Note 7 - Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Due in one year or less, Amortized Cost	$ 199,497
Due in one year or less, Fair Value	199,383
Due after one year through five years, Amortized Cost	465,892
Due after one year through five years, Fair Value	450,363
Due after five years through ten years, Amortized Cost	6,434
Due after five years through ten years, Fair Value	6,163
Due after ten years, Amortized Cost	649,020
Due after ten years, Fair Value	627,151
Total, Amortized Cost	1,320,843	$ 1,336,345
Total, Fair Value	$ 1,283,060	$ 1,333,626