INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 940,580	$ 1,380,438
Allowance for Credit Losses	(112)	0
Gross Unrealized Gains	42,854	24,261
Gross Unrealized Losses	(443)	(1,296)
Estimated Fair Value	982,879	1,403,403
U.S. government sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,161	22,246
Allowance for Credit Losses	0	0
Gross Unrealized Gains	343	116
Gross Unrealized Losses	0	0
Estimated Fair Value	17,504	22,362
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	63,286	102,952
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,492	2,310
Gross Unrealized Losses	0	(2)
Estimated Fair Value	66,778	105,260
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51,639	51,720
Allowance for Credit Losses	(112)	0
Gross Unrealized Gains	602	1,281
Gross Unrealized Losses	(233)	(2)
Estimated Fair Value	51,896	52,999
SBA pool securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	59,973	73,704
Allowance for Credit Losses	0	0
Gross Unrealized Gains	2,620	617
Gross Unrealized Losses	(96)	(409)
Estimated Fair Value	62,497	73,912
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	748,521	1,129,816
Allowance for Credit Losses	0	0
Gross Unrealized Gains	35,797	19,937
Gross Unrealized Losses	(114)	(883)
Estimated Fair Value	$ 784,204	$ 1,148,870

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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