Pension and Other Postretirement Obligations - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017
Pension Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,717	$ 1,206	$ 3,792	$ 3,549
Interest cost	2,011	1,822	5,792	5,391
Expected return on plan assets	(4,660)	(2,487)	(9,688)	(7,415)
Amortization of prior service credit	(37)	(10)	(59)	(30)
Actuarial losses	1,073	633	2,350	1,866
Net periodic cost	104	1,164	2,187	3,361
Other Postretirement Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	14	40	41
Interest cost	257	344	781	1,000
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	0	0	0	0
Actuarial losses	0	23	0	68
Net periodic cost	$ 271	$ 381	$ 821	$ 1,109