INVESTMENTS - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	33,142	22,136
After five years but within ten years	217,301	194,197
After ten years	1,846,671	1,693,150
Amortized Cost	2,097,114	1,909,483
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	34,759	22,207
After five years but within ten years	229,175	194,376
After ten years	1,931,455	1,728,331
Available-for-sale securities, fair value total	2,195,389	1,944,914
Held to Maturity, Amortized Cost
Within one year	1,130	2,649
After one year but within five years	5,197	4,239
After five years but within ten years	31,881	35,288
After ten years	89,401	91,425
Debt Securities, Held-to-maturity, Total	127,609	133,601
Held to Maturity, Fair Value
Within one year	1,131	2,653
After one year but within five years	5,286	4,270
After five years but within ten years	33,015	35,967
After ten years	92,766	93,735
Held-to-maturity securities, fair value total	$ 132,198	$ 136,625