INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Total	$ 443,957	$ 654,008
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	949	5,572
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(9,121)	(6,479)
Held-to-Maturity Market Value	435,785	653,101
Total	2,677,257	1,348,227
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	7,500	8,328
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(29,848)	(7,147)
Investment securities available-for-sale	2,654,909	1,349,408
U.S. Treasuries
Investment Holdings [Line Items]
Total	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-Maturity Market Value	0	0
Total	98	98
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	0	0
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(3)	(1)
Investment securities available-for-sale	95	97
Securities of U.S. government agencies and corporations
Investment Holdings [Line Items]
Total	0	11,168
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	(76)
Held-to-Maturity Market Value	0	11,092
Total	34,896	15,695
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	84	220
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(452)	0
Investment securities available-for-sale	34,528	15,915
Residential Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	31,857	162,093
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	2,042
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(1,515)	(1,535)
Held-to-Maturity Market Value	30,342	162,600
Total	644,358	290,793
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,314	849
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(10,152)	(2,599)
Investment securities available-for-sale	635,520	289,043
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	152,906	255,027
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	13	1,372
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(5,503)	(3,000)
Held-to-Maturity Market Value	147,416	253,399
Total	290,930	150,356
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	106	164
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(5,059)	(1,417)
Investment securities available-for-sale	285,977	149,103
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	13,982	143,545
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	354
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(675)	(1,602)
Held-to-Maturity Market Value	13,307	142,297
Total	873,923	306,095
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	965	1,158
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(10,313)	(1,861)
Investment securities available-for-sale	864,575	305,392
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	245,212	82,175
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	936	1,804
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(1,428)	(266)
Held-to-Maturity Market Value	244,720	83,713
Total	250,993	124,269
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	2,214	2,162
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1,910)	(676)
Investment securities available-for-sale	251,297	125,755
Asset-backed Securities [Member]
Investment Holdings [Line Items]
Total	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-Maturity Market Value	0	0
Total	492,702	377,655
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	879	1,628
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1,354)	(306)
Investment securities available-for-sale	492,227	378,977
Other securities
Investment Holdings [Line Items]
Total	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-Maturity Market Value	0	0
Total	89,357	83,266
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,938	2,147
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(605)	(287)
Investment securities available-for-sale	$ 90,690	$ 85,126

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

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