INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale, Amortized Cost
Due in one year or less	$ 3,726
Due after one year through five years	5,665
Due after five years through ten years	78,395
Due after ten years	49,569
Subtotal	137,355
Mortgage-related securities	1,306,445
Available-for-sale securities, book value	1,443,800	$ 1,078,474
Available-for-sale, Fair Value
Due in one year or less	3,773
Due after one year through five years	6,063
Due after five years through ten years	81,078
Due after ten years	51,477
Subtotal	142,391
Mortgage-related securities	1,312,819
Fair value, total	$ 1,455,210	$ 1,115,813