Note 3 - Investment Securities Available for Sale - Investment Securities With Unrealized Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Less than 12 months fair value	$ 92,764	$ 62,691
Less than 12 months unrealized losses	2,008	472
12 months or more fair value	32,817	34,616
12 months or more unrealized losses	1,506	845
Total fair value	125,581	97,307
Total unrealized losses	3,514	1,317
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Less than 12 months fair value	72,580	60,070
Less than 12 months unrealized losses	1,632	441
12 months or more fair value	29,541	31,213
12 months or more unrealized losses	1,267	732
Total fair value	102,121	91,283
Total unrealized losses	2,899	1,173
US States and Political Subdivisions Debt Securities [Member]
Less than 12 months fair value	20,184	2,621
Less than 12 months unrealized losses	376	31
12 months or more fair value	3,276	3,403
12 months or more unrealized losses	239	113
Total fair value	23,460	6,024
Total unrealized losses	$ 615	$ 144