INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale, Amortized Cost
Due in one year or less	$ 0
Due after one year through five years	69,611
Due after five years through ten years	231,573
Due after ten years	613,916
Amortized cost, total	915,100
Available-for-sale, Fair Value
Due in one year or less	0
Due after one year through five years	69,758
Due after five years through ten years	234,217
Due after ten years	616,108
Fair value, total	920,083
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	0
Due after five years through ten years	1,304
Due after ten years	0
Amortized Cost	1,304	$ 1,307
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	0
Due after five years through ten years	1,335
Due after ten years	0
Debt Securities, Held-to-maturity, Fair Value	$ 1,335	$ 1,291