MARKETABLE SECURITIES - Narrative (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($) security position	Jun. 30, 2018 USD ($) security position	Jun. 30, 2017 security	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Other than temporary impairment losses on debt securities related to credit losses	$ 0	$ 0		$ 0
Number of investment positions in an unrealized loss position | position	175	175
Fair value of securities pledged as collateral	$ 255,163,000	$ 255,163,000		$ 245,600,000
Number of held-to-maturity securities sold | security	0	0	3