INVESTMENTS (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Sep. 30, 2018 USD ($)
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less	$ 36,645
Over one through five years	212,426
Over five through ten years	181,102
Over ten years	1,878
Amortized cost	432,051
Available-for-sale Securities, Debt maturities, Fair value:
One year or less	36,575
Over one through five years	209,585
Over five through ten years	176,126
Over ten years	1,862
Fair value	424,148
Held-to-maturity Securities, Debt maturities, Amortized cost [Abstract]
One year or less	750
Over one through five years	4,033
Over five through ten years	472
Amortized cost	5,255
Held-to-maturity Securities, Debt Maturities, Fair value:
One year or less	751
Over one through five years	3,869
Over five through ten years	458
Fair value	5,078
Total Investments
Amortized cost	437,306
Fair value	$ 429,226