Pension and Other Post-Retirement Benefit Plans (Details) - Pension Plans - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 25, 2016	Jun. 27, 2015	Jun. 25, 2016	Jun. 27, 2015
Employee benefits
Service cost	$ 639	$ 1,150	$ 1,193	$ 2,299
Interest cost	3,220	3,336	6,584	6,671
Expected return on plan assets	(3,998)	(4,382)	(7,990)	(8,764)
Amortization of prior service credit	(144)	(150)	(288)	(301)
Amortization of net loss	545	911	1,091	1,791
Net periodic cost	$ 262	$ 865	$ 590	$ 1,696