Pension and Other Postretirement Benefits - Pension Benefits - Net Periodic Pension Cost (Details) - Pension Plan - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2021	Jun. 28, 2020	Jun. 27, 2021	Jun. 28, 2020
Pension Benefits
Service cost	$ 2,276	$ 2,607	$ 4,552	$ 5,214
Interest cost	8,717	13,391	17,434	26,781
Expected return on plan assets	(12,678)	(17,745)	(25,355)	(35,481)
Amortization of actuarial loss	6,879	7,177	13,755	14,353
Amortization of prior service credit	(486)	(486)	(972)	(972)
Net periodic postretirement benefit cost/(income)	4,708	4,944	9,414	9,895
Qualified Plans
Pension Benefits
Service cost	2,276	2,607	4,552	5,214
Interest cost	7,629	11,742	15,258	23,484
Expected return on plan assets	(12,678)	(17,745)	(25,355)	(35,481)
Amortization of actuarial loss	5,057	5,655	10,113	11,310
Amortization of prior service credit	(486)	(486)	(972)	(972)
Net periodic postretirement benefit cost/(income)	1,798	1,773	3,596	3,555
Pension contributions			4,200	4,600
Expected future employer contributions	9,000		9,000
Non- Qualified Plans
Pension Benefits
Service cost	0	0	0	0
Interest cost	1,088	1,649	2,176	3,297
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss	1,822	1,522	3,642	3,043
Amortization of prior service credit	0	0	0	0
Net periodic postretirement benefit cost/(income)	$ 2,910	$ 3,171	$ 5,818	$ 6,340