Note 5 - Investment Securities - Unrealized Gain (Loss) on Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment securities available for sale, at amortized cost	$ 291,440	$ 326,740
Investment securities available for sale, gross unrealized gains	1,322	723
Investment securities available for sale, gross unrealized losses	(5,068)	(6,449)
Investment securities available for sale, at fair value	287,694	321,014
Investment securities held to maturity, at amortized cost	742,435	761,563
Investment securities held to maturity, gross unrealized gains	3,294	570
Investment securities held to maturity, gross unrealized losses	(8,994)	(14,810)
Investment securities held to maturity, at fair value	736,735	747,323
Collateralized Mortgage Obligations [Member]
Investment securities available for sale, at amortized cost	177,000	197,812
Investment securities available for sale, gross unrealized gains	913	567
Investment securities available for sale, gross unrealized losses	(1,405)	(2,120)
Investment securities available for sale, at fair value	176,508	196,259
Investment securities held to maturity, at amortized cost	488,402	500,690
Investment securities held to maturity, gross unrealized gains	2,665	570
Investment securities held to maturity, gross unrealized losses	(4,021)	(5,793)
Investment securities held to maturity, at fair value	487,046	495,467
Agency Mortgage-backed Securities [Member]
Investment securities available for sale, at amortized cost	38,380	39,105
Investment securities available for sale, gross unrealized gains	176	5
Investment securities available for sale, gross unrealized losses	(325)	(611)
Investment securities available for sale, at fair value	38,231	38,499
Investment securities held to maturity, at amortized cost	149,923	153,483
Investment securities held to maturity, gross unrealized gains	588
Investment securities held to maturity, gross unrealized losses	(2,727)	(5,245)
Investment securities held to maturity, at fair value	147,784	148,238
US States and Political Subdivisions Debt Securities [Member]
Investment securities available for sale, at amortized cost	15,061	20,807
Investment securities available for sale, gross unrealized gains	190	64
Investment securities available for sale, gross unrealized losses	(3)	(232)
Investment securities available for sale, at fair value	15,248	20,639
Corporate Debt Securities [Member]
Investment securities available for sale, at amortized cost	60,999	62,583
Investment securities available for sale, gross unrealized gains	43	87
Investment securities available for sale, gross unrealized losses	(3,335)	(3,396)
Investment securities available for sale, at fair value	57,707	59,274
Asset-backed Securities [Member]
Investment securities available for sale, at amortized cost		6,433
Investment securities available for sale, gross unrealized gains
Investment securities available for sale, gross unrealized losses		(90)
Investment securities available for sale, at fair value		6,343
US Government Agencies Debt Securities [Member]
Investment securities held to maturity, at amortized cost	104,110	107,390
Investment securities held to maturity, gross unrealized gains	41
Investment securities held to maturity, gross unrealized losses	(2,246)	(3,772)
Investment securities held to maturity, at fair value	$ 101,905	$ 103,618

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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