Note 8 - Employee Benefit Plans - Change in Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at beginning of year	$ 132,270	$ 120,237
Service cost	4,786	4,309	$ 4,216
Interest cost	5,356	5,229	4,823
Amendments	2,571
Change due to discount rate change	5,529	5,088
Actuarial loss (gain)	1,733	2,412
Benefits paid	(5,222)	(5,006)
Benefit obligation at end of year	147,023	132,270	120,237
Fair value of plan assets at beginning of year	87,357	73,321
Actual return (loss) on plan assets	10,560	9,950
Employer contributions	7,227	9,092
Benefits paid	(5,222)	(5,006)
Fair value of plan assets at end of year	99,922	87,357	$ 73,321
Underfunded status at end of year	$ (47,101)	$ (44,913)