INVESTMENTS - Amortized Cost and Estimated Fair Values of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 1,288,818	$ 1,366,843
Gross Unrealized Gains	41	77
Gross Unrealized Losses	(145,171)	(152,382)
Investments available-for-sale	1,143,688	1,214,538
Held-to-maturity debt securities:
Total held-to-maturity debt securities	247,814	259,452
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39,152)	(39,329)
Total held-to-maturity debt securities	208,662	220,123
Total debt securities
Amortized Cost	1,536,632	1,626,295
Gross Unrealized Gains	41	77
Gross Unrealized Losses	(184,323)	(191,711)
Estimated Fair Value	1,352,350	1,434,661
U.S. treasuries and government agencies
Available-for-sale debt securities:
Total available-for-sale debt securities	101,299	100,926
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(6,954)	(7,304)
Investments available-for-sale	94,345	93,622
State and municipal
Available-for-sale debt securities:
Total available-for-sale debt securities	318,815	322,519
Gross Unrealized Gains	1	4
Gross Unrealized Losses	(50,565)	(56,526)
Investments available-for-sale	268,251	265,997
Mortgage-backed and asset-backed
Available-for-sale debt securities:
Total available-for-sale debt securities	868,704	943,398
Gross Unrealized Gains	40	73
Gross Unrealized Losses	(87,652)	(88,552)
Investments available-for-sale	781,092	854,919
Held-to-maturity debt securities:
Total held-to-maturity debt securities	247,814	259,452
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39,152)	(39,329)
Total held-to-maturity debt securities	$ 208,662	$ 220,123