Pension and Other Postretirement Obligations - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jul. 01, 2018	Jun. 30, 2019	Jul. 01, 2018
Pension Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,047	$ 942	$ 2,106	$ 2,075
Interest cost	2,939	1,905	5,948	3,781
Expected return on plan assets	(4,020)	(2,508)	(8,135)	(5,028)
Amortization of prior service cost (credit)	40	(12)	26	(22)
Actuarial losses (gains)	316	612	642	1,277
Net periodic benefit cost	322	939	587	2,083
Other Postretirement Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	10	13	19	26
Interest cost	270	260	542	524
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	0	0	0
Actuarial losses (gains)	(26)	0	(51)	0
Net periodic benefit cost	$ 254	$ 273	$ 510	$ 550