MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due within one year	$ 0
Due after one year through five years	14,645
Due after five years through ten years	29,473
Due after ten years	0
Available-for-sale debt securities, amortized cost basis	265,236	$ 265,236	$ 363,097
Estimated Fair Value
Due within one year	0
Due after one year through five years	15,430
Due after five years through ten years	29,875
Due after ten years	0
Available-for-sale debt securities, estimated fair value	269,070		380,795
Held to Maturity Amortized Cost
Due within one year	4,802
Due after one year through five years	48,612
Due after five years through ten years	50,369
Due after ten years	66,851
Held to Maturity debt securities, amortized cost	173,676		0
Held to Maturity Estimated Fair Value
Due within one year	4,868
Due after one year through five years	50,466
Due after five years through ten years	53,350
Due after ten years	70,942
Held to maturity debt securities, estimated fair value	182,631
Mortgage-backed securities
Amortized Cost
Available-for-sale debt securities, amortized cost basis	157,846
Available-for-sale debt securities, amortized cost basis	157,846		104,210
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	159,142
Available-for-sale debt securities, estimated fair value	159,142		107,164
Held to Maturity Amortized Cost
Held to Maturity debt securities, amortized cost	3,042
Held to Maturity debt securities, amortized cost	3,042
Held to Maturity Estimated Fair Value
Held to maturity debt securities, estimated fair value	3,005
Held to maturity debt securities, estimated fair value	3,005
Collateralized mortgage obligations
Amortized Cost
Available-for-sale debt securities, amortized cost basis	63,272
Available-for-sale debt securities, amortized cost basis	63,272		64,611
Estimated Fair Value
Available-for-sale debt securities, estimated fair value	64,623
Available-for-sale debt securities, estimated fair value	64,623		$ 66,945
Held to Maturity Amortized Cost
Held to Maturity debt securities, amortized cost	0
Held to Maturity Estimated Fair Value
Held to maturity debt securities, estimated fair value	$ 0