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. 28, 2014	Dec. 29, 2013	Dec. 30, 2012
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 369	$ 529	$ 545
Interest cost on benefit obligation	4,711	5,656	7,744
Amortization of prior service costs	(11,648)	(9,165)	(19,190)
Amortization of actuarial loss	758	1,169	1,943
Pension expense (benefit) for company-sponsored retirement plans	$ (5,810)	$ (1,811)	$ (8,958)