INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 826,729	$ 940,580
Allowance for Credit Losses	(101)	(112)	$ 0	$ 0
Gross Unrealized Gains	33,386	42,854
Gross Unrealized Losses	(362)	(443)
Estimated Fair Value	859,652	982,879
U.S. government sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,138	17,161
Allowance for Credit Losses	0	0
Gross Unrealized Gains	256	343
Gross Unrealized Losses	0	0
Estimated Fair Value	12,394	17,504
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	60,140	63,286
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,845	3,492
Gross Unrealized Losses	0	0
Estimated Fair Value	62,985	66,778
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	46,131	51,639
Allowance for Credit Losses	(101)	(112)
Gross Unrealized Gains	946	602
Gross Unrealized Losses	(229)	(233)
Estimated Fair Value	46,747	51,896
SBA pool securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	54,305	59,973
Allowance for Credit Losses	0	0
Gross Unrealized Gains	1,488	2,620
Gross Unrealized Losses	(102)	(96)
Estimated Fair Value	55,691	62,497
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	654,015	748,521
Allowance for Credit Losses	0	0
Gross Unrealized Gains	27,851	35,797
Gross Unrealized Losses	(31)	(114)
Estimated Fair Value	$ 681,835	$ 784,204

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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Amount, before tax, of unrealized loss 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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Amortized cost of 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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Amount of 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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Amount of allowance for credit loss 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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