INVESTMENT SECURITIES - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($)	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Available-for-Sale Debt Securities
Amortized Cost	$ 4,290,823,000		$ 4,504,342,000
Gross Unrealized Gain	1,400,000		168,000
Gross Unrealized Loss	555,104,000		657,973,000
Allowance for Credit Losses	0		0
Fair Value	3,737,119,000		3,846,537,000
Held-to-Maturity Debt Securities
Amortized Cost	1,026,312,000		1,058,565,000
Gross Unrealized Gain	1,795,000		2,665,000
Gross Unrealized Loss	69,131,000		75,291,000
Allowance for Credit Losses	7,000		8,000
Fair Value	958,969,000		985,931,000
Available for sale securities transfers to held to maturity unrealized losses	105,400,000		120,400,000
Collateralized mortgage obligation (CMO)
Available-for-Sale Debt Securities
Amortized Cost	537,666,000		560,952,000
Gross Unrealized Gain	566,000		0
Gross Unrealized Loss	82,749,000		96,333,000
Allowance for Credit Losses	0		0
Fair Value	455,483,000		464,619,000
Fannie Mae (FNMA) mortgage-backed securities (MBS)
Available-for-Sale Debt Securities
Amortized Cost	3,362,926,000		3,544,762,000
Gross Unrealized Gain	793,000		162,000
Gross Unrealized Loss	423,832,000		502,574,000
Allowance for Credit Losses	0		0
Fair Value	2,939,887,000		3,042,350,000
Held-to-Maturity Debt Securities
Amortized Cost	841,697,000		872,653,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	68,348,000		74,332,000
Allowance for Credit Losses	0		0
Fair Value	773,349,000		798,321,000
Freddie Mac (FHLMC) MBS
Available-for-Sale Debt Securities
Amortized Cost	120,570,000		126,856,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	8,763,000		11,324,000
Allowance for Credit Losses	0		0
Fair Value	111,807,000		115,532,000
Ginnie Mae (GNMA) MBS
Available-for-Sale Debt Securities
Amortized Cost	46,150,000		46,333,000
Gross Unrealized Gain	41,000		6,000
Gross Unrealized Loss	2,368,000		2,999,000
Allowance for Credit Losses	0		0
Fair Value	43,823,000		43,340,000
Government-sponsored enterprises (GSE) agency notes
Available-for-Sale Debt Securities
Amortized Cost	223,511,000		225,439,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	37,392,000		44,743,000
Allowance for Credit Losses	0		0
Fair Value	186,119,000		180,696,000
State and political subdivisions
Held-to-Maturity Debt Securities
Amortized Cost	184,615,000		185,912,000
Gross Unrealized Gain	1,795,000		2,665,000
Gross Unrealized Loss	783,000		959,000
Allowance for Credit Losses	7,000	$ 7,000	8,000	$ 8,000	$ 8,000	$ 10,000
Fair Value	$ 185,620,000		$ 187,610,000