Note 2 - Investment Securities - Unrealized Loss Positions of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	$ 24,402	$ 24,374
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(635)	(281)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	13,452	12,713
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(536)	(302)
Securities available-for-sale in a continuous unrealized loss position, total fair value	37,854	37,087
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(1,171)	(583)
US Government Agencies Debt Securities [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	7,401	6,859
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(119)	(44)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	2,817	2,995
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(67)	(57)
Securities available-for-sale in a continuous unrealized loss position, total fair value	10,218	9,854
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(186)	(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	12,916	15,624
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(438)	(192)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	8,388	7,386
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(329)	(183)
Securities available-for-sale in a continuous unrealized loss position, total fair value	21,304	23,010
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(767)	(375)
Corporate Debt Securities [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	2,419	1,438
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(61)	(42)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss
Securities available-for-sale in a continuous unrealized loss position, total fair value	2,419	1,438
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(61)	(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	1,666	453
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(17)	(3)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	2,247	2,332
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(140)	(62)
Securities available-for-sale in a continuous unrealized loss position, total fair value	3,913	2,785
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	$ (157)	$ (65)