Note 7 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
U.S. federal statutory income tax rate	21.00%	21.00%	21.00%	21.00%
State income taxes, net of federal benefit	3.20%	(1.10%)	3.10%	(1.10%)
Excess tax benefit from stock-based compensation arrangements	(2.90%)	(0.40%)	(3.20%)	0.00%
Change in cash surrender value of life insurance	(4.40%)	(2.00%)	(1.10%)	(0.40%)
Tax credits	(1.20%)	(5.90%)	(0.60%)	(5.90%)
Tax exempt interest income	(1.30%)	(0.20%)	(0.30%)	(0.30%)
Non-deductible merger expenses	4.20%	0.00%	0.90%	0.00%
Insurance captive	(0.40%)	0.00%	(0.10%)	0.00%
Other, net	(1.10%)	3.50%	(0.70%)	1.40%
Effective tax rate	17.10%	14.90%	19.00%	14.70%