INVESTMENTS - Amortized Cost and Estimated Fair Values of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Available-for-sale debt securities:
Amortized Cost	$ 1,220,819	$ 1,366,843
Gross Unrealized Gains	182	77
Gross Unrealized Losses	(118,320)	(152,382)
Investments available-for-sale (at fair value)	1,102,681	1,214,538
Held-to-maturity debt securities
Amortized Cost	236,165	259,452
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(35,754)	(39,329)
Investments held-to-maturity (at fair value)	200,411	220,123
Total debt securities
Book value of debt securities pledged as collateral	1,456,984	1,626,295
Gross Unrealized Gains	182	77
Gross Unrealized Losses	(154,074)	(191,711)
Estimated Fair Value	1,303,092	1,434,661
U.S. treasuries and government agencies
Available-for-sale debt securities:
Amortized Cost	101,678	100,926
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4,751)	(7,304)
Investments available-for-sale (at fair value)	96,927	93,622
State and municipal
Available-for-sale debt securities:
Amortized Cost	311,505	322,519
Gross Unrealized Gains	1	4
Gross Unrealized Losses	(43,292)	(56,526)
Investments available-for-sale (at fair value)	268,214	265,997
Mortgage-backed and asset-backed
Available-for-sale debt securities:
Amortized Cost	807,636	943,398
Gross Unrealized Gains	181	73
Gross Unrealized Losses	(70,277)	(88,552)
Investments available-for-sale (at fair value)	737,540	854,919
Held-to-maturity debt securities
Amortized Cost	236,165	259,452
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(35,754)	(39,329)
Investments held-to-maturity (at fair value)	$ 200,411	$ 220,123