INVESTMENT SECURITIES - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	19,792	20,051
After five years but within ten years	170,009	179,812
After ten years	847,104	647,928
Amortized Cost	1,036,905	847,791
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	19,160	19,825
After five years but within ten years	159,754	175,583
After ten years	818,217	642,091
Available-for-sale securities, fair value total	997,131	837,499
Held to Maturity, Amortized Cost
Within one year	1,027	322
After one year but within five years	6,636	5,895
After five years but within ten years	29,148	18,751
After ten years	115,766	136,218
Amortized Cost	152,577	161,186
Held to Maturity, Fair Value
Within one year	1,023	320
After one year but within five years	6,599	5,894
After five years but within ten years	28,902	18,873
After ten years	114,059	137,766
Held-to-maturity securities, fair value total	$ 150,583	$ 162,853