Note 5 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
U.S. federal statutory income tax rate	21.00%	35.00%	21.00%	35.00%	35.00%
Excess tax benefits from stock-based compensation arrangements		(1.50%)	(1.00%)	(3.00%)
Increase in cash surrender value of life insurance	(0.70%)	(1.10%)	(0.60%)	(1.40%)
Tax exempt interest income	(0.40%)	(1.00%)	(0.50%)	(1.10%)
Tax credits	(0.20%)	(3.20%)	(0.40%)	(4.60%)
Other, net	(0.10%)	(2.90%)	0.10%	0.30%
State income taxes, net of federal benefit	0.80%	0.60%	0.70%	0.70%
Effective income tax rate	20.40%	25.90%	19.30%	25.90%