INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investment Holdings [Line Items]
Total	$ 136,744	$ 142,862
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	3,696	1,497
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(375)	(1,538)
Held-to-Maturity Market Value	140,065	142,821
Total	2,857,355	2,798,298
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	85,992	58,555
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(34,659)	(4,769)
Investment securities available-for-sale	2,908,688	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	5	1
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Investment securities available-for-sale	104	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	116	156
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	2	2
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Investment securities available-for-sale	118	158
Residential Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	19,832	20,818
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1,046	122
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(174)
Held-to-Maturity Market Value	20,878	20,766
Total	419,423	421,945
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	19,215	9,709
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(219)	(99)
Investment securities available-for-sale	438,419	431,555
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	97,073	101,267
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1,671	571
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(375)	(1,225)
Held-to-Maturity Market Value	98,369	100,613
Total	506,343	474,174
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	7,034	4,988
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(9,082)	(2,644)
Investment securities available-for-sale	504,295	476,518
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	9,043	9,763
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	149	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(108)
Held-to-Maturity Market Value	9,192	9,655
Total	745,649	769,076
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	28,845	16,753
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(2,479)	(385)
Investment securities available-for-sale	772,015	785,444
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	10,796	11,014
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	830	804
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(31)
Held-to-Maturity Market Value	11,626	11,787
Total	723,470	652,986
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	30,576	23,729
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(152)	(462)
Investment securities available-for-sale	753,894	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	382,415	400,081
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	225	1,414
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(18,841)	(1,064)
Investment securities available-for-sale	363,799	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	79,840	79,781
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	90	1,959
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(3,886)	(115)
Investment securities available-for-sale	$ 76,044	$ 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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