Note 3 - Investment Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Continuous unrealized loss, less than 12 months, fair value	$ 14,674	$ 41,704
Continuous unrealized loss, less than 12 months, gross unrealized losses	(86)	(528)
Continuous unrealized loss, greater than 12 months, fair value	255,900	258,492
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(7,194)	(10,160)
Continuous unrealized loss, fair value	270,574	300,196
Continuous unrealized loss, gross unrealized losses	(7,280)	(10,688)
US Government Agencies Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	12,669	39,083
Continuous unrealized loss, less than 12 months, gross unrealized losses	(66)	(504)
Continuous unrealized loss, greater than 12 months, fair value	253,449	255,747
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(7,159)	(10,092)
Continuous unrealized loss, fair value	266,118	294,830
Continuous unrealized loss, gross unrealized losses	(7,225)	(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	1,600	1,882
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(12)	(34)
Continuous unrealized loss, fair value	1,600	2,494
Continuous unrealized loss, gross unrealized losses	(12)	(37)
Corporate Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	2,005	2,009
Continuous unrealized loss, less than 12 months, gross unrealized losses	(20)	(21)
Continuous unrealized loss, greater than 12 months, fair value	851	863
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(23)	(34)
Continuous unrealized loss, fair value	2,856	2,872
Continuous unrealized loss, gross unrealized losses	$ (43)	$ (55)