Note 8 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Service cost	$ 1,252	$ 1,196	$ 3,756	$ 3,588
Interest cost	1,377	1,339	4,131	4,017
Expected return on plan assets	(1,634)	(1,462)	(4,902)	(4,386)
Amortization of prior service cost	15	(84)	45	(252)
Amortization of net loss	931	1,033	2,793	3,099
Net periodic pension cost	$ 1,941	$ 2,022	$ 5,823	$ 6,066