Note 11 - Federal Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax at statutory rate	$ 10,883,000	$ 16,129,000	$ 16,410,000
Tax-exempt interest	(620,000)	(1,030,000)	(876,000)
Bank owned life insurance	(201,000)	(948,000)	(440,000)
Effect of federal tax law change	0	1,336,000	0
Change in valuation allowance	0	0	(113,000)
Other	(264,000)	(678,000)	(7,000)
Tax expense	$ 9,798,000	$ 14,809,000	$ 14,974,000