Note 4 - Securities - Securities Available-for-sale (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Securities available-for-sale, amortized costs	$ 458,605	$ 336,075
Securities available-for-sale, gross unrealized gains	1,600	954
Securities available-for-sale, gross unrealized losses	11,720	5,642
Securities available-for-sale, fair values	448,485	331,387
US Government Agencies Debt Securities [Member]
Securities available-for-sale, amortized costs	329,455	312,604
Securities available-for-sale, gross unrealized gains	267	609
Securities available-for-sale, gross unrealized losses	9,751	5,494
Securities available-for-sale, fair values	319,971	307,719
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, amortized costs	121,515	16,853
Securities available-for-sale, gross unrealized gains	1,272	100
Securities available-for-sale, gross unrealized losses	1,704	119
Securities available-for-sale, fair values	121,083	16,834
Collateralized Mortgage Backed Securities [Member]
Securities available-for-sale, amortized costs	632	602
Securities available-for-sale, gross unrealized gains	47	57
Securities available-for-sale, gross unrealized losses	0	0
Securities available-for-sale, fair values	679	659
Corporate Debt Securities [Member]
Securities available-for-sale, amortized costs	7,003	6,016
Securities available-for-sale, gross unrealized gains	14	188
Securities available-for-sale, gross unrealized losses	265	29
Securities available-for-sale, fair values	$ 6,752	$ 6,175