Note 4 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Service cost	$ 1,455	$ 1,334
Interest cost	1,248	1,404
Expected return on plan assets	(2,313)	(1,872)
Amortization of prior service cost	99	29
Amortization of net loss	1,125	1,163
Net periodic pension cost	$ 1,614	$ 2,058