Pension and Other Postretirement Obligations - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Apr. 01, 2018
Pension Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,058	$ 1,133
Interest cost	3,010	1,876
Expected return on plan assets	(4,116)	(2,520)
Amortization of prior service credit	(14)	(10)
Actuarial losses (gains)	327	665
Net periodic benefit cost	265	1,144
Other Postretirement Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	10	13
Interest cost	272	264
Expected return on plan assets	0	0
Amortization of prior service credit	0	0
Actuarial losses (gains)	(26)	0
Net periodic benefit cost	$ 256	$ 277