Note 7 - Investment Securities - Investment Securities With Unrealized Losses Not Recognized In Income (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Less than 12 Months Fair Value	$ 169,923	$ 145,433
Less than 12 Months Unrealized Losses	3,202	1,039
12 Months or More Fair Value	187,566	193,588
12 Months or More Unrealized Losses	7,175	4,655
Total Fair Value	357,489	339,021
Total Unrealized Losses	10,377	5,694
US Government-sponsored Enterprises Debt Securities [Member]
Less than 12 Months Fair Value	13,336	11,544
Less than 12 Months Unrealized Losses	182	43
12 Months or More Fair Value	27,010	25,298
12 Months or More Unrealized Losses	509	394
Total Fair Value	40,346	36,842
Total Unrealized Losses	691	437
US States and Political Subdivisions Debt Securities [Member]
Less than 12 Months Fair Value	52,756	40,402
Less than 12 Months Unrealized Losses	956	413
12 Months or More Fair Value	33,058	33,965
12 Months or More Unrealized Losses	1,409	860
Total Fair Value	85,814	74,367
Total Unrealized Losses	2,365	1,273
Residential Mortgage Backed Securities [Member]
Less than 12 Months Fair Value	74,591	77,312
Less than 12 Months Unrealized Losses	1,642	481
12 Months or More Fair Value	93,924	99,986
12 Months or More Unrealized Losses	3,516	2,254
Total Fair Value	168,515	177,298
Total Unrealized Losses	5,158	2,735
Commercial Mortgage Backed Securities [Member]
Less than 12 Months Fair Value	14,798	7,758
Less than 12 Months Unrealized Losses	378	62
12 Months or More Fair Value	33,076	34,139
12 Months or More Unrealized Losses	1,738	1,146
Total Fair Value	47,874	41,897
Total Unrealized Losses	2,116	1,208
Asset-backed Securities [Member]
Less than 12 Months Fair Value	11,721	1,166
Less than 12 Months Unrealized Losses	29	4
12 Months or More Fair Value
12 Months or More Unrealized Losses
Total Fair Value	11,721	1,166
Total Unrealized Losses	29	4
Corporate Debt Securities [Member]
Less than 12 Months Fair Value	2,721	7,251
Less than 12 Months Unrealized Losses	15	36
12 Months or More Fair Value	498	200
12 Months or More Unrealized Losses	3	1
Total Fair Value	3,219	7,451
Total Unrealized Losses	$ 18	$ 37