Pension and Other Postretirement Benefits - Pension Benefits - Net Periodic Pension Cost (Details) - Pension Plan - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2021	Sep. 27, 2020	Sep. 26, 2021	Sep. 27, 2020
Pension Benefits
Service cost	$ 2,276	$ 2,608	$ 6,828	$ 7,822
Interest cost	8,717	13,390	26,151	40,171
Expected return on plan assets	(12,678)	(17,741)	(38,033)	(53,222)
Amortization of actuarial loss	6,877	7,176	20,632	21,530
Amortization of prior service credit	(486)	(486)	(1,458)	(1,459)
Net periodic postretirement benefit cost/(income)	4,706	4,947	14,120	14,842
Qualified Plans
Pension Benefits
Service cost	2,276	2,608	6,828	7,822
Interest cost	7,630	11,742	22,888	35,226
Expected return on plan assets	(12,678)	(17,741)	(38,033)	(53,222)
Amortization of actuarial loss	5,056	5,655	15,169	16,966
Amortization of prior service credit	(486)	(486)	(1,458)	(1,459)
Net periodic postretirement benefit cost/(income)	1,798	1,778	5,394	5,333
Pension contributions			6,500	6,900
Expected future employer contributions	9,000		9,000
Non- Qualified Plans
Pension Benefits
Service cost	0	0	0	0
Interest cost	1,087	1,648	3,263	4,945
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss	1,821	1,521	5,463	4,564
Amortization of prior service credit	0	0	0	0
Net periodic postretirement benefit cost/(income)	$ 2,908	$ 3,169	$ 8,726	$ 9,509