INVESTMENT SECURITIES - Amortized Cost and Estimated Fair Value of Investment Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,502,212		$ 1,559,005
Allowance for Credit Losses	(80)	$ (82)	(75)	$ (79)	$ (88)	$ 0
Gross Unrealized Gains	18		264
Gross Unrealized Losses	(78,069)		(59,134)
Estimated Fair Value	1,424,081		1,500,060
U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	781,154		775,784
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		131
Gross Unrealized Losses	(20,558)		(16,381)
Estimated Fair Value	760,596		759,534
U.S. government-sponsored agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,026		1,036
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(57)		(57)
Estimated Fair Value	969		979
State, county and municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,492		35,358
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		17
Gross Unrealized Losses	(1,789)		(1,180)
Estimated Fair Value	27,703		34,195
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	16,171		16,397
Allowance for Credit Losses	(80)		(75)
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(1,055)		(396)
Estimated Fair Value	15,036		15,926
SBA pool securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,834		29,422
Allowance for Credit Losses	0		0
Gross Unrealized Gains	2		3
Gross Unrealized Losses	(2,085)		(2,027)
Estimated Fair Value	21,751		27,398
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	650,535		701,008
Allowance for Credit Losses	0		0
Gross Unrealized Gains	16		113
Gross Unrealized Losses	(52,525)		(39,093)
Estimated Fair Value	$ 598,026		$ 662,028