INVESTMENTS (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less	$ 50,526
Over one through five years	207,292
Over five through ten years	175,529
Over ten years	1,320
Amortized cost	434,667
Available-for-sale Securities, Debt maturities, Fair value:
One year or less	50,394
Over one through five years	205,249
Over five through ten years	172,538
Over ten years	1,276
Fair value	429,457
Held-to-maturity Securities, Debt maturities, Amortized cost [Abstract]
One year or less	730
Over one through five years	3,940
Over five through ten years	628
Amortized cost	5,298
Held-to-maturity Securities, Debt Maturities, Fair value:
One year or less	732
Over one through five years	3,812
Over five through ten years	620
Fair value	5,164
Total Investments [Abstract]
Amortized cost	439,965
Fair value	$ 434,621