Note 9 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
U.S. federal statutory income tax rate	21.00%	21.00%	35.00%
Kentucky state income tax enactments	(5.20%)	0.00%	0.00%
Net deferred tax asset remeasurement	0.00%	(0.50%)	10.80%
Excess tax benefits from stock-based compensation arrangements	(1.00%)	(0.80%)	(2.60%)
Increase in cash surrender value of life insurance	(0.90%)	(0.40%)	(1.50%)
Tax credits	(1.90%)	(1.80%)	(14.40%)
Tax exempt interest income	(0.30%)	(0.40%)	(1.20%)
Amortization/impairment of investments in tax credit partnerships	0.30%	0.40%	3.40%
State income taxes, net of federal benefit	0.70%	0.80%	0.70%
Other, net	0.00%	(0.50%)	0.90%
Effective tax rate	12.70%	17.80%	31.10%