Employee Benefit Plans - Components Of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019
Pension and SERPA Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,806	$ 6,072	$ 20,418	$ 19,336
Interest cost	19,112	21,371	57,335	64,113
Expected return on plan assets	(33,764)	(35,581)	(101,292)	(106,743)
Amortization of unrecognized: Prior service credit	(272)	(483)	(816)	(1,449)
Amortization of unrecognized: Net loss	16,372	11,128	49,116	33,384
Settlement loss	0	1,500	0	1,500
Net periodic benefit cost	8,254	4,007	24,761	10,141
Postretirement Healthcare Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,202	1,040	3,605	3,409
Interest cost	2,336	2,938	7,008	8,814
Expected return on plan assets	(3,467)	(3,507)	(10,401)	(10,521)
Amortization of unrecognized: Prior service credit	(595)	(595)	(1,785)	(1,785)
Amortization of unrecognized: Net loss	123	69	369	207
Special retirement benefit cost	0	0	0	1,583
Curtailment gain	0	0	0	(960)
Net periodic benefit cost	$ (401)	$ (55)	$ (1,204)	$ 747