Pension and Other Postretirement Benefits - Net Periodic Pension Cost (Details) - Pension Plan - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Pension Benefits
Service cost	$ 2,607	$ 1,279	$ 5,214	$ 2,557
Interest cost	13,391	16,796	26,781	33,593
Expected return on plan assets	(17,745)	(20,259)	(35,481)	(40,517)
Amortization of actuarial loss	7,177	5,729	14,353	11,458
Amortization of prior service credit	(486)	(486)	(972)	(972)
Net periodic postretirement benefit (income)/cost	4,944	3,059	9,895	6,119
Qualified Plans
Pension Benefits
Service cost	2,607	1,279	5,214	2,557
Interest cost	11,742	14,708	23,484	29,417
Expected return on plan assets	(17,745)	(20,259)	(35,481)	(40,517)
Amortization of actuarial loss	5,655	4,635	11,310	9,270
Amortization of prior service credit	(486)	(486)	(972)	(972)
Net periodic postretirement benefit (income)/cost	1,773	(123)	3,555	(245)
Pension contributions			4,600	4,300
Expected contributions in 2020	9,000		9,000
Non- Qualified Plans
Pension Benefits
Service cost	0	0	0	0
Interest cost	1,649	2,088	3,297	4,176
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss	1,522	1,094	3,043	2,188
Amortization of prior service credit	0	0	0	0
Net periodic postretirement benefit (income)/cost	$ 3,171	$ 3,182	$ 6,340	$ 6,364