Note 4 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Service cost	$ 1,455	$ 1,334	$ 2,910	$ 2,668
Interest cost	1,248	1,404	2,496	2,808
Expected return on plan assets	(2,313)	(1,872)	(4,626)	(3,744)
Amortization of prior service cost	99	29	198	58
Amortization of net loss	1,125	1,163	2,250	2,326
Net periodic pension cost	$ 1,614	$ 2,058	$ 3,228	$ 4,116