Note 8 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Service cost	$ 4,401	$ 5,009	$ 4,786
Interest cost	6,482	5,508	5,356
Expected return on plan assets	(5,982)	(6,536)	(5,849)
Amortization of prior service benefit	61	61	(337)
Amortization of net gain from earlier periods	4,389	3,726	4,132
Net periodic pension cost	$ 9,351	$ 7,768	$ 8,088