Note 2 - Securities - Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Securities available-for-sale, less than 12 months, fair value	$ 434,707	$ 336,779
Securities available-for-sale, less than 12 months, unrealized losses	(4,049)	(1,145)
Securities available-for-sale, 12 months or more, fair value	116,795	121,981
Securities available-for-sale, 12 months or more, unrealized losses	(4,055)	(2,959)
Securities available-for-sale, fair value	551,502	458,760
Securities available-for-sale, unrealized losses	(8,104)	(4,104)
US Government-sponsored Enterprises Debt Securities [Member]
Securities available-for-sale, less than 12 months, fair value	340,018	95,139
Securities available-for-sale, less than 12 months, unrealized losses	(2,188)	(586)
Securities available-for-sale, 12 months or more, fair value	48,470	49,870
Securities available-for-sale, 12 months or more, unrealized losses	(1,185)	(896)
Securities available-for-sale, fair value	388,488	145,009
Securities available-for-sale, unrealized losses	(3,373)	(1,482)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available-for-sale, less than 12 months, fair value	75,107	69,290
Securities available-for-sale, less than 12 months, unrealized losses	(1,669)	(440)
Securities available-for-sale, 12 months or more, fair value	63,083	67,047
Securities available-for-sale, 12 months or more, unrealized losses	(2,783)	(2,007)
Securities available-for-sale, fair value	138,190	136,337
Securities available-for-sale, unrealized losses	(4,452)	(2,447)
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, less than 12 months, fair value	19,582	22,366
Securities available-for-sale, less than 12 months, unrealized losses	(192)	(107)
Securities available-for-sale, 12 months or more, fair value	5,242	5,064
Securities available-for-sale, 12 months or more, unrealized losses	(87)	(56)
Securities available-for-sale, fair value	24,824	27,430
Securities available-for-sale, unrealized losses	$ (279)	(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)