INVESTMENTS - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($)	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Available-for-Sale Debt Securities
Amortized Cost	$ 5,903,550,000	$ 5,249,882,000
Gross Unrealized Gain	3,706,000	34,626,000
Gross Unrealized Loss	411,327,000	79,197,000
Allowance for Credit Losses	0	0
Fair Value	5,495,929,000	5,205,311,000
Held-to-Maturity Debt Securities
Amortized Cost	84,902,000	90,646,000
Gross Unrealized Gain	875,000	3,489,000
Gross Unrealized Loss	19,000	0
Allowance for Credit Losses	4,000	4,000
Fair Value	85,754,000	94,131,000
State and political subdivisions
Held-to-Maturity Debt Securities
Amortized Cost	84,102,000	90,146,000
Gross Unrealized Gain	875,000	3,489,000
Gross Unrealized Loss	18,000	0
Allowance for Credit Losses	4,000	4,000	$ 5,000	$ 6,000
Fair Value	84,955,000	93,631,000
Available for sale securities transfers to held to maturity unrealized gains	200,000	200,000
Foreign bonds
Held-to-Maturity Debt Securities
Amortized Cost	800,000	500,000
Gross Unrealized Gain	0	0
Gross Unrealized Loss	1,000	0
Allowance for Credit Losses	0	0
Fair Value	799,000	500,000
Collateralized mortgage obligation (CMO)
Available-for-Sale Debt Securities
Amortized Cost	607,150,000	586,830,000
Gross Unrealized Gain	187,000	3,569,000
Gross Unrealized Loss	44,247,000	14,633,000
Allowance for Credit Losses	0	0
Fair Value	563,090,000	575,766,000
Fannie Mae (FNMA) mortgage-backed securities (MBS)
Available-for-Sale Debt Securities
Amortized Cost	4,921,188,000	4,275,307,000
Gross Unrealized Gain	2,549,000	24,170,000
Gross Unrealized Loss	336,759,000	53,793,000
Allowance for Credit Losses	0	0
Fair Value	4,586,978,000	4,245,684,000
Freddie Mac (FHLMC) MBS
Available-for-Sale Debt Securities
Amortized Cost	128,918,000	139,708,000
Gross Unrealized Gain	871,000	6,336,000
Gross Unrealized Loss	3,383,000	516,000
Allowance for Credit Losses	0	0
Fair Value	126,406,000	145,528,000
Ginnie Mae (GNMA) MBS
Available-for-Sale Debt Securities
Amortized Cost	16,356,000	17,456,000
Gross Unrealized Gain	99,000	551,000
Gross Unrealized Loss	396,000	71,000
Allowance for Credit Losses	0	0
Fair Value	16,059,000	17,936,000
Government-sponsored enterprises (GSE) agency notes
Available-for-Sale Debt Securities
Amortized Cost	229,938,000	230,581,000
Gross Unrealized Gain	0	0
Gross Unrealized Loss	26,542,000	10,184,000
Allowance for Credit Losses	0	0
Fair Value	$ 203,396,000	$ 220,397,000

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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