MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due within one year	$ 2,611
Due after one year through five years	54,004
Due after five years through ten years	63,588
Due after ten years	75,799
Available-for-sale debt securities, amortized cost basis	392,869	$ 363,097
Estimated Fair Value
Due within one year	2,657
Due after one year through five years	56,579
Due after five years through ten years	66,693
Due after ten years	80,537
Available-for-sale debt securities, estimated fair value	407,736	380,795
Mortgage-backed securities
Amortized Cost
Available-for-sale debt securities, amortized cost basis	139,186
Available-for-sale debt securities, amortized cost basis	139,186	104,210
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	141,468
Available-for-sale debt securities, estimated fair value	141,468	107,164
Collateralized mortgage obligations
Amortized Cost
Available-for-sale debt securities, amortized cost basis	57,681
Available-for-sale debt securities, amortized cost basis	57,681	64,611
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	59,802
Available-for-sale debt securities, estimated fair value	$ 59,802	$ 66,945