Note 8 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.S. federal income tax rate	21.00%	35.00%	35.00%
Tax credits	(1.80%)	(14.40%)	(9.70%)
Stock based compensation	(0.80%)	(2.60%)
Net deferred tax asset remeasurement	(0.50%)	10.80%
Other, net	(0.50%)	0.90%	0.30%
Cash surrender value of life insurance	(0.40%)	(1.50%)	(0.90%)
Tax exempt interest income	(0.40%)	(1.20%)	(1.20%)
Amortization/impairment of investments in tax credit partnerships	(0.40%)	(3.40%)	(2.80%)
State income taxes	0.80%	0.70%	0.80%
Effective income tax rate	17.80%	31.10%	27.10%