INVESTMENTS (Schedule of Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale, Amortized Cost
Due in one year or less	$ 5,307
Due after one year through five years	112,412
Due after five years through ten years	213,112
Due after ten years	491,624
Amortized cost, total	822,455
Available-for-sale, Fair Value
Due in one year or less	5,302
Due after one year through five years	110,253
Due after five years through ten years	206,368
Due after ten years	474,764
Fair value, total	796,687
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due in one year or less	1,418
Due after one year through five years	3,783
Due after five years through ten years	13,035
Due after ten years	74,956
Held-to-maturity securities, at amortized cost	93,192	$ 94,073
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract]
Due in one year or less	1,418
Due after one year through five years	3,796
Due after five years through ten years	12,954
Due after ten years	73,706
Held-to-maturity Securities, Fair Value	$ 91,874	$ 94,913