Postretirement benefits other than pensions Postretirement Benefit Costs for Health Care and Life Insurance (Details) (Other Postretirement Benefit Plans, Defined Benefit, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 29, 2013	Dec. 30, 2012	Dec. 25, 2011
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 529	$ 545	$ 611
Defined Benefit Plan, Interest Cost	5,656	7,744	9,205
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(9,165)	(19,190)	(19,510)
Defined Benefit Plan, Amortization of Gains (Losses)	(1,169)	(1,943)	(5,444)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ (1,811)	$ (8,958)	$ (4,250)