Note 7 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Service cost	$ 1,334	$ 1,100	$ 4,002	$ 3,300
Interest cost	1,404	1,620	4,212	4,860
Expected return on plan assets	(1,872)	(1,496)	(5,616)	(4,488)
Amortization of prior service cost	29	15	87	45
Amortization of net loss	1,163	1,097	3,489	3,291
Net periodic pension cost	$ 2,058	$ 2,336	$ 6,174	$ 7,008