INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Held-to-Maturity Amortized Cost
Amortized Cost	$ 633,722	$ 654,008
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	623,967	653,101
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	1,370,844	1,348,227
Available-for-Sale Market Value
Investment securities available-for-sale	1,359,381	1,349,408
One Year or Less [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	165
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	165
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	2,296
Available-for-Sale Market Value
Investment securities available-for-sale	2,292
After One Year Through Five Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	4,277
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	4,279
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	37,023
Available-for-Sale Market Value
Investment securities available-for-sale	36,960
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	2,499
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	2,632
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	82,431
Available-for-Sale Market Value
Investment securities available-for-sale	83,521
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	85,177
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	84,711
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	109,128
Available-for-Sale Market Value
Investment securities available-for-sale	109,448
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	155,208	162,093
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	153,616	162,600
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	288,757	290,793
Available-for-Sale Market Value
Investment securities available-for-sale	282,443	289,043
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	251,253	255,027
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	246,133	253,399
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	153,770	150,356
Available-for-Sale Market Value
Investment securities available-for-sale	150,942	149,103
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	135,143	143,545
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	132,431	142,297
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	305,496	306,095
Available-for-Sale Market Value
Investment securities available-for-sale	301,263	305,392
Asset-backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	0	0
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	0	0
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	391,943	377,655
Available-for-Sale Market Value
Investment securities available-for-sale	$ 392,512	$ 378,977