Note 7 - Investment Securities - Investment Securities With Unrealized Losses Not Recognized In Income (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Less than 12 Months Fair Value	$ 156,063	$ 145,433
Less than 12 Months Unrealized Losses	4,014	1,039
12 Months or More Fair Value	178,869	193,588
12 Months or More Unrealized Losses	7,664	4,655
Total Fair Value	334,932	339,021
Total Unrealized Losses	11,678	5,694
US Government-sponsored Enterprises Debt Securities [Member]
Less than 12 Months Fair Value	10,175	11,544
Less than 12 Months Unrealized Losses	282	43
12 Months or More Fair Value	22,987	25,298
12 Months or More Unrealized Losses	575	394
Total Fair Value	33,162	36,842
Total Unrealized Losses	857	437
US States and Political Subdivisions Debt Securities [Member]
Less than 12 Months Fair Value	50,261	40,402
Less than 12 Months Unrealized Losses	1,051	413
12 Months or More Fair Value	33,147	33,965
12 Months or More Unrealized Losses	1,399	860
Total Fair Value	83,408	74,367
Total Unrealized Losses	2,450	1,273
Residential Mortgage Backed Securities [Member]
Less than 12 Months Fair Value	74,287	77,312
Less than 12 Months Unrealized Losses	2,010	481
12 Months or More Fair Value	90,087	99,986
12 Months or More Unrealized Losses	3,820	2,254
Total Fair Value	164,374	177,298
Total Unrealized Losses	5,830	2,735
Commercial Mortgage Backed Securities [Member]
Less than 12 Months Fair Value	14,509	7,758
Less than 12 Months Unrealized Losses	611	62
12 Months or More Fair Value	32,400	34,139
12 Months or More Unrealized Losses	1,867	1,146
Total Fair Value	46,909	41,897
Total Unrealized Losses	2,478	1,208
Asset-backed Securities [Member]
Less than 12 Months Fair Value	3,729	1,166
Less than 12 Months Unrealized Losses	26	4
12 Months or More Fair Value
12 Months or More Unrealized Losses
Total Fair Value	3,729	1,166
Total Unrealized Losses	26	4
Corporate Debt Securities [Member]
Less than 12 Months Fair Value	3,102	7,251
Less than 12 Months Unrealized Losses	34	36
12 Months or More Fair Value	248	200
12 Months or More Unrealized Losses	3	1
Total Fair Value	3,350	7,451
Total Unrealized Losses	$ 37	$ 37