Pension and Other Postretirement Obligations - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Pension Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 892	$ 967	$ 1,824	$ 1,974
Interest cost	2,410	2,918	4,747	5,878
Expected return on plan assets	(4,004)	(4,020)	(7,944)	(8,137)
Amortization of prior service cost	45	40	89	26
Actuarial losses (gains)	673	281	1,350	602
Net periodic benefit cost	16	186	66	343
Other Postretirement Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	10	16	19
Interest cost	195	270	397	542
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	0	0	0	0
Actuarial losses (gains)	(19)	(26)	(38)	(51)
Net periodic benefit cost	$ 184	$ 254	$ 375	$ 510