Note 5 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
U.S. federal statutory income tax rate	35.00%	35.00%
Tax credits	(4.60%)	(9.40%)
Excess tax benefits from share-based compensation arrangements	(3.00%)
Increase in cash surrender value of life insurance	(1.40%)	(0.90%)
Tax exempt interest income	(1.10%)	(1.30%)
State income taxes, net of federal benefit	0.70%	0.90%
Other, net	0.30%	2.70%
Effective income tax rate	25.90%	27.00%