Note 6 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale, amortized cost, due in one year or less	$ 30,884
Available-for-sale, estimated fair value, due in one year or less	30,911
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	71,412
Available-for-sale, estimated fair value, due after one year through five years	71,703
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	76,611
Available-for-sale, estimated fair value, due after five years through ten years	76,197
Held-to-maturity, amortized cost, due after five years through ten years	1,302
Held-to-maturity, estimated fair value, due after five years through ten years	1,402
Available-for-sale, amortized cost, due after ten years	23,017
Available-for-sale, estimated fair value, due after ten years	23,140
Held-to-maturity, amortized cost, due after ten years	2,146
Held-to-maturity, estimated fair value, due after ten years	2,195
Available-for-sale, amortized cost, mortgage-backed securities	246,231
Available-for-sale, estimated fair value, mortgage-backed securities	244,659
Held-to-maturity, amortized cost, mortgage-backed securities	0
Held-to-maturity, estimated fair value, mortgage-backed securities	0
Available-for-sale, amortized cost	448,155
Available-for-sale, estimated fair value	446,610
Held-to-maturity, amortized cost	3,448	$ 3,488
Held-to-maturity, estimated fair value	$ 3,597	$ 3,597