Note 3 - Investment Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Continuous unrealized loss, less than 12 months, fair value	$ 11,906	$ 41,704
Continuous unrealized loss, less than 12 months, gross unrealized losses	(25)	(528)
Continuous unrealized loss, greater than 12 months, fair value	183,086	258,492
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(3,151)	(10,160)
Continuous unrealized loss, fair value	194,992	300,196
Continuous unrealized loss, gross unrealized losses	(3,176)	(10,688)
US Government Agencies Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	9,891	39,083
Continuous unrealized loss, less than 12 months, gross unrealized losses	(19)	(504)
Continuous unrealized loss, greater than 12 months, fair value	182,244	255,747
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(3,144)	(10,092)
Continuous unrealized loss, fair value	192,135	294,830
Continuous unrealized loss, gross unrealized losses	(3,163)	(10,596)
US States and Political Subdivisions Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value		612
Continuous unrealized loss, less than 12 months, gross unrealized losses		(3)
Continuous unrealized loss, greater than 12 months, fair value		1,882
Continuous unrealized loss, greater than 12 months, gross unrealized losses		(34)
Continuous unrealized loss, fair value		2,494
Continuous unrealized loss, gross unrealized losses		(37)
Corporate Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	2,015	2,009
Continuous unrealized loss, less than 12 months, gross unrealized losses	(6)	(21)
Continuous unrealized loss, greater than 12 months, fair value	842	863
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(7)	(34)
Continuous unrealized loss, fair value	2,857	2,872
Continuous unrealized loss, gross unrealized losses	$ (13)	$ (55)