INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Held-to-Maturity Amortized Cost
Amortized Cost	$ 434,369	$ 654,008
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	422,638	653,101
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	2,662,329	1,348,227
Available-for-Sale Market Value
Investment securities available-for-sale	2,626,726	1,349,408
One Year or Less [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	3,932
Available-for-Sale Market Value
Investment securities available-for-sale	3,922
After One Year Through Five Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	275
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	277
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	72,831
Available-for-Sale Market Value
Investment securities available-for-sale	72,633
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	18,337
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	18,274
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	138,584
Available-for-Sale Market Value
Investment securities available-for-sale	139,591
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	225,749
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	222,799
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	146,942
Available-for-Sale Market Value
Investment securities available-for-sale	145,192
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	26,603	162,093
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	24,988	162,600
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	581,657	290,793
Available-for-Sale Market Value
Investment securities available-for-sale	567,946	289,043
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	150,223	255,027
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	143,845	253,399
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	362,874	150,356
Available-for-Sale Market Value
Investment securities available-for-sale	357,346	149,103
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	13,182	143,545
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	12,455	142,297
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	864,824	306,095
Available-for-Sale Market Value
Investment securities available-for-sale	850,328	305,392
Asset-backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0	0
Held-to-Maturity Market Value
Debt Securities, Held-to-maturity, Fair Value	0	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	490,685	377,655
Available-for-Sale Market Value
Investment securities available-for-sale	$ 489,768	$ 378,977