MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due within one year	$ 0
Due after one year through five years	6,145
Due after five years through ten years	12,658
Due after ten years	7,722
Available-for-sale debt securities, amortized cost basis	242,415
Estimated Fair Value
Due within one year	0
Due after one year through five years	5,994
Due after five years through ten years	12,251
Due after ten years	7,465
Available-for-sale debt securities, estimated fair value	235,075	$ 232,372
Amortized Cost
Due within one year	401
Due after one year through five years	12,093
Due after five years through ten years	42,238
Due after ten years	89,040
Amortized Cost	170,408	174,684
Estimated Fair Value
Due within one year	403
Due after one year through five years	12,173
Due after five years through ten years	42,994
Due after ten years	87,752
Held-to-maturity securities, estimated fair value	169,681	$ 176,790
Mortgage-backed securities
Amortized Cost
Available-for-sale securities without single maturity date	97,654
Estimated Fair Value
Available-for-sale securities without single maturity date	94,164
Amortized Cost
Held-to-maturity securities, without single maturity date	20,919
Estimated Fair Value
Held-to-maturity securities, without single maturity date	20,584
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities without single maturity date	118,236
Estimated Fair Value
Available-for-sale securities without single maturity date	115,201
Amortized Cost
Held-to-maturity securities, without single maturity date	5,717
Estimated Fair Value
Held-to-maturity securities, without single maturity date	$ 5,775