MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due within one year	$ 4,139
Due after one year through five years	52,050
Due after five years through ten years	60,132
Due after ten years	77,955
Available-for-sale debt securities, amortized cost basis	363,097	$ 210,212
Estimated Fair Value
Due within one year	4,154
Due after one year through five years	54,688
Due after five years through ten years	63,966
Due after ten years	83,878
Available-for-sale debt securities, estimated fair value	380,795	212,716
Mortgage-backed securities
Amortized Cost
Available-for-sale debt securities, amortized cost basis	104,210
Available-for-sale debt securities, amortized cost basis	104,210	83,967
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	107,164
Available-for-sale debt securities, estimated fair value	107,164	84,182
Collateralized mortgage obligations
Amortized Cost
Available-for-sale debt securities, amortized cost basis	64,611
Available-for-sale debt securities, amortized cost basis	64,611	89,798
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	66,945
Available-for-sale debt securities, estimated fair value	$ 66,945	$ 90,927