Net Periodic Benefit Expense - Defined Benefit Pension Plans (Details) - Pension plans, defined benefit [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Defined Benefit Plan Disclosure [Line Items]
Service costs--increase in benefit obligation earned	$ 100	$ 132	$ 200	$ 265
Interest cost on projected benefit obligation	179	264	359	527
Expected return on plan assets	(57)	(170)	(115)	(339)
Amortization of net actuarial loss	182	238	364	475
Amortization of prior service cost	16	15	32	31
Net pension expense of defined benefit pension plans	$ 420	$ 479	$ 840	$ 959