INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Total	$ 158,305	$ 429,328
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	512	2,212
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(5,742)	(7,422)
Held-to-Maturity Market Value	153,075	424,118
Total	3,096,085	2,792,326
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	30,204	9,409
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(12,478)	(22,480)
Investment securities available-for-sale	3,113,811	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	0	0
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1)	(2)
Investment securities available-for-sale	98	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	30,880	32,095
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	74	57
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(355)	(233)
Investment securities available-for-sale	30,599	31,919
Residential Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	24,937	25,565
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	72	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(671)	(1,045)
Held-to-Maturity Market Value	24,338	24,520
Total	557,730	565,071
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	4,566	691
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(3,449)	(7,163)
Investment securities available-for-sale	558,847	558,599
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	109,826	147,780
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	258
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(4,414)	(4,385)
Held-to-Maturity Market Value	105,412	143,653
Total	464,611	423,797
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,821	819
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(2,988)	(3,581)
Investment securities available-for-sale	463,444	421,035
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	11,936	12,540
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(429)	(633)
Held-to-Maturity Market Value	11,507	11,907
Total	919,486	928,586
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	9,214	4,319
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(3,067)	(6,158)
Investment securities available-for-sale	925,633	926,747
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	11,606	243,443
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	440	1,954
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(228)	(1,359)
Held-to-Maturity Market Value	11,818	244,038
Total	542,265	257,300
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	12,496	2,554
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(504)	(1,429)
Investment securities available-for-sale	554,257	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	502,018	511,430
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,180	611
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1,790)	(2,810)
Investment securities available-for-sale	501,408	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	78,996	73,948
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	853	358
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(324)	(1,104)
Investment securities available-for-sale	$ 79,525	$ 73,202

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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