Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Pension Benefits [Member]
Components of net periodic benefit cost (income) [Abstract]
Interest cost	$ 862	$ 1,455	$ 2,586	$ 4,365
Expected return on plan assets	(1,496)	(2,077)	(4,486)	(6,230)
Amortization of net loss (gain)	494	485	1,482	1,455
Net periodic benefit income	(140)	(137)	(418)	(410)
Other Benefits [Member]
Components of net periodic benefit cost (income) [Abstract]
Service cost	59	64	176	191
Interest cost	106	153	321	459
Amortization of prior service credit	(117)	(226)	(401)	(677)
Amortization of net loss (gain)	(50)	(10)	(150)	(31)
Curtailment gain	0	0	(2,591)	0
Net periodic benefit income	$ (2)	$ (19)	$ (2,645)	$ (58)