Note 8 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
U.S. federal income tax rate	35.00%	35.00%	35.00%
Tax credits	(14.40%)	(9.70%)	(2.50%)
Net deferred tax asset remeasurement	10.80%
Amortization/impairment of investments in tax credit partnerships	3.40%	2.80%	0.40%
Stock based compensation	(2.60%)
Cash surrender value of life insurance	(1.50%)	(0.90%)	(0.80%)
Tax exempt interest income	(1.20%)	(1.20%)	(1.40%)
Other, net	0.90%	0.30%	(0.20%)
State income taxes	0.70%	0.80%	0.80%
Effective tax rate	31.10%	27.10%	31.30%