Note 3 - Available for Sale Debt Securities - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available for sale debt securities, amortized cost	$ 416,767	$ 469,313
Securities available-for-sale, unrealized gains	12,884	2,559
Securities available-for-sale, unrealized losses	(467)	(1,134)
Securities available-for-sale, allowance for credit losses	0	0
Securities available-for-sale, fair value	429,184	470,738
US Treasury Securities [Member]
Available for sale debt securities, amortized cost	0	49,887
Securities available-for-sale, unrealized gains	0	10
Securities available-for-sale, unrealized losses	0	0
Securities available-for-sale, allowance for credit losses	0	0
Securities available-for-sale, fair value	0	49,897
US Government-sponsored Enterprises Debt Securities [Member]
Available for sale debt securities, amortized cost	136,965	208,933
Securities available-for-sale, unrealized gains	5,294	1,189
Securities available-for-sale, unrealized losses	(378)	(178)
Securities available-for-sale, allowance for credit losses	0	0
Securities available-for-sale, fair value	141,881	209,944
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Available for sale debt securities, amortized cost	272,145	193,574
Securities available-for-sale, unrealized gains	7,396	1,243
Securities available-for-sale, unrealized losses	(89)	(956)
Securities available-for-sale, allowance for credit losses	0	0
Securities available-for-sale, fair value	279,452	193,861
US States and Political Subdivisions Debt Securities [Member]
Available for sale debt securities, amortized cost	7,657	16,919
Securities available-for-sale, unrealized gains	194	117
Securities available-for-sale, unrealized losses	0	0
Securities available-for-sale, allowance for credit losses	0	0
Securities available-for-sale, fair value	$ 7,851	$ 17,036

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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