Note 4 - Available for Sale Debt Securities - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available for sale debt securities, amortized cost	$ 574,722	$ 469,313
Securities available-for-sale, unrealized gains	12,785	2,559
Securities available-for-sale, unrealized losses	(529)	(1,134)
Securities available-for-sale, fair value	586,978	470,738
US Government-sponsored Enterprises Debt Securities [Member]
Available for sale debt securities, amortized cost	133,436	208,933
Securities available-for-sale, unrealized gains	5,003	1,189
Securities available-for-sale, unrealized losses	(361)	(178)
Securities available-for-sale, fair value	138,078	209,944
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available for sale debt securities, amortized cost	430,198	193,574
Securities available-for-sale, unrealized gains	7,555	1,243
Securities available-for-sale, unrealized losses	(168)	(956)
Securities available-for-sale, fair value	437,585	193,861
US States and Political Subdivisions Debt Securities [Member]
Available for sale debt securities, amortized cost	11,088	16,919
Securities available-for-sale, unrealized gains	227	117
Securities available-for-sale, unrealized losses	0	0
Securities available-for-sale, fair value	$ 11,315	17,036
US Treasury Securities [Member]
Available for sale debt securities, 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

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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