Note 7 - Investment Securities - Investments by Contractual Maturity Date (Details) - USD ($)		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Due in one year or less, Amortized Cost		$ 140,165,000
Due in one year or less, Fair Value		139,922,000
Due after one year through five years, Amortized Cost		466,090,000
Due after one year through five years, Fair Value		453,280,000
Due after five years through ten years, Amortized Cost		5,783,000
Due after five years through ten years, Fair Value		5,624,000
Due after ten years (1), Amortized Cost	[1]	659,253,000
Due after ten years (1), Fair Value	[1]	642,279,000
Total, Amortized Cost		1,271,291,000	$ 1,336,345,000
Total, Fair Value		$ 1,241,105,000	$ 1,333,626,000	$ 1,300,000,000

[1] Equity securities are reported in this category