Note 4 - Securities - Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities available-for-sale, less than 12 months, fair value	$ 336,779	$ 300,218
Securities available-for-sale, less than 12 months, unrealized losses	(1,145)	(3,597)
Securities available-for-sale, 12 months or more, fair value	121,981	14,877
Securities available-for-sale, 12 months or more, unrealized losses	(2,959)	(529)
Securities available-for-sale, fair value	458,760	315,095
Securities available-for-sale, unrealized losses	(4,104)	(4,126)
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)
US Government-sponsored Enterprises Debt Securities [Member]
Securities available-for-sale, less than 12 months, fair value	95,139	154,951
Securities available-for-sale, less than 12 months, unrealized losses	(586)	(1,344)
Securities available-for-sale, 12 months or more, fair value	49,870	3,485
Securities available-for-sale, 12 months or more, unrealized losses	(896)	(150)
Securities available-for-sale, fair value	145,009	158,436
Securities available-for-sale, unrealized losses	(1,482)	(1,494)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available-for-sale, less than 12 months, fair value	69,290	115,374
Securities available-for-sale, less than 12 months, unrealized losses	(440)	(1,873)
Securities available-for-sale, 12 months or more, fair value	67,047	9,914
Securities available-for-sale, 12 months or more, unrealized losses	(2,007)	(363)
Securities available-for-sale, fair value	136,337	125,288
Securities available-for-sale, unrealized losses	(2,447)	(2,236)
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale, less than 12 months, fair value	22,366	29,893
Securities available-for-sale, less than 12 months, unrealized losses	(107)	(380)
Securities available-for-sale, 12 months or more, fair value	5,064	1,478
Securities available-for-sale, 12 months or more, unrealized losses	(56)	(16)
Securities available-for-sale, fair value	27,430	31,371
Securities available-for-sale, unrealized losses	$ (163)	$ (396)