Retirement plans (Details) (Pension Plans, Defined Benefit [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 25, 2012
Pension Plans, Defined Benefit [Member]
Company's pension costs
Service cost-benefits earned during the period	$ 2,125	$ 2,032
Interest cost on benefit obligation	35,183	39,654
Expected return on plan assets	(49,543)	(47,649)
Amortization of prior service cost	1,878	1,928
Amortization of actuarial loss	15,360	13,505
Net periodic cost (benefit)	$ 5,003	$ 9,470