Note 7 - Employee Benefit Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Service cost	$ 1,252	$ 1,196	$ 2,504	$ 2,392
Interest cost	1,377	1,339	2,754	2,678
Expected return on plan assets	(1,634)	(1,462)	(3,268)	(2,924)
Amortization of prior service cost	15	(84)	30	(168)
Amortization of net loss	931	1,033	1,862	2,066
Net periodic pension cost	$ 1,941	$ 2,022	$ 3,882	$ 4,044