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 Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost	$ 250	$ 199	$ 750	$ 596
Interest cost	2,041	2,064	6,122	6,191
Expected return on plan assets	(2,654)	(3,202)	(7,963)	(9,604)
Amortization of prior service cost (credit)	0	1	0	1
Amortization of net loss (gain)	685	188	2,055	565
Net periodic benefit (income) cost	322	(750)	964	(2,251)
Other Benefits [Member]
Components of net periodic benefit cost [Abstract]
Service cost	90	88	270	264
Interest cost	193	181	579	544
Amortization of prior service cost (credit)	2	0	5	(1)
Amortization of net loss (gain)	(7)	(55)	(20)	(164)
Net periodic benefit (income) cost	$ 278	$ 214	$ 834	$ 643