Employee Benefit Plans - Components Of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2021	Jun. 28, 2020	Jun. 27, 2021	Jun. 28, 2020
Pension and SERPA Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,348	$ 6,806	$ 12,696	$ 13,612
Interest cost	15,472	19,111	30,942	38,223
Expected return on plan assets	(32,720)	(33,764)	(65,440)	(67,528)
Amortization of unrecognized: Prior service credit	(312)	(272)	(624)	(544)
Amortization of unrecognized: Net loss	18,386	16,372	36,772	32,744
Settlement loss	0	0	816	0
Net periodic benefit cost	7,174	8,253	15,162	16,507
Postretirement Healthcare Benefits:
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,288	1,202	2,576	2,403
Interest cost	1,626	2,336	3,252	4,672
Expected return on plan assets	(3,495)	(3,467)	(6,990)	(6,934)
Amortization of unrecognized: Prior service credit	(581)	(595)	(1,162)	(1,190)
Amortization of unrecognized: Net loss	264	123	528	246
Net periodic benefit cost	$ (898)	$ (401)	$ (1,796)	$ (803)