Note 2 - Securities - Available-for-sale Securities (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Securities available for sale, amortized cost	$ 343,434,000	$ 341,948,000
Securities available for sale, gross unrealized gains	462,000	1,355,000
Securities available for sale, gross unrealized losses	(17,365,000)	(7,559,000)
Securities available for sale, fair value	326,531,000	335,744,000
US Government Agencies Debt Securities [Member]
Securities available for sale, amortized cost	194,900,000	175,953,000
Securities available for sale, gross unrealized gains	0	99,000
Securities available for sale, gross unrealized losses	(14,086,000)	(6,352,000)
Securities available for sale, fair value	180,814,000	169,700,000
Collateralized Mortgage Backed Securities [Member]
Securities available for sale, amortized cost	42,719,000	38,967,000
Securities available for sale, gross unrealized gains	189,000	335,000
Securities available for sale, gross unrealized losses	(1,271,000)	(510,000)
Securities available for sale, fair value	41,637,000	38,792,000
Municipal General Obligation Bonds [Member]
Securities available for sale, amortized cost	101,514,000	121,040,000
Securities available for sale, gross unrealized gains	272,000	891,000
Securities available for sale, gross unrealized losses	(1,921,000)	(638,000)
Securities available for sale, fair value	99,865,000	121,293,000
Municipal Revenue Bonds [Member]
Securities available for sale, amortized cost	3,801,000	3,978,000
Securities available for sale, gross unrealized gains	1,000	30,000
Securities available for sale, gross unrealized losses	(87,000)	(30,000)
Securities available for sale, fair value	3,715,000	3,978,000
Other Debt and Equity Securities [Member]
Securities available for sale, amortized cost	500,000	2,010,000
Securities available for sale, gross unrealized gains	0	0
Securities available for sale, gross unrealized losses	0	(29,000)
Securities available for sale, fair value	$ 500,000	$ 1,981,000