Note 11 - Federal Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Tax at statutory rate (35%)	$ 16,129,000	$ 16,410,000	$ 13,591,000
Tax-exempt interest	(1,030,000)	(876,000)	(781,000)
Bank owned life insurance	(948,000)	(440,000)	(384,000)
Effect of federal tax law change	1,336,000	0	0
Change in valuation allowance	0	(113,000)	(180,000)
Other	(678,000)	(7,000)	(435,000)
Tax expense	$ 14,809,000	$ 14,974,000	$ 11,811,000