Employee Benefit Plans - Components Of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Pension and SERPA Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,806	$ 6,632	$ 13,612	$ 13,264
Interest cost	19,111	21,371	38,223	42,742
Expected return on plan assets	(33,764)	(35,581)	(67,528)	(71,162)
Amortization of unrecognized: Prior service credit	(272)	(483)	(544)	(966)
Amortization of unrecognized: Net loss	16,372	11,128	32,744	22,256
Net periodic benefit cost	8,253	3,067	16,507	6,134
Postretirement Healthcare Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,202	1,185	2,403	2,369
Interest cost	2,336	2,938	4,672	5,876
Expected return on plan assets	(3,467)	(3,507)	(6,934)	(7,014)
Amortization of unrecognized: Prior service credit	(595)	(595)	(1,190)	(1,190)
Amortization of unrecognized: Net loss	123	69	246	138
Special retirement benefit cost	0	1,583	0	1,583
Curtailment gain	0	(960)	0	(960)
Net periodic benefit cost	$ (401)	$ 713	$ (803)	$ 802