INVESTMENT SECURITIES - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($)	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Available-for-Sale Debt Securities
Amortized Cost	$ 4,584,190,000		$ 4,834,550,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	892,649,000		741,490,000
Allowance for Credit Losses	0		0
Fair Value	3,691,541,000		4,093,060,000
Held-to-Maturity Debt Securities
Amortized Cost	1,068,879,000		1,111,629,000
Gross Unrealized Gain	24,000		534,000
Gross Unrealized Loss	141,215,000		72,049,000
Allowance for Credit Losses	8,000		10,000
Fair Value	927,680,000		1,040,104,000
Available for sale securities transfers to held to maturity unrealized losses	(125,800,000)
Collateralized mortgage obligation (CMO)
Available-for-Sale Debt Securities
Amortized Cost	573,393,000		608,834,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	118,729,000		102,454,000
Allowance for Credit Losses	0		0
Fair Value	454,664,000		506,380,000
Fannie Mae (FNMA) mortgage-backed securities (MBS)
Available-for-Sale Debt Securities
Amortized Cost	3,608,095,000		3,823,036,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	694,683,000		572,778,000
Allowance for Credit Losses	0		0
Fair Value	2,913,412,000		3,250,258,000
Held-to-Maturity Debt Securities
Amortized Cost	881,844,000		909,498,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	129,388,000		68,677,000
Allowance for Credit Losses	0		0
Fair Value	752,456,000		840,821,000
Freddie Mac (FHLMC) MBS
Available-for-Sale Debt Securities
Amortized Cost	128,999,000		135,554,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	18,280,000		13,555,000
Allowance for Credit Losses	0		0
Fair Value	110,719,000		121,999,000
Ginnie Mae (GNMA) MBS
Available-for-Sale Debt Securities
Amortized Cost	47,621,000		39,116,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	4,870,000		2,978,000
Allowance for Credit Losses	0		0
Fair Value	42,751,000		36,138,000
Government-sponsored enterprises (GSE) agency notes
Available-for-Sale Debt Securities
Amortized Cost	226,082,000		228,010,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	56,087,000		49,725,000
Allowance for Credit Losses	0		0
Fair Value	169,995,000		178,285,000
State and political subdivisions
Held-to-Maturity Debt Securities
Amortized Cost	187,035,000		201,631,000
Gross Unrealized Gain	24,000		532,000
Gross Unrealized Loss	11,827,000		3,372,000
Allowance for Credit Losses	8,000	$ 8,000	10,000	$ 10,000	$ 6,000	$ 4,000
Fair Value	$ 175,224,000		198,781,000
Available for sale securities transfers to held to maturity unrealized losses			(142,800,000)
Foreign bonds
Held-to-Maturity Debt Securities
Amortized Cost			500,000
Gross Unrealized Gain			2,000
Gross Unrealized Loss			0
Allowance for Credit Losses			0
Fair Value			$ 502,000