Benefit Plans - Benefit Plan Expenses (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	$ 533	$ 408	$ 1,599	$ 1,224
Interest cost	501	360	1,503	1,080
Expected return on plan assets	(540)	(476)	(1,620)	(1,428)
Amortization of prior service	74	84	222	252
Net periodic benefit cost	$ 568	$ 376	$ 1,704	$ 1,128