Note 3 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale, amortized cost, due in one year or less	$ 27,301
Available-for-sale, estimated fair value, due in one year or less	27,276
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	50,953
Available-for-sale, estimated fair value, due after one year through five years	50,676
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	63,345
Available-for-sale, estimated fair value, due after five years through ten years	62,451
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	1,322
Available-for-sale, amortized cost, due after ten years	40,109
Available-for-sale, estimated fair value, due after ten years	40,170
Held-to-maturity, amortized cost, due after ten years	2,132
Held-to-maturity, estimated fair value, due after ten years	2,156
Available-for-sale, amortized cost, mortgage-backed securities	246,123
Available-for-sale, estimated fair value, mortgage-backed securities	242,745
Held-to-maturity, amortized cost, mortgage-backed securities	0
Held-to-maturity, estimated fair value, mortgage-backed securities	0
Available-for-sale, amortized cost	427,831
Available-for-sale, estimated fair value	423,318
Held-to-maturity, amortized cost	3,364	$ 3,488
Held-to-maturity, estimated fair value	$ 3,478	$ 3,597