost (benefit)	5,616	7,515	10,619	16,985
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments	0	(5,423)	0	(5,423)
Pension Cost	$ 5,616	$ 12,938	$ 10,619	$ 22,408