Note 7 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
U.S. federal statutory income tax rate	21.00%	21.00%	21.00%	21.00%
State income taxes, net of federal benefit	3.60%	0.60%	3.40%	0.70%
Excess tax benefit from stock-based compensation arrangements	(0.10%)	(2.60%)	(1.70%)	(0.90%)
Change in cash surrender value of life insurance	(0.20%)	(1.10%)	(0.70%)	(0.60%)
Tax credits	(0.30%)	(5.50%)	(0.80%)	(5.80%)
Tax exempt interest income	(0.40%)	(0.30%)	(0.30%)	(0.30%)
Non-deductible merger expenses	0.10%	0.00%	0.50%	0.00%
Insurance captive	(0.30%)	0.00%	(0.20%)	0.00%
Other, net	(0.40%)	(2.20%)	(0.30%)	(1.00%)
Effective tax rate	23.00%	9.90%	20.90%	13.10%