Post-Retirement Health Care (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 213	$ 185	$ 639	$ 556
Interest cost	197	206	591	619
Amortization of net (gain) loss	32	6	95	19
Net periodic post-retirement benefit cost	$ 442	$ 397	$ 1,325	$ 1,194