Note 9 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Service cost	$ 5,334	$ 4,401	$ 5,009
Interest cost	5,618	6,482	5,508
Expected return on plan assets	(7,489)	(5,982)	(6,536)
Amortization of prior service benefit	117	61	61
Amortization of net gain from earlier periods	4,652	4,389	3,726
Net periodic pension cost	$ 8,232	$ 9,351	$ 7,768