INVESTMENTS (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Jun. 30, 2018 USD ($)
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less	$ 44,627
Over one through five years	210,431
Over five through ten years	176,276
Over ten years	1,321
Amortized cost	432,655
Available-for-sale Securities, Debt maturities, Fair value:
One year or less	44,515
Over one through five years	207,761
Over five through ten years	171,943
Over ten years	1,270
Fair value	425,489
Held-to-maturity Securities, Debt maturities, Amortized cost [Abstract]
One year or less	785
Over one through five years	4,000
Over five through ten years	503
Amortized cost	5,288
Held-to-maturity Securities, Debt Maturities, Fair value:
One year or less	786
Over one through five years	3,854
Over five through ten years	492
Fair value	5,132
Total Investments
Amortized cost	437,943
Fair value	$ 430,621