Pension and Other Postretirement Obligations - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019
Pension Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 885	$ 978	$ 2,709	$ 2,952
Interest cost	2,493	2,901	7,240	8,779
Expected return on plan assets	(4,091)	(3,841)	(12,035)	(11,978)
Amortization of prior service cost (credit)	47	(143)	136	(117)
Actuarial losses (gains)	688	292	2,038	894
Net periodic benefit cost	22	187	88	530
Other Postretirement Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	10	24	29
Interest cost	202	273	599	815
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	0	0	0
Actuarial losses (gains)	(19)	(25)	(57)	(76)
Net periodic benefit cost	$ 191	$ 258	$ 566	$ 768