MARKETABLE SECURITIES - Contractual Maturities of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Due within one year	$ 0
Due after one year through five years	1,094
Due after five years through ten years	17,767
Due after ten years	7,793
Available-for-sale debt securities, amortized cost basis	246,830
Estimated Fair Value
Due within one year	0
Due after one year through five years	1,104
Due after five years through ten years	17,929
Due after ten years	7,493
Available-for-sale debt securities, estimated fair value	246,233
Amortized Cost
Due within one year	2,831
Due after one year through five years	5,553
Due after five years through ten years	40,659
Due after ten years	98,978
Amortized Cost	182,248	$ 189,371
Estimated Fair Value
Due within one year	2,838
Due after one year through five years	5,731
Due after five years through ten years	42,175
Due after ten years	99,616
Held-to-maturity securities, estimated fair value	184,527	$ 186,155
Mortgage-backed securities
Amortized Cost
Available-for-sale securities without single maturity date	95,343
Estimated Fair Value
Available-for-sale securities without single maturity date	94,465
Amortized Cost
Held-to-maturity securities, without single maturity date	24,642
Estimated Fair Value
Held-to-maturity securities, without single maturity date	24,869
Collateralized mortgage obligations
Amortized Cost
Available-for-sale securities without single maturity date	124,833
Estimated Fair Value
Available-for-sale securities without single maturity date	125,242
Amortized Cost
Held-to-maturity securities, without single maturity date	9,585
Estimated Fair Value
Held-to-maturity securities, without single maturity date	$ 9,298