INVESTMENTS - Amortized Cost and Estimated Fair Values of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Available-for-sale debt securities:
Total available-for-sale debt securities	$ 1,411,343	$ 1,466,350
Gross Unrealized Gains	99	14,966
Gross Unrealized Losses	(167,107)	(15,420)
Investments available-for-sale	1,244,335	1,465,896
Held-to-maturity debt securities:
Total held-to-maturity debt securities	265,648	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39,618)	0
Total held-to-maturity debt securities	226,030	0
Total debt securities
Amortized Cost	1,676,991	1,466,350
Gross Unrealized Gains	99	14,966
Gross Unrealized Losses	(206,725)	(15,420)
Estimated Fair Value	1,470,365	1,465,896
Investments held-to-maturity, at cost (fair value of $226,030)	265,648	0
U.S. treasuries and government agencies
Available-for-sale debt securities:
Total available-for-sale debt securities	100,740	68,487
Gross Unrealized Gains	0	202
Gross Unrealized Losses	(7,935)	(150)
Investments available-for-sale	92,805	68,539
State and municipal
Available-for-sale debt securities:
Total available-for-sale debt securities	342,492	323,286
Gross Unrealized Gains	5	6,561
Gross Unrealized Losses	(65,558)	(3,445)
Investments available-for-sale	276,939	326,402
Mortgage-backed and asset-backed
Available-for-sale debt securities:
Total available-for-sale debt securities	968,111	1,074,577
Gross Unrealized Gains	94	8,203
Gross Unrealized Losses	(93,614)	(11,825)
Investments available-for-sale	874,591	1,070,955
Held-to-maturity debt securities:
Total held-to-maturity debt securities	265,648	0
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39,618)	0
Total held-to-maturity debt securities	226,030	0
Total debt securities
Investments held-to-maturity, at cost (fair value of $226,030)	$ 265,648	$ 0