Note 4 - Securities - Securities Held-to-maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Securities held-to-maturity, amortized costs	$ 122,905	$ 127,164
Securities held-to-maturity, gross unrealized gains	2,301	3,122
Securities held-to-maturity, gross unrealized losses	657	173
Securities held-to-maturity, fair values	124,549	130,113
US Government Agencies Debt Securities [Member]
Securities held-to-maturity, amortized costs	3,934	3,934
Securities held-to-maturity, gross unrealized gains	99	167
Securities held-to-maturity, gross unrealized losses	1	0
Securities held-to-maturity, fair values	4,032	4,101
US States and Political Subdivisions Debt Securities [Member]
Securities held-to-maturity, amortized costs	117,791	122,039
Securities held-to-maturity, gross unrealized gains	2,184	2,929
Securities held-to-maturity, gross unrealized losses	649	173
Securities held-to-maturity, fair values	119,326	124,795
Collateralized Mortgage Backed Securities [Member]
Securities held-to-maturity, amortized costs	196	209
Securities held-to-maturity, gross unrealized gains	18	21
Securities held-to-maturity, gross unrealized losses	0	0
Securities held-to-maturity, fair values	214	230
Corporate Debt Securities [Member]
Securities held-to-maturity, amortized costs	984	982
Securities held-to-maturity, gross unrealized gains		5
Securities held-to-maturity, gross unrealized losses	7	0
Securities held-to-maturity, fair values	$ 977	$ 987