Note 8 - Retirement Plans - Components of Net Periodic Benefit Cost for Pension Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net periodic pension cost:
Service cost	$ 0	$ 0	$ 3,130
Interest cost	4,669	4,398	4,159
Expected return on plan assets	(5,648)	(4,836)	(4,782)
Recognized net actuarial loss	484	406	1,580
Net periodic pension (benefit) cost	$ (494)	$ (32)	$ 4,087