Note 6 - Retirement Plans - Components of Net Periodic Benefit Cost for Pension Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Service cost
Interest cost	1,106	1,167	3,318	3,502
Expected return on plan assets	(1,536)	(1,412)	(4,609)	(4,236)
Loss amortization	169	121	509	363
Net periodic (benefit) cost	$ (261)	$ (124)	$ (782)	$ (371)