Pension and Other Post-Retirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2015	Sep. 27, 2014	Sep. 26, 2015	Sep. 27, 2014
Pension Plans
Employee benefits
Service cost	$ 935	$ 880	$ 2,806	$ 2,565
Interest cost	3,070	3,211	9,210	9,633
Expected return on plan assets	(4,383)	(4,540)	(13,147)	(13,095)
Amortization of prior service cost (credit)	(150)	(166)	(451)	(484)
Amortization of net loss	780	238	2,433	684
Net periodic cost (benefit)	252	(377)	851	(697)
Other Post-Retirement Benefit Plans
Employee benefits
Service cost	214	190	642	569
Interest cost	265	252	796	757
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	159	0	489
Amortization of net loss	65	60	203	185
Net periodic cost (benefit)	$ 544	$ 661	$ 1,641	$ 2,000