INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment Holdings [Line Items]
Total	$ 121,945	$ 131,687
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	3,226	5,011
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(877)	0
Held-to-Maturity Market Value	124,294	136,698
Total	3,698,779	3,330,029
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	82,242	99,942
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(27,258)	(5,391)
Investment securities available-for-sale	3,753,763	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	10,257	99
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	3	4
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Investment securities available-for-sale	10,260	103
Securities of U.S. government agencies and corporations
Investment Holdings [Line Items]
Total		0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain		0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss		0
Held-to-Maturity Market Value		0
Total		60
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments		0
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Investment securities available-for-sale		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	784,407	704,482
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	12,488	15,938
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(11,255)	(237)
Investment securities available-for-sale	785,640	720,183
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	63,481	71,737
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	2,034	3,485
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	65,515	75,222
Total	591,629	584,125
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	7,096	10,395
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(3,680)	(3,584)
Investment securities available-for-sale	595,045	590,936
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	17,580	5,799
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	109	79
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(411)	0
Held-to-Maturity Market Value	17,278	5,878
Total	661,322	634,418
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	17,796	21,148
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(2,404)	(445)
Investment securities available-for-sale	676,714	655,121
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	9,634	9,911
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1,001	1,239
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held-to-Maturity Market Value	10,635	11,150
Total	971,325	856,054
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	38,835	46,755
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(8,541)	(291)
Investment securities available-for-sale	1,001,619	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	601,836	478,539
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	4,075	4,158
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1,190)	(826)
Investment securities available-for-sale	604,721	481,871
Other securities
Investment Holdings [Line Items]
Total	31,250	30,250
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	82	11
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(466)	0
Held-to-Maturity Market Value	30,866	30,261
Total	78,003	72,252
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,949	1,544
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(188)	(8)
Investment securities available-for-sale	$ 79,764	$ 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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