INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Total	$ 148,778	$ 429,328
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1,994	2,212
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(1,048)	(7,422)
Held-to-Maturity Market Value	149,724	424,118
Total	2,789,151	2,792,326
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	65,125	9,409
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(3,774)	(22,480)
Investment securities available-for-sale	2,850,502	2,779,255
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	1	0
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	(2)
Investment securities available-for-sale	100	97
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	156	32,095
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	3	57
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	0	(233)
Investment securities available-for-sale	159	31,919
Residential Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	22,367	25,565
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	184	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(41)	(1,045)
Held-to-Maturity Market Value	22,510	24,520
Total	416,446	565,071
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	9,679	691
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(103)	(7,163)
Investment securities available-for-sale	426,022	558,599
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	104,586	147,780
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	1,105	258
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(828)	(4,385)
Held-to-Maturity Market Value	104,863	143,653
Total	440,457	423,797
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	6,929	819
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(2,052)	(3,581)
Investment securities available-for-sale	445,334	421,035
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	10,545	12,540
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(118)	(633)
Held-to-Maturity Market Value	10,427	11,907
Total	799,072	928,586
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	20,067	4,319
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(208)	(6,158)
Investment securities available-for-sale	818,931	926,747
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	11,280	243,443
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	705	1,954
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(61)	(1,359)
Held-to-Maturity Market Value	11,924	244,038
Total	588,634	257,300
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	25,140	2,554
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(61)	(1,429)
Investment securities available-for-sale	613,713	258,425
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	465,542	511,430
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,652	611
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1,107)	(2,810)
Investment securities available-for-sale	466,087	509,231
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	78,745	73,948
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,654	358
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(243)	(1,104)
Investment securities available-for-sale	$ 80,156	$ 73,202

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