Note 5 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Service cost	$ 1,455	$ 1,334	$ 4,365	$ 4,002
Interest cost	1,248	1,404	3,744	4,212
Expected return on plan assets	(2,313)	(1,872)	(6,939)	(5,616)
Amortization of prior service cost	99	29	297	87
Amortization of net loss	1,125	1,163	3,375	3,489
Net periodic pension cost	$ 1,614	$ 2,058	$ 4,842	$ 6,174