Note 9 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Service cost	$ 5,009	$ 4,786	$ 4,309
Interest cost	5,508	5,356	5,229
Expected return on plan assets	(6,536)	(5,849)	(5,299)
Amortization of prior service benefit	61	(337)	(337)
Amortization of net gain from earlier periods	3,726	4,132	4,372
Net periodic pension cost	$ 7,768	$ 8,088	$ 8,274