Note 8 - Retirement Plans - Summary of Funded Status and Amounts Recognized on Balance Sheets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Projected benefit obligation at beginning of year	$ 114,976	$ 103,199
Interest cost	4,669	4,398	$ 4,159
Actuarial losses	0	1,839
Benefits paid	(3,529)	(2,790)
Merger of assumed plans	9,970	8,330
Projected benefit obligation at end of year	126,086	114,976	103,199
Fair value of pension plan assets at beginning of year	80,929	69,246
Actual return on plan assets	7,724	5,918
Company contributions	3,124	2,775
Benefits paid	(3,529)	(2,790)
Merger of assumed plans	0	5,780
Fair value of pension plan assets at end of year	88,248	80,929	$ 69,246
Funded status of pension plan	(37,838)	(34,047)
Accrued benefit cost	(1,502)	(5,121)
Accumulated other comprehensive loss	(36,336)	(28,926)
Net liability recognized	$ (37,838)	$ (34,047)