Summary of Significant Accounting Policies - Life Insurance Policies (Details) $ in Thousands	6 Months Ended
	Jun. 28, 2019 USD ($) Individual	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Number of former employees covered under insurance policies | Individual	18
Average age of former employees covered under insurance policies	75 years
Gross cash surrender values of life insurance	$ 28,800	$ 28,400
Loans on cash surrender value	26,400	25,800
Net cash surrender values of life insurance	2,393	2,626
Gross death benefit of life insurance contracts	37,600	$ 37,600
Life insurance proceeds to be received upon the death of participants	10,900
Life insurance gain to be recognized upon death of participants	$ 8,500