INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale, Amortized Cost
Due in one year or less	$ 14,702
Due after one year through five years	68,822
Due after five years through ten years	241,455
Due after ten years	608,156
Amortized cost, total	933,135
Available-for-sale, Fair Value
Due in one year or less	14,669
Due after one year through five years	67,998
Due after five years through ten years	240,454
Due after ten years	601,190
Fair value, total	924,311
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due in one year or less	0
Due after one year through five years	0
Due after five years through ten years	1,306
Due after ten years	0
Amortized Cost	1,306	$ 1,307
Held-to-maturity Securities, Debt Maturities, 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	1,324
Due after ten years	0
Held-to-maturity Securities, Fair Value	$ 1,324	$ 1,291