MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due within one year	$ 0
Due after one year through five years	7,373
Due after five years through ten years	21,924
Due after ten years	7,698
Amortized Cost	251,428	$ 235,849
Estimated Fair Value
Due within one year	0
Due after one year through five years	7,216
Due after five years through ten years	21,469
Due after ten years	7,410
Available-for-sale debt securities, estimated fair value	243,490	232,372
Amortized Cost
Due within one year	1,175
Due after one year through five years	19,089
Due after five years through ten years	40,520
Due after ten years	82,511
Amortized Cost	167,239	174,684
Estimated Fair Value
Due within one year	1,175
Due after one year through five years	19,295
Due after five years through ten years	41,178
Due after ten years	81,207
Held-to-maturity securities, estimated fair value	166,441	176,790
Mortgage-backed securities
Amortized Cost
Available-for-sale securities without single maturity date	95,062
Amortized Cost	95,062	92,471
Estimated Fair Value
Available-for-sale securities without single maturity date	91,242
Available-for-sale debt securities, estimated fair value	91,242	90,678
Amortized Cost
Held-to-maturity securities, without single maturity date	18,880
Estimated Fair Value
Held-to-maturity securities, without single maturity date	18,496
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities without single maturity date	119,371
Amortized Cost	119,371	116,809
Estimated Fair Value
Available-for-sale securities without single maturity date	116,153
Available-for-sale debt securities, estimated fair value	116,153	$ 115,311
Amortized Cost
Held-to-maturity securities, without single maturity date	5,064
Estimated Fair Value
Held-to-maturity securities, without single maturity date	$ 5,090