Note 3 - Investment Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Continuous unrealized loss, less than 12 months, fair value	$ 43,500	$ 41,704
Continuous unrealized loss, less than 12 months, gross unrealized losses	(169)	(528)
Continuous unrealized loss, greater than 12 months, fair value	130,237	258,492
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(2,004)	(10,160)
Continuous unrealized loss, fair value	173,737	300,196
Continuous unrealized loss, gross unrealized losses	(2,173)	(10,688)
US Government Agencies Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	43,500	39,083
Continuous unrealized loss, less than 12 months, gross unrealized losses	(169)	(504)
Continuous unrealized loss, greater than 12 months, fair value	127,407	255,747
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(1,994)	(10,092)
Continuous unrealized loss, fair value	170,907	294,830
Continuous unrealized loss, gross unrealized losses	(2,163)	(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		2,009
Continuous unrealized loss, less than 12 months, gross unrealized losses		(21)
Continuous unrealized loss, greater than 12 months, fair value	2,830	863
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(10)	(34)
Continuous unrealized loss, fair value	2,830	2,872
Continuous unrealized loss, gross unrealized losses	$ (10)	$ (55)