Note 8 - Income Taxes - Difference Between Statutory and Effective Tax Rates (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
U.S. federal income tax rate	35.00%	35.00%	35.00%
Tax credits	(9.70%)	(2.50%)	(3.10%)
Tax credit amortization expense	2.80%	0.40%
Tax exempt interest income	(1.20%)	(1.40%)	(1.50%)
Cash surrender value of life insurance	(0.90%)	(0.80%)	(1.40%)
State income taxes	0.80%	0.80%	0.90%
Other, net	0.30%	(0.20%)	0.60%
Effective tax rate	27.10%	31.30%	30.50%