INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Mar. 31, 2021 USD ($)
Available-for-sale, Amortized Cost
Due in one year or less	$ 3,025
Due after one year through five years	61,558
Due after five years through ten years	248,264
Due after ten years	787,667
Amortized cost, total	1,100,514
Available-for-sale, Fair Value
Due in one year or less	3,102
Due after one year through five years	63,040
Due after five years through ten years	255,924
Due after ten years	793,482
Fair value, total	$ 1,115,548