Note 5 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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 share-based compensation arrangements	(1.20%)	(0.70%)	(1.50%)	(3.80%)
Tax credits	(0.70%)	(5.10%)	(0.50%)	(5.30%)
Tax exempt interest income	(0.50%)	(1.10%)	(0.50%)	(1.10%)
Increase in cash surrender value of life insurance	(1.20%)	(1.90%)	(0.60%)	(1.60%)
State income taxes, net of federal benefit	0.70%	0.70%	0.70%	0.70%
Other, net	0.80%	2.20%	0.10%	2.10%
Effective income tax rate	18.90%	29.10%	18.70%	26.00%