MARKETABLE SECURITIES - Narrative (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security position	Mar. 31, 2017 security	Dec. 31, 2017 USD ($)
Investments, Debt and Equity Securities [Abstract]
Other than temporary impairment losses on debt securities related to credit losses	$ 0		$ 0
Number of investment positions in an unrealized loss position | position	174
Fair value of securities pledged as collateral	$ 270,716,000		$ 245,600,000
Number of held-to-maturity securities sold | security	0	0