INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Total	$ 434,369	$ 654,008
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	275	5,572
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(12,006)	(6,479)
Held-to-Maturity Market Value	422,638	653,101
Total	2,662,329	1,348,227
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	4,770	8,328
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(40,373)	(7,147)
Investment securities available-for-sale	2,626,726	1,349,408
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	98
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	0	0
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(4)	(1)
Investment securities available-for-sale	95	97
Securities of U.S. government agencies and corporations
Investment Holdings [Line Items]
Total	0	11,168
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(76)
Held-to-Maturity Market Value	0	11,092
Total	33,438	15,695
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	7	220
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(600)	0
Investment securities available-for-sale	32,845	15,915
Residential Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	26,603	162,093
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	2,042
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(1,615)	(1,535)
Held-to-Maturity Market Value	24,988	162,600
Total	581,657	290,793
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	551	849
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(14,262)	(2,599)
Investment securities available-for-sale	567,946	289,043
Commercial Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	150,223	255,027
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	1,372
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(6,378)	(3,000)
Held-to-Maturity Market Value	143,845	253,399
Total	362,874	150,356
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	118	164
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(5,646)	(1,417)
Investment securities available-for-sale	357,346	149,103
Collateralized Mortgage Backed Securities [Member]
Investment Holdings [Line Items]
Total	13,182	143,545
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	354
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(727)	(1,602)
Held-to-Maturity Market Value	12,455	142,297
Total	864,824	306,095
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	940	1,158
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(15,436)	(1,861)
Investment securities available-for-sale	850,328	305,392
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Total	244,361	82,175
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	275	1,804
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(3,286)	(266)
Held-to-Maturity Market Value	241,350	83,713
Total	247,206	124,269
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,533	2,162
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(2,626)	(676)
Investment securities available-for-sale	246,113	125,755
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	490,685	377,655
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	582	1,628
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(1,499)	(306)
Investment securities available-for-sale	489,768	378,977
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	81,546	83,266
Available for Sale Securities Gross Unrealized Gain Accumulated In Investments	1,039	2,147
Available for Sale Securities Gross Unrealized Loss Accumulated In Investments	(300)	(287)
Investment securities available-for-sale	$ 82,285	$ 85,126

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

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