Pension and Other Post-Retirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 01, 2017	Jun. 25, 2016	Jul. 01, 2017	Jun. 25, 2016
Pension
Employee benefits
Service cost	$ 774	$ 639	$ 1,528	$ 1,193
Interest cost	2,939	3,220	5,765	6,584
Expected return on plan assets	(3,485)	(3,998)	(6,935)	(7,990)
Amortization of prior service credit	(139)	(144)	(258)	(288)
Amortization of net loss	1,047	545	2,013	1,091
Net periodic cost	1,136	262	2,113	590
Post-retirement Benefit Plan
Employee benefits
Net periodic cost	$ 0	$ 0	$ 0	$ 0