INVESTMENTS (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less	$ 17,108
Over one through five years	198,410
Over five through ten years	218,177
Over ten years	5,559
Amortized cost	439,254
Available-for-sale Securities, Debt maturities, Fair value:
One year or less	17,094
Over one through five years	199,858
Over five through ten years	220,854
Over ten years	5,652
Fair value	443,458
Held-to-maturity Securities, Debt maturities, Amortized cost [Abstract]
One year or less	135
Over one through five years	4,042
Over five through ten years	375
Amortized cost	4,552
Held-to-maturity Securities, Debt Maturities, Fair value:
One year or less	135
Over one through five years	3,963
Over five through ten years	371
Fair value	4,469
Total Investments
Amortized cost	443,806
Fair value	$ 447,927