MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Due within one year	$ 0
Due after one year through five years	11,234
Due after five years through ten years	16,668
Due after ten years	7,674
Amortized Cost	241,909	$ 235,849
Estimated Fair Value
Due within one year	0
Due after one year through five years	10,926
Due after five years through ten years	16,262
Due after ten years	7,289
Available-for-sale debt securities, estimated fair value	232,378	232,372
Amortized Cost
Due within one year	769
Due after one year through five years	19,153
Due after five years through ten years	42,448
Due after ten years	80,049
Amortized Cost	164,839	174,684
Estimated Fair Value
Due within one year	768
Due after one year through five years	19,244
Due after five years through ten years	42,774
Due after ten years	77,923
Held-to-maturity securities, estimated fair value	162,596	176,790
Mortgage-backed securities
Amortized Cost
Available-for-sale securities without single maturity date	91,208
Amortized Cost	91,208	92,471
Estimated Fair Value
Available-for-sale securities without single maturity date	86,761
Available-for-sale debt securities, estimated fair value	86,761	90,678
Amortized Cost
Held-to-maturity securities, without single maturity date	17,871
Amortized Cost	17,871	22,026
Estimated Fair Value
Held-to-maturity securities, without single maturity date	17,337
Held-to-maturity securities, estimated fair value	17,337	21,995
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities without single maturity date	115,125
Amortized Cost	115,125	116,809
Estimated Fair Value
Available-for-sale securities without single maturity date	111,140
Available-for-sale debt securities, estimated fair value	111,140	115,311
Amortized Cost
Held-to-maturity securities, without single maturity date	4,549
Amortized Cost	4,549	6,162
Estimated Fair Value
Held-to-maturity securities, without single maturity date	4,550
Held-to-maturity securities, estimated fair value	$ 4,550	$ 6,273