INVESTMENTS - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	13,677	22,136
After five years but within ten years	188,785	194,197
After ten years	1,750,627	1,693,150
Amortized Cost	1,953,089	1,909,483
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	14,000	22,207
After five years but within ten years	198,340	194,376
After ten years	1,836,060	1,728,331
Available-for-sale securities, fair value total	2,048,400	1,944,914
Held to Maturity, Amortized Cost
Within one year	1,140	2,649
After one year but within five years	4,226	4,239
After five years but within ten years	37,241	35,288
After ten years	91,448	91,425
Debt Securities, Held-to-maturity, Total	134,055	133,601
Held to Maturity, Fair Value
Within one year	1,142	2,653
After one year but within five years	4,252	4,270
After five years but within ten years	37,995	35,967
After ten years	93,985	93,735
Held-to-maturity securities, fair value total	$ 137,374	$ 136,625