Note 4 - Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Due in one year or less, Amortized Cost	$ 124,807
Due in one year or less, Fair Value	124,757
Due after one year through five years, Amortized Cost	465,813
Due after one year through five years, Fair Value	455,010
Due after five years through ten years, Amortized Cost	6,078
Due after five years through ten years, Fair Value	5,856
Due after ten years, Amortized Cost	671,033
Due after ten years, Fair Value	656,886
Total, Amortized Cost	1,267,731	$ 1,336,345
Total, Fair Value	$ 1,242,509	$ 1,333,626