Note 4 - Securities - Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Securities available-for-sale, less than 12 months, fair value	$ 100,662	$ 336,779
Securities available-for-sale, less than 12 months, unrealized losses	(44)	(1,145)
Securities available-for-sale, 12 months or more, fair value	287,807	121,981
Securities available-for-sale, 12 months or more, unrealized losses	(7,248)	(2,959)
Securities available-for-sale, fair value	388,469	458,760
Securities available-for-sale, unrealized losses	(7,292)	(4,104)
US Government-sponsored Enterprises Debt Securities [Member]
Securities available-for-sale, less than 12 months, fair value	96,740	95,139
Securities available-for-sale, less than 12 months, unrealized losses	(38)	(586)
Securities available-for-sale, 12 months or more, fair value	149,320	49,870
Securities available-for-sale, 12 months or more, unrealized losses	(3,313)	(896)
Securities available-for-sale, fair value	246,060	145,009
Securities available-for-sale, unrealized losses	(3,351)	(1,482)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available-for-sale, less than 12 months, fair value	3,108	69,290
Securities available-for-sale, less than 12 months, unrealized losses	(5)	(440)
Securities available-for-sale, 12 months or more, fair value	120,848	67,047
Securities available-for-sale, 12 months or more, unrealized losses	(3,748)	(2,007)
Securities available-for-sale, fair value	123,956	136,337
Securities available-for-sale, unrealized losses	(3,753)	(2,447)
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, less than 12 months, fair value	814	22,366
Securities available-for-sale, less than 12 months, unrealized losses	(1)	(107)
Securities available-for-sale, 12 months or more, fair value	17,639	5,064
Securities available-for-sale, 12 months or more, unrealized losses	(187)	(56)
Securities available-for-sale, fair value	18,453	27,430
Securities available-for-sale, unrealized losses	$ (188)	(163)
US Treasury Securities [Member]
Securities available-for-sale, less than 12 months, fair value		149,984
Securities available-for-sale, less than 12 months, unrealized losses		(12)
Securities available-for-sale, 12 months or more, fair value
Securities available-for-sale, 12 months or more, unrealized losses
Securities available-for-sale, fair value		149,984
Securities available-for-sale, unrealized losses		$ (12)