Note 7 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale, amortized cost, due in one year or less	$ 28,921
Available-for-sale, estimated fair value, due in one year or less	28,872
Held-to-maturity, amortized cost, due in one year or less	0
Held-to-maturity, estimated fair value, due in one year or less	0
Available-for-sale, amortized cost, due after one year through five years	52,222
Available-for-sale, estimated fair value, due after one year through five years	51,572
Held-to-maturity, amortized cost, due after one year through five years	0
Held-to-maturity, estimated fair value, due after one year through five years	0
Available-for-sale, amortized cost, due after five years through ten years	55,496
Available-for-sale, estimated fair value, due after five years through ten years	53,726
Held-to-maturity, amortized cost, due after five years through ten years	1,232
Held-to-maturity, estimated fair value, due after five years through ten years	1,289
Available-for-sale, amortized cost, due after ten years	38,925
Available-for-sale, estimated fair value, due after ten years	38,643
Held-to-maturity, amortized cost, due after ten years	2,118
Held-to-maturity, estimated fair value, due after ten years	2,135
Available-for-sale, amortized cost, mortgage-backed securities	236,431
Available-for-sale, estimated fair value, mortgage-backed securities	229,475
Held-to-maturity, amortized cost, mortgage-backed securities	0
Held-to-maturity, estimated fair value, mortgage-backed securities	0
Available-for-sale, amortized cost	411,995	$ 427,831
Available-for-sale, estimated fair value	402,288	423,318
Held-to-maturity, amortized cost	3,350	3,364
Held-to-maturity, estimated fair value	$ 3,424	$ 3,478