INVESTMENTS - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	32,729	22,136
After five years but within ten years	219,962	194,197
After ten years	1,991,850	1,693,150
Amortized Cost	2,244,541	1,909,483
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	34,393	22,207
After five years but within ten years	233,169	194,376
After ten years	2,067,360	1,728,331
Available-for-sale securities, fair value total	2,334,922	1,944,914
Held to Maturity, Amortized Cost
Within one year	1,152	2,649
After one year but within five years	1,549	4,239
After five years but within ten years	32,533	35,288
After ten years	78,382	91,425
Debt Securities, Held-to-maturity, Total	113,616	133,601
Held to Maturity, Fair Value
Within one year	1,168	2,653
After one year but within five years	1,567	4,270
After five years but within ten years	33,763	35,967
After ten years	81,615	93,735
Held-to-maturity securities, fair value total	$ 118,113	$ 136,625