Note 2 - Investment Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
US Government Agencies Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	$ 62,927	$ 139,765
Continuous unrealized loss, less than 12 months, gross unrealized losses	(301)	(1,270)
Continuous unrealized loss, greater than 12 months, fair value	110,587	139,720
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(1,815)	(5,522)
Continuous unrealized loss, fair value	173,514	279,485
Continuous unrealized loss, gross unrealized losses	(2,116)	(6,792)
US States and Political Subdivisions Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value		1,035
Continuous unrealized loss, less than 12 months, gross unrealized losses		(20)
Continuous unrealized loss, greater than 12 months, fair value
Continuous unrealized loss, greater than 12 months, gross unrealized losses
Continuous unrealized loss, fair value		1,035
Continuous unrealized loss, gross unrealized losses		(20)
Continuous unrealized loss, less than 12 months, fair value	62,927	140,800
Continuous unrealized loss, less than 12 months, gross unrealized losses	(301)	(1,290)
Continuous unrealized loss, greater than 12 months, fair value	110,587	139,720
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(1,815)	(5,522)
Continuous unrealized loss, fair value	173,514	280,520
Continuous unrealized loss, gross unrealized losses	$ (2,116)	$ (6,812)