INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,070,349	$ 1,380,438
Allowance for Credit Losses	(68)	0
Gross Unrealized Gains	47,636	24,261
Gross Unrealized Losses	(481)	(1,296)
Estimated Fair Value	1,117,436	1,403,403
U.S. government sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,184	22,246
Allowance for Credit Losses	0	0
Gross Unrealized Gains	420	116
Gross Unrealized Losses	0	0
Estimated Fair Value	17,604	22,362
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	84,219	102,952
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,429	2,310
Gross Unrealized Losses	0	(2)
Estimated Fair Value	87,648	105,260
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	51,659	51,720
Allowance for Credit Losses	(68)	0
Gross Unrealized Gains	826	1,281
Gross Unrealized Losses	(276)	(2)
Estimated Fair Value	52,141	52,999
SBA pool securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	62,204	73,704
Allowance for Credit Losses	0	0
Gross Unrealized Gains	3,019	617
Gross Unrealized Losses	(107)	(409)
Estimated Fair Value	65,116	73,912
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	855,083	1,129,816
Allowance for Credit Losses	0	0
Gross Unrealized Gains	39,942	19,937
Gross Unrealized Losses	(98)	(883)
Estimated Fair Value	$ 894,927	$ 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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