Note 8 - Income Taxes - Reconciliation of Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Tax at federal statutory rate	21.00%	21.00%	35.00%
Federal statutory rate change on deferred tax liability			(35.20%)
State income taxes, net of federal benefit	(0.20%)	5.90%	2.70%
Compensation-based credits	(1.20%)	(1.90%)	(1.00%)
Nondeductible expenses	0.40%	0.70%	0.50%
Tax exempt interest	(0.20%)	(0.20%)	(0.30%)
Company provided benefits	1.60%	0.10%	0.50%
Other, net	0.40%	0.40%	(0.40%)
Effective tax rate	21.80%	26.00%	1.80%