INVESTMENTS - (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Available-for-sale Securities, Debt maturities, Amortized cost:
One year or less	$ 45,949
Over one through five years	194,427
Over five through ten years	177,657
Over ten years	4,267
Amortized cost	422,300
Available-for-sale Securities, Debt maturities, Fair value:
One year or less	45,879
Over one through five years	194,463
Over five through ten years	178,664
Over ten years	4,232
Amortized cost	423,238
Held-to-maturity Securities, Debt maturities, Amortized cost [Abstract]
One year or less	180
Over one through five years	4,097
Over five through ten years	1,072
Amortized cost	5,349
Held-to-maturity Securities, Debt Maturities, Fair value:
One year or less	180
Over one through five years	4,028
Over five through ten years	1,066
Estimated fair value	5,274
Total Investments [Abstract]
Amortized cost	427,649
Estimated fair value	$ 428,512