Note 4 - Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Due in one year or less, Amortized Cost		$ 265,550
Due in one year or less, Fair Value		265,231
Due after one year through five years, Amortized Cost		465,195
Due after one year through five years, Fair Value		456,804
Due after five years through ten years, Amortized Cost		8,056
Due after five years through ten years, Fair Value		7,962
Due after ten years (1), Amortized Cost	[1]	597,544
Due after ten years (1), Fair Value	[1]	603,629
Total, Amortized Cost		1,336,345	$ 1,317,012
Total, Fair Value		$ 1,333,626	$ 1,314,345

[1] Equity securities are reported in this category