Note 2 - Investment Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Continuous unrealized loss, less than 12 months, fair value	$ 125,545	$ 123,708
Continuous unrealized loss, less than 12 months, gross unrealized losses	(3,600)	(1,700)
Continuous unrealized loss, greater than 12 months, fair value	194,329	190,196
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(10,300)	(6,811)
Continuous unrealized loss, fair value	319,874	313,904
Continuous unrealized loss, gross unrealized losses	(13,900)	(8,511)
US Government Agencies Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	121,705	120,139
Continuous unrealized loss, less than 12 months, gross unrealized losses	(3,542)	(1,655)
Continuous unrealized loss, greater than 12 months, fair value	193,486	190,196
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(10,267)	(6,811)
Continuous unrealized loss, fair value	315,191	310,335
Continuous unrealized loss, gross unrealized losses	(13,809)	(8,466)
US States and Political Subdivisions Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	2,931	2,598
Continuous unrealized loss, less than 12 months, gross unrealized losses	(22)	(23)
Continuous unrealized loss, greater than 12 months, fair value	843
Continuous unrealized loss, greater than 12 months, gross unrealized losses	(33)
Continuous unrealized loss, fair value	3,774	2,598
Continuous unrealized loss, gross unrealized losses	(55)	(23)
Corporate Debt Securities [Member]
Continuous unrealized loss, less than 12 months, fair value	909
Continuous unrealized loss, less than 12 months, gross unrealized losses	(36)
Continuous unrealized loss, greater than 12 months, fair value
Continuous unrealized loss, greater than 12 months, gross unrealized losses
Continuous unrealized loss, fair value	909
Continuous unrealized loss, gross unrealized losses	$ (36)
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)