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INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 656,802		$ 940,580
Allowance for Credit Losses	0	$ (81)	(112)	$ (68)	$ 0	$ 0
Gross Unrealized Gains	27,894		42,854
Gross Unrealized Losses	(192)		(443)
Estimated Fair Value	684,504		982,879
U.S. government sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,102		17,161
Allowance for Credit Losses	0		0
Gross Unrealized Gains	150		343
Gross Unrealized Losses	0		0
Estimated Fair Value	12,252		17,504
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51,541		63,286
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2,675		3,492
Gross Unrealized Losses	0		0
Estimated Fair Value	54,216		66,778
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	33,397		51,639
Allowance for Credit Losses	0		(112)
Gross Unrealized Gains	722		602
Gross Unrealized Losses	(121)		(233)
Estimated Fair Value	33,998		51,896
SBA pool securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	46,354		59,973
Allowance for Credit Losses	0		0
Gross Unrealized Gains	1,629		2,620
Gross Unrealized Losses	(66)		(96)
Estimated Fair Value	47,917		62,497
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	513,408		748,521
Allowance for Credit Losses	0		0
Gross Unrealized Gains	22,718		35,797
Gross Unrealized Losses	(5)		(114)
Estimated Fair Value	$ 536,121		$ 784,204