INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investment Holdings [Line Items]
Total	$ 103,886	$ 131,687
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	2,445	5,011
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(652)	0
Held-to-Maturity Market Value	105,679	136,698
Total	4,065,301	3,330,029
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	69,420	99,942
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(20,627)	(5,391)
Investment securities available-for-sale	4,114,094	3,424,580
U.S. Treasuries
Investment Holdings [Line Items]
Total	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	0	0
Total	20,223	99
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	107	4
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Investment securities available-for-sale	20,330	103
Securities of U.S. government agencies and corporations
Investment Holdings [Line Items]
Total	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	0	0
Total	78,653	60
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	455	0
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(75)	0
Investment securities available-for-sale	79,033	60
Residential Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	0	13,990
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	197
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	0	14,187
Total	711,442	704,482
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	8,743	15,938
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(6,300)	(237)
Investment securities available-for-sale	713,885	720,183
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	50,456	71,737
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1,337	3,485
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	51,793	75,222
Total	702,387	584,125
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	6,762	10,395
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1,236)	(3,584)
Investment securities available-for-sale	707,913	590,936
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	13,030	5,799
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	66	79
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(274)	0
Held-to-Maturity Market Value	12,822	5,878
Total	730,190	634,418
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	11,300	21,148
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(3,167)	(445)
Investment securities available-for-sale	738,323	655,121
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	9,150	9,911
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	932	1,239
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	10,082	11,150
Total	1,045,192	856,054
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	36,213	46,755
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(8,879)	(291)
Investment securities available-for-sale	1,072,526	902,518
Asset-backed Securities [Member]
Investment Holdings [Line Items]
Total	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	0	0
Total	666,213	478,539
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	3,501	4,158
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(808)	(826)
Investment securities available-for-sale	668,906	481,871
Other securities
Investment Holdings [Line Items]
Total	31,250	30,250
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	110	11
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(378)	0
Held-to-Maturity Market Value	30,982	30,261
Total	111,001	72,252
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	2,339	1,544
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(162)	(8)
Investment securities available-for-sale	$ 113,178	$ 73,788

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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