INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Held-to-Maturity Amortized Cost
Amortized Cost	$ 696,269	$ 763,254
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	698,722	763,575
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	1,295,377	1,045,337
Available-for-Sale Market Value
Investment securities available-for-sale	1,298,578	1,039,870
One Year or Less [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	170
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	170
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	2,236
Available-for-Sale Market Value
Investment securities available-for-sale	2,238
After One Year Through Five Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	3,657
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	3,660
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	25,504
Available-for-Sale Market Value
Investment securities available-for-sale	25,743
After Five Years Through Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	2,691
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	2,790
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	73,547
Available-for-Sale Market Value
Investment securities available-for-sale	75,012
After Ten Years [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	80,834
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	81,228
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	122,624
Available-for-Sale Market Value
Investment securities available-for-sale	123,835
Residential Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	181,553	205,522
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	182,211	206,096
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	262,052	184,960
Available-for-Sale Market Value
Investment securities available-for-sale	261,173	183,395
Commercial Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	261,739	278,728
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	262,983	280,165
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	157,138	154,239
Available-for-Sale Market Value
Investment securities available-for-sale	157,408	153,601
Collateralized Mortgage Backed Securities [Member]
Held-to-Maturity Amortized Cost
Amortized Cost	165,625	195,408
Held-to-Maturity Market Value
Held-to-maturity Securities, Fair Value	165,680	195,057
Available-for-Sale Amortized Cost
Available-for-sale Securities, Amortized Cost Basis	324,712	232,701
Available-for-Sale Market Value
Investment securities available-for-sale	324,392	231,014
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	327,564	322,708
Available-for-Sale Market Value
Investment securities available-for-sale	$ 328,777	$ 321,212