Note 8 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Service cost	$ 4,786	$ 4,309	$ 4,216
Interest cost	5,356	5,229	4,823
Expected return on plan assets	(5,849)	(5,299)	(5,382)
Amortization of prior service benefit	(337)	(337)	(337)
Amortization of net gain from earlier periods	4,132	4,372	4,260
Net periodic pension cost	$ 8,088	$ 8,274	$ 7,580