INVESTMENTS - (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Available-for-sale Securities, Debt maturities, Amortized cost:
One year or less	$ 20,349
Over one through five years	194,166
Over five through ten years	216,543
Over ten years	2,606
Amortized cost	433,664
Available-for-sale Securities, Debt maturities, Fair value:
One year or less	20,285
Over one through five years	192,491
Over five through ten years	213,427
Over ten years	2,438
Amortized cost	428,641
Held-to-maturity Securities, Debt maturities, Amortized cost [Abstract]
One year or less	650
Over one through five years	4,088
Over five through ten years	388
Amortized cost	5,126
Held-to-maturity Securities, Debt Maturities, Fair value:
One year or less	650
Over one through five years	3,935
Over five through ten years	379
Estimated fair value	4,964
Total Investments [Abstract]
Amortized cost	438,790
Estimated fair value	$ 433,605