INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available-for-sale, Amortized Cost
Due in one year or less	$ 4,439
Due after one year through five years	64,370
Due after five years through ten years	225,186
Due after ten years	716,330
Amortized cost, total	1,010,325
Available-for-sale, Fair Value
Due in one year or less	4,487
Due after one year through five years	66,655
Due after five years through ten years	238,559
Due after ten years	737,962
Fair value, total	1,047,663
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	788
Due after ten years	0
Amortized Cost	1,299	$ 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	542
Due after five years through ten years	846
Due after ten years	0
Debt Securities, Held-to-maturity, Fair Value	$ 1,388	$ 1,359