Note 2 - Investment Securities - Unrealized Loss Positions of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	$ 11,726	$ 24,374
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(361)	(281)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	25,204	12,713
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(1,199)	(302)
Securities available-for-sale in a continuous unrealized loss position, total fair value	36,930	37,087
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(1,560)	(583)
US Government Agencies Debt Securities [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	3,717	6,859
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(76)	(44)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	6,330	2,995
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(263)	(57)
Securities available-for-sale in a continuous unrealized loss position, total fair value	10,047	9,854
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(339)	(101)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	4,567	15,624
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(211)	(192)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	14,758	7,386
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(743)	(183)
Securities available-for-sale in a continuous unrealized loss position, total fair value	19,325	23,010
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(954)	(375)
Corporate Debt Securities [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	1,661	1,438
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(38)	(42)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	1,446
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(36)
Securities available-for-sale in a continuous unrealized loss position, total fair value	3,107	1,438
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(74)	(42)
US States and Political Subdivisions Debt Securities [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	1,781	453
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(36)	(3)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	2,670	2,332
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(157)	(62)
Securities available-for-sale in a continuous unrealized loss position, total fair value	4,451	2,785
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	$ (193)	$ (65)