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INVESTMENTS - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Details) - USD ($)	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Available-for-Sale Debt Securities
Amortized Cost	$ 5,016,103,000		$ 5,249,882,000
Gross Unrealized Gain	1,583,000		34,626,000
Gross Unrealized Loss	521,599,000		79,197,000
Allowance for Credit Losses	0		0
Fair Value	4,496,087,000		5,205,311,000
Held-to-Maturity Debt Securities
Amortized Cost	1,064,188,000		90,646,000
Gross Unrealized Gain	322,000		3,489,000
Gross Unrealized Loss	1,152,000		0
Allowance for Credit Losses	6,000		4,000
Fair Value	1,063,352,000		94,131,000
Available for sale securities transfers to held to maturity unrealized (losses) gains	(157,400,000)
FNMA MBS
Held-to-Maturity Debt Securities
Amortized Cost	931,421,000
Gross Unrealized Gain	0
Gross Unrealized Loss	0
Allowance for Credit Losses	0
Fair Value	931,421,000
Available for sale securities transfers to held to maturity unrealized (losses) gains	157,600,000
State and political subdivisions
Held-to-Maturity Debt Securities
Amortized Cost	132,267,000		90,146,000
Gross Unrealized Gain	322,000		3,489,000
Gross Unrealized Loss	1,151,000		0
Allowance for Credit Losses	6,000	$ 4,000	4,000	$ 5,000	$ 5,000	$ 6,000
Fair Value	131,432,000		93,631,000
Available for sale securities transfers to held to maturity unrealized (losses) gains			200,000
Foreign bonds
Held-to-Maturity Debt Securities
Amortized Cost	500,000		500,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	1,000		0
Allowance for Credit Losses	0		0
Fair Value	499,000		500,000
Collateralized mortgage obligation (CMO)
Available-for-Sale Debt Securities
Amortized Cost	624,579,000		586,830,000
Gross Unrealized Gain	64,000		3,569,000
Gross Unrealized Loss	66,988,000		14,633,000
Allowance for Credit Losses	0		0
Fair Value	557,655,000		575,766,000
FNMA MBS
Available-for-Sale Debt Securities
Amortized Cost	3,997,394,000		4,275,307,000
Gross Unrealized Gain	1,473,000		24,170,000
Gross Unrealized Loss	406,409,000		53,793,000
Allowance for Credit Losses	0		0
Fair Value	3,592,458,000		4,245,684,000
Freddie Mac (FHLMC) MBS
Available-for-Sale Debt Securities
Amortized Cost	123,756,000		139,708,000
Gross Unrealized Gain	25,000		6,336,000
Gross Unrealized Loss	7,385,000		516,000
Allowance for Credit Losses	0		0
Fair Value	116,396,000		145,528,000
Ginnie Mae (GNMA) MBS
Available-for-Sale Debt Securities
Amortized Cost	41,078,000		17,456,000
Gross Unrealized Gain	21,000		551,000
Gross Unrealized Loss	1,938,000		71,000
Allowance for Credit Losses	0		0
Fair Value	39,161,000		17,936,000
Government-sponsored enterprises (GSE) agency notes
Available-for-Sale Debt Securities
Amortized Cost	229,296,000		230,581,000
Gross Unrealized Gain	0		0
Gross Unrealized Loss	38,879,000		10,184,000
Allowance for Credit Losses	0		0
Fair Value	$ 190,417,000		$ 220,397,000