Note 2 - Investment Securities - Unrealized Loss Positions of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	$ 23,703	$ 24,374
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(726)	(281)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	14,061	12,713
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(583)	(302)
Securities available-for-sale in a continuous unrealized loss position, total fair value	37,764	37,087
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(1,309)	(583)
US Government Agencies Debt Securities [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	7,521	6,859
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(186)	(44)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	2,881	2,995
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(78)	(57)
Securities available-for-sale in a continuous unrealized loss position, total fair value	10,402	9,854
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(264)	(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	11,599	15,624
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(467)	(192)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	8,724	7,386
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(376)	(183)
Securities available-for-sale in a continuous unrealized loss position, total fair value	20,323	23,010
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(843)	(375)
Corporate Debt Securities [Member]
Securities available-for-sale in a continuous unrealized loss position, less than 12 months fair value	3,118	1,438
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(62)	(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	3,118	1,438
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	(62)	(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	1,465	453
Securities available-for-sale in a continuous unrealized loss position, less than 12 months unrealized loss	(11)	(3)
Securities available-for-sale in a continuous unrealized loss position, 12 months or more fair value	2,456	2,332
Securities available-for-sale in a continuous unrealized loss position, 12 months or more unrealized loss	(129)	(62)
Securities available-for-sale in a continuous unrealized loss position, total fair value	3,921	2,785
Securities available-for-sale in a continuous unrealized loss position, total unrealized loss	$ (140)	$ (65)