MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Amortized Cost
Due within one year	$ 1,023
Due after one year through five years	13,421
Due after five years through ten years	13,333
Due after ten years	7,579
Available-for-sale debt securities, amortized cost basis	218,599	$ 239,945
Estimated Fair Value
Due within one year	1,027
Due after one year through five years	13,748
Due after five years through ten years	13,907
Due after ten years	7,810
Available-for-sale debt securities, estimated fair value	221,345	232,975
Amortized Cost
Due within one year	2,150
Due after one year through five years	29,272
Due after five years through ten years	33,943
Due after ten years	73,511
Amortized Cost	156,925	163,164
Estimated Fair Value
Due within one year	2,164
Due after one year through five years	30,148
Due after five years through ten years	35,375
Due after ten years	76,166
Held-to-maturity securities, estimated fair value	162,165	162,800
Mortgage-backed securities
Amortized Cost
Available-for-sale securities without single maturity date	87,946
Available-for-sale debt securities, amortized cost basis	87,946	93,083
Estimated Fair Value
Available-for-sale securities without single maturity date	88,067
Available-for-sale debt securities, estimated fair value	88,067	89,459
Amortized Cost
Held-to-maturity securities, without single maturity date	15,119
Amortized Cost	15,119	17,163
Estimated Fair Value
Held-to-maturity securities, without single maturity date	15,264
Held-to-maturity securities, estimated fair value	15,264	16,717
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities without single maturity date	95,297
Available-for-sale debt securities, amortized cost basis	95,297	111,341
Estimated Fair Value
Available-for-sale securities without single maturity date	96,786
Available-for-sale debt securities, estimated fair value	96,786	108,969
Amortized Cost
Held-to-maturity securities, without single maturity date	2,930
Amortized Cost	2,930	4,059
Estimated Fair Value
Held-to-maturity securities, without single maturity date	3,048
Held-to-maturity securities, estimated fair value	$ 3,048	$ 4,079