Note 9 - Retirement Plans - Summary of Funded Status and Amounts Recognized on Balance Sheets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Projected benefit obligation at beginning of year	$ 126,086	$ 114,976
Interest cost	4,424	4,669	$ 4,398
Actuarial gains	(10,940)
Benefits paid	(2,916)	(3,529)
Merger of assumed plans		9,970
Projected benefit obligation at end of year	116,654	126,086	114,976
Fair value of pension plan assets at beginning of year	88,248	80,929
Actual return on plan assets	(4,417)	7,724
Company contributions	2,500	3,124
Benefits paid	(2,916)	(3,529)
Fair value of pension plan assets at end of year	83,415	88,248	$ 80,929
Funded status of pension plan	(33,239)	(37,838)
Accrued benefit cost	2,041
Accrued benefit cost		(1,502)
Accumulated other comprehensive loss	(35,280)	(36,336)
Net liability recognized	$ (33,239)	$ (37,838)