Note 7 - Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Due in one year or less, Amortized Cost	$ 358,743
Due in one year or less, Fair Value	358,622
Due after one year through five years, Amortized Cost	465,994
Due after one year through five years, Fair Value	450,736
Due after five years through ten years, Amortized Cost	4,478
Due after five years through ten years, Fair Value	4,314
Due after ten years, Amortized Cost	680,927
Due after ten years, Fair Value	662,277
Total, Amortized Cost	1,510,142	$ 1,336,345
Total, Fair Value	$ 1,475,949	$ 1,333,626