INVESTMENT SECURITIES - Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 17,228
Due from one year to five years	34,212
Due from five to ten years	52,456
Due after ten years	39,498
Mortgage-backed securities	513,408
Amortized Cost	656,802	$ 940,580
Estimated Fair Value
Due in one year or less	17,357
Due from one year to five years	35,780
Due from five to ten years	54,242
Due after ten years	41,004
Mortgage-backed securities	536,121
Estimated Fair Value	684,504	982,879
Amortized Cost
Due in one year or less	0
Due from one year to five years	0
Due from five to ten years	0
Due after ten years	3,905
Mortgage-backed securities	60,546
Amortized Cost	64,451	0
Estimated Fair Value
Due in one year or less	0
Due from one year to five years	0
Due from five to ten years	0
Due after ten years	3,784
Mortgage-backed securities	59,520
Estimated Fair Value	$ 63,304	$ 0