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INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 746,572		$ 940,580
Allowance for Credit Losses	(81)	$ (101)	(112)	$ 0	$ 0	$ 0
Gross Unrealized Gains	32,008		42,854
Gross Unrealized Losses	(332)		(443)
Estimated Fair Value	778,167		982,879
U.S. government sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,120		17,161
Allowance for Credit Losses	0		0
Gross Unrealized Gains	207		343
Gross Unrealized Losses	0		0
Estimated Fair Value	12,327		17,504
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	58,016		63,286
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,820		3,492
Gross Unrealized Losses	0		0
Estimated Fair Value	60,836		66,778
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	43,132		51,639
Allowance for Credit Losses	(81)		(112)
Gross Unrealized Gains	810		602
Gross Unrealized Losses	(249)		(233)
Estimated Fair Value	43,612		51,896
SBA pool securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	52,283		59,973
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,071		2,620
Gross Unrealized Losses	(73)		(96)
Estimated Fair Value	54,281		62,497
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	581,021		748,521
Allowance for Credit Losses	0		0
Gross Unrealized Gains	26,100		35,797
Gross Unrealized Losses	(10)		(114)
Estimated Fair Value	$ 607,111		$ 784,204