Note 7 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale, amortized cost, due in one year or less	$ 19,847
Available-for-sale, estimated fair value, due in one year or less	19,828
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	53,443
Available-for-sale, estimated fair value, due after one year through five years	52,717
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	51,363
Available-for-sale, estimated fair value, due after five years through ten years	49,493
Held-to-maturity, amortized cost, due after five years through ten years	732
Held-to-maturity, estimated fair value, due after five years through ten years	748
Available-for-sale, amortized cost, due after ten years	38,627
Available-for-sale, estimated fair value, due after ten years	38,194
Held-to-maturity, amortized cost, due after ten years	2,105
Held-to-maturity, estimated fair value, due after ten years	2,120
Available-for-sale, amortized cost, mortgage-backed securities	227,813
Available-for-sale, estimated fair value, mortgage-backed securities	219,747
Held-to-maturity, amortized cost, mortgage-backed securities	0
Held-to-maturity, estimated fair value, mortgage-backed securities	0
Available-for-sale, amortized cost	391,093	$ 427,831
Available-for-sale, estimated fair value	379,979	423,318
Held-to-maturity, amortized cost	2,837	3,364
Held-to-maturity, estimated fair value	$ 2,868	$ 3,478