INVESTMENTS (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Available-for-sale Securities, Debt maturities, Amortized cost:
One year or less	$ 37,940
Over one through five years	196,630
Over five through ten years	182,560
Over ten years	2,113
Amortized cost	419,243
Available-for-sale Securities, Debt maturities, Fair value:
One year or less	37,968
Over one through five years	198,180
Over five through ten years	184,199
Over ten years	2,012
Fair value	422,359
Held-to-maturity Securities, Debt maturities, Amortized cost [Abstract]
One year or less	170
Over one through five years	4,114
Over five through ten years	1,126
Amortized cost	5,410
Held-to-maturity Securities, Debt Maturities, Fair value:
One year or less	170
Over one through five years	4,065
Over five through ten years	1,131
Fair value	5,366
Total Investments [Abstract]
Amortized cost	424,653
Fair value	$ 427,725