Note 10 - Income Taxes - A Reconciliation of Income Tax Expense (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Computed at statutory rate	34.00%	34.00%	34.00%
State financial institution tax and credits	(10.20%)	(4.20%)	(4.80%)
Cash surrender value of life insurance	(2.10%)	(2.20%)	(1.50%)
Tax exempt interest	(3.00%)	(3.30%)	(1.70%)
Impact of 2017 Tax Act	13.10%
Other	1.50%	2.20%	4.10%
Actual effective rate	33.30%	26.50%	30.10%