INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale, Amortized Cost
Due in one year or less	$ 7,933
Due after one year through five years	94,474
Due after five years through ten years	193,165
Due after ten years	518,174
Amortized cost, total	813,746
Available-for-sale, Fair Value
Due in one year or less	7,907
Due after one year through five years	91,482
Due after five years through ten years	184,931
Due after ten years	496,023
Fair value, total	780,343
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due in one year or less	4,610
Due after one year through five years	1,529
Due after five years through ten years	19,960
Due after ten years	66,834
Amortized Cost	92,933	$ 94,073
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]
Due in one year or less	4,609
Due after one year through five years	1,538
Due after five years through ten years	19,670
Due after ten years	64,733
Held-to-maturity Securities, Fair Value	$ 90,550	$ 94,913