INVESTMENT SECURITIES - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($)	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Trading securities	$ 0
Available-for-Sale Debt Securities
Amortized Cost	4,029,355,000		$ 4,218,266,000
Gross Unrealized Gain	2,415,000		285,000
Gross Unrealized Loss	529,611,000		707,903,000
Allowance for Credit Losses	0		0
Fair Value	3,502,159,000		3,510,648,000
Held-to-Maturity Debt Securities
Amortized Cost	979,704,000		1,015,168,000
Gross Unrealized Gain	504,000		247,000
Gross Unrealized Loss	94,020,000		119,897,000
Beginning balance	6,000	$ 6,000	7,000	$ 7,000	$ 7,000	$ 8,000
Fair Value	886,182,000		895,511,000
Available for sale securities transfers to held to maturity unrealized losses	87,600,000		100,500,000
Collateralized mortgage obligations (CMO)
Available-for-Sale Debt Securities
Amortized Cost	490,692,000		526,796,000
Gross Unrealized Gain	425,000		113,000
Gross Unrealized Loss	74,326,000		95,967,000
Allowance for Credit Losses	0		0
Fair Value	416,791,000		430,942,000
Fannie Mae (FNMA) mortgage-backed securities (MBS)
Available-for-Sale Debt Securities
Amortized Cost	3,154,279,000		3,305,418,000
Gross Unrealized Gain	1,892,000		172,000
Gross Unrealized Loss	410,013,000		550,011,000
Allowance for Credit Losses	0		0
Fair Value	2,746,158,000		2,755,579,000
Held-to-Maturity Debt Securities
Amortized Cost	799,548,000		831,325,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	92,293,000		116,600,000
Beginning balance	0		0
Fair Value	707,255,000		714,725,000
Freddie Mac (FHLMC) MBS
Available-for-Sale Debt Securities
Amortized Cost	117,850,000		118,605,000
Gross Unrealized Gain	54,000		0
Gross Unrealized Loss	8,357,000		13,091,000
Allowance for Credit Losses	0		0
Fair Value	109,547,000		105,514,000
Ginnie Mae (GNMA) MBS
Available-for-Sale Debt Securities
Amortized Cost	45,594,000		44,578,000
Gross Unrealized Gain	44,000		0
Gross Unrealized Loss	2,740,000		3,902,000
Allowance for Credit Losses	0		0
Fair Value	42,898,000		40,676,000
Government-sponsored enterprises (GSE) agency notes
Available-for-Sale Debt Securities
Amortized Cost	220,940,000		222,869,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	34,175,000		44,932,000
Allowance for Credit Losses	0		0
Fair Value	186,765,000		177,937,000
State and political subdivisions
Held-to-Maturity Debt Securities
Amortized Cost	180,156,000		183,843,000
Gross Unrealized Gain	504,000		247,000
Gross Unrealized Loss	1,727,000		3,297,000
Beginning balance	6,000	$ 6,000	7,000	$ 7,000	$ 7,000	$ 8,000
Fair Value	$ 178,927,000		$ 180,786,000