INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale, Amortized Cost
Due in one year or less	$ 0
Due after one year through five years	76,517
Due after five years through ten years	215,515
Due after ten years	639,844
Amortized cost, total	931,876
Available-for-sale, Fair Value
Due in one year or less	0
Due after one year through five years	78,367
Due after five years through ten years	223,272
Due after ten years	658,492
Fair value, total	960,131
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Due in one year or less	0
Due after one year through five years	511
Due after five years through ten years	789
Due after ten years	0
Amortized Cost	1,300	$ 1,302
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Fair Value [Abstract]
Due in one year or less	0
Due after one year through five years	532
Due after five years through ten years	826
Due after ten years	0
Debt Securities, Held-to-maturity, Fair Value	$ 1,358	$ 1,359