MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Amortized Cost
Due within one year	$ 0
Due after one year through five years	12,110
Due after five years through ten years	15,729
Due after ten years	7,627
Available-for-sale debt securities, amortized cost basis	238,790	$ 239,945
Estimated Fair Value
Due within one year	0
Due after one year through five years	12,150
Due after five years through ten years	15,909
Due after ten years	7,567
Available-for-sale debt securities, estimated fair value	236,979	232,975
Amortized Cost
Due within one year	990
Due after one year through five years	28,477
Due after five years through ten years	34,557
Due after ten years	76,688
Amortized Cost	160,980	163,164
Estimated Fair Value
Due within one year	996
Due after one year through five years	29,093
Due after five years through ten years	35,627
Due after ten years	77,267
Held-to-maturity securities, estimated fair value	163,110	162,800
Mortgage-backed securities
Amortized Cost
Available-for-sale securities without single maturity date	95,322
Available-for-sale debt securities, amortized cost basis	95,322	93,083
Estimated Fair Value
Available-for-sale securities without single maturity date	93,746
Available-for-sale debt securities, estimated fair value	93,746	89,459
Amortized Cost
Amortized Cost	16,521	17,163
Held-to-maturity securities, without single maturity date	16,521
Estimated Fair Value
Held-to-maturity securities, estimated fair value	16,301	16,717
Held-to-maturity securities, without single maturity date	16,301
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities without single maturity date	108,002
Available-for-sale debt securities, amortized cost basis	108,002	111,341
Estimated Fair Value
Available-for-sale securities without single maturity date	107,607
Available-for-sale debt securities, estimated fair value	107,607	108,969
Amortized Cost
Amortized Cost	3,747	4,059
Held-to-maturity securities, without single maturity date	3,747
Estimated Fair Value
Held-to-maturity securities, estimated fair value	3,826	$ 4,079
Held-to-maturity securities, without single maturity date	$ 3,826