Note 2 - Investment Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Continuous unrealized loss, less than 12 months, fair value	$ 100,593	$ 123,708
Continuous unrealized loss, less than 12 months, gross unrealized losses	(3,426)	(1,700)
Continuous unrealized loss, greater than 12 months, fair value	211,591	190,196
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(12,254)	(6,811)
Continuous unrealized loss, fair value	312,184	313,904
Continuous unrealized loss, gross unrealized losses	(15,680)	(8,511)
US Government Agencies Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	96,095	120,139
Continuous unrealized loss, less than 12 months, gross unrealized losses	(3,370)	(1,655)
Continuous unrealized loss, greater than 12 months, fair value	209,451	190,196
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(12,180)	(6,811)
Continuous unrealized loss, fair value	305,546	310,335
Continuous unrealized loss, gross unrealized losses	(15,550)	(8,466)
US States and Political Subdivisions Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	2,498	2,598
Continuous unrealized loss, less than 12 months, gross unrealized losses	(21)	(23)
Continuous unrealized loss, greater than 12 months, fair value	1,258
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(35)
Continuous unrealized loss, fair value	3,756	2,598
Continuous unrealized loss, gross unrealized losses	(56)	(23)
Corporate Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	2,000
Continuous unrealized loss, less than 12 months, gross unrealized losses	(35)
Continuous unrealized loss, greater than 12 months, fair value	882
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(39)
Continuous unrealized loss, fair value	2,882
Continuous unrealized loss, gross unrealized losses	$ (74)
Asset-backed Securities [Member]
Continuous unrealized loss, less than 12 months, fair value		971
Continuous unrealized loss, less than 12 months, gross unrealized losses		(22)
Continuous unrealized loss, greater than 12 months, fair value
Continuous unrealized loss, greater than 12 months, gross unrealized losses
Continuous unrealized loss, fair value		971
Continuous unrealized loss, gross unrealized losses		$ (22)