Note 4 - Securities Available for Sale - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Securities available for sale, amortized cost	$ 469,313	$ 443,742
Securities available-for-sale, unrealized gains	2,559	545
Securities available-for-sale, unrealized losses	(1,134)	(7,292)
Securities available-for-sale, fair value	470,738	436,995
US Treasury Securities [Member]
Securities available for sale, amortized cost	49,887
Securities available-for-sale, unrealized gains	10
Securities available-for-sale, unrealized losses	0
Securities available-for-sale, fair value	49,897
US Government-sponsored Enterprises Debt Securities [Member]
Securities available for sale, amortized cost	208,933	264,234
Securities available-for-sale, unrealized gains	1,189	156
Securities available-for-sale, unrealized losses	(178)	(3,351)
Securities available-for-sale, fair value	209,944	261,039
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available for sale, amortized cost	193,574	149,748
Securities available-for-sale, unrealized gains	1,243	282
Securities available-for-sale, unrealized losses	(956)	(3,753)
Securities available-for-sale, fair value	193,861	146,277
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale, amortized cost	16,919	29,760
Securities available-for-sale, unrealized gains	117	107
Securities available-for-sale, unrealized losses	0	(188)
Securities available-for-sale, fair value	$ 17,036	$ 29,679

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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