INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Total	$ 118,072	$ 142,862
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	5,369	1,497
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(1,538)
Held-to-Maturity Market Value	123,441	142,821
Total	2,915,937	2,798,298
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	98,389	58,555
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(9,363)	(4,769)
Investment securities available-for-sale	3,004,963	2,852,084
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	99	99
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	4	1
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Investment securities available-for-sale	103	100
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	59	156
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1	2
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Investment securities available-for-sale	60	158
Residential Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	16,069	20,818
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	250	122
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(174)
Held-to-Maturity Market Value	16,319	20,766
Total	393,395	421,945
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	16,084	9,709
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(59)	(99)
Investment securities available-for-sale	409,420	431,555
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	84,701	101,267
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	3,834	571
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(1,225)
Held-to-Maturity Market Value	88,535	100,613
Total	550,408	474,174
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	10,943	4,988
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(4,791)	(2,644)
Investment securities available-for-sale	556,560	476,518
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	6,987	9,763
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	93	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(108)
Held-to-Maturity Market Value	7,080	9,655
Total	680,581	769,076
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	25,891	16,753
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(339)	(385)
Investment securities available-for-sale	706,133	785,444
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	10,315	11,014
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1,192	804
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(31)
Held-to-Maturity Market Value	11,507	11,787
Total	761,589	652,986
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	41,007	23,729
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(599)	(462)
Investment securities available-for-sale	801,997	676,253
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	433,811	400,081
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	2,963	1,414
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(2,978)	(1,064)
Investment securities available-for-sale	433,796	400,431
Other securities
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	95,995	79,781
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,496	1,959
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(597)	(115)
Investment securities available-for-sale	$ 96,894	$ 81,625

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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