MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Amortized Cost
Due within one year	$ 1,030
Due after one year through five years	11,060
Due after five years through ten years	15,717
Due after ten years	7,604
Available-for-sale debt securities, amortized cost basis	227,200	$ 239,945
Estimated Fair Value
Due within one year	1,035
Due after one year through five years	11,222
Due after five years through ten years	16,262
Due after ten years	7,705
Available-for-sale debt securities, estimated fair value	228,714	232,975
Amortized Cost
Due within one year	984
Due after one year through five years	28,359
Due after five years through ten years	34,497
Due after ten years	76,000
Amortized Cost	158,915	163,164
Estimated Fair Value
Due within one year	990
Due after one year through five years	29,166
Due after five years through ten years	35,860
Due after ten years	77,801
Held-to-maturity securities, estimated fair value	162,972	162,800
Mortgage-backed securities
Amortized Cost
Available-for-sale securities without single maturity date	91,782
Available-for-sale debt securities, amortized cost basis	91,782	93,083
Estimated Fair Value
Available-for-sale securities without single maturity date	91,279
Available-for-sale debt securities, estimated fair value	91,279	89,459
Amortized Cost
Held-to-maturity securities, without single maturity date	15,832
Amortized Cost	15,832	17,163
Estimated Fair Value
Held-to-maturity securities, without single maturity date	15,808
Held-to-maturity securities, estimated fair value	15,808	16,717
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities without single maturity date	100,007
Available-for-sale debt securities, amortized cost basis	100,007	111,341
Estimated Fair Value
Available-for-sale securities without single maturity date	101,211
Available-for-sale debt securities, estimated fair value	101,211	108,969
Amortized Cost
Held-to-maturity securities, without single maturity date	3,243
Amortized Cost	3,243	4,059
Estimated Fair Value
Held-to-maturity securities, without single maturity date	3,347
Held-to-maturity securities, estimated fair value	$ 3,347	$ 4,079