INVESTMENTS - Schedule of Maturities of Investment Securities Available-for-Sale and Held-to-Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available for Sale Amortized Cost
Within one year	$ 0	$ 0
After one year but within five years	57,993	37,852
After five years but within ten years	233,748	239,845
After ten years	2,709,245	2,172,568
Amortized Cost	3,000,986	2,450,265
Available for Sale Fair Value
Within one year	0	0
After one year but within five years	60,781	39,985
After five years but within ten years	235,304	251,874
After ten years	2,691,800	2,237,198
Available-for-sale securities, fair value total	2,987,885	2,529,057
Held to Maturity, Amortized Cost
Within one year	1,135	1,144
After one year but within five years	971	972
After five years but within ten years	41,075	35,967
After ten years	60,347	73,664
Debt Securities, Held-to-maturity, Total	103,528	111,747
Held to Maturity, Fair Value
Within one year	1,140	1,154
After one year but within five years	985	990
After five years but within ten years	42,558	37,317
After ten years	62,946	76,960
Held-to-maturity securities, fair value total	$ 107,629	$ 116,421