Note 9 - Employee Benefit Plans - Change in Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Benefit obligation at beginning of year	$ 147,023	$ 132,270
Service cost	5,009	4,786	$ 4,309
Interest cost	5,508	5,356	5,229
Amendments		2,571
Change due to mortality change	2,713
Change due to discount rate change	(13,716)	5,529
Actuarial loss	3,487	1,733
Benefits paid	(5,386)	(5,222)
Benefit obligation at end of year	144,638	147,023	132,270
Fair value of plan assets at beginning of year	99,922	87,357
Actual (loss) return on plan assets	(8,772)	10,560
Employer contributions	10,565	7,227
Benefits paid	(5,386)	(5,222)
Fair value of plan assets at end of year	96,329	99,922	$ 87,357
Underfunded status at end of year	$ (48,309)	$ (47,101)