MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Amortized Cost
Due within one year	$ 6,235
Due after one year through five years	51,011
Due after five years through ten years	51,214
Due after ten years	80,987
Available-for-sale debt securities, amortized cost basis	359,126	$ 210,212
Estimated Fair Value
Due within one year	6,280
Due after one year through five years	53,759
Due after five years through ten years	54,905
Due after ten years	86,187
Available-for-sale debt securities, estimated fair value	376,381	212,716
Mortgage-backed securities
Amortized Cost
Available-for-sale debt securities, amortized cost basis	89,123
Available-for-sale debt securities, amortized cost basis	89,123	83,967
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	91,821
Available-for-sale debt securities, estimated fair value	91,821	84,182
Collateralized mortgage obligations
Amortized Cost
Available-for-sale debt securities, amortized cost basis	80,556
Available-for-sale debt securities, amortized cost basis	80,556	89,798
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	83,429
Available-for-sale debt securities, estimated fair value	$ 83,429	$ 90,927