Pension and Other Postretirement Benefits - Pension Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019	Oct. 09, 2020
Pension Benefits
Gain from pension liability adjustment	$ 0	$ 2,045	$ 0	$ 2,045
Pension Plan
Pension Benefits
Service cost	2,608	1,278	7,822	3,835
Interest cost	13,390	16,797	40,171	50,390
Expected return on plan assets	(17,741)	(20,258)	(53,222)	(60,775)
Amortization of actuarial loss	7,176	5,729	21,530	17,187
Amortization of prior service credit	(486)	(487)	(1,459)	(1,459)
Net periodic postretirement benefit income/cost	4,947	3,059	14,842	9,178
Pension Plan | Subsequent Event
Pension Benefits
Pension obligation					$ 235,000
Qualified Plans | Pension Plan
Pension Benefits
Service cost	2,608	1,278	7,822	3,835
Interest cost	11,742	14,708	35,226	44,125
Expected return on plan assets	(17,741)	(20,258)	(53,222)	(60,775)
Amortization of actuarial loss	5,655	4,635	16,966	13,905
Amortization of prior service credit	(486)	(487)	(1,459)	(1,459)
Net periodic postretirement benefit income/cost	1,778	(124)	5,333	(369)
Pension contributions			6,900	6,300
Expected future employer contributions	9,000		9,000
Qualified Plans | Pension Plan | Subsequent Event
Pension Benefits
Pension obligation					$ 235,000
Non- Qualified Plans | Pension Plan
Pension Benefits
Service cost	0	0	0	0
Interest cost	1,648	2,089	4,945	6,265
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss	1,521	1,094	4,564	3,282
Amortization of prior service credit	0	0	0	0
Net periodic postretirement benefit income/cost	$ 3,169	$ 3,183	$ 9,509	$ 9,547