Note 7 - Benefit Plans - Benefit Plan Expenses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Service cost	$ 533	$ 408	$ 1,066	$ 816
Interest cost	501	360	1,002	720
Expected return on plan assets	(540)	(476)	(1,080)	(952)
Amortization of prior service (credit)	74	84	148	168
Net periodic benefit cost	$ 568	$ 376	$ 1,136	$ 752