MARKETABLE SECURITIES - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) Position	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Position security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 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	26		26
Number of securities other than agencies amount | security			0
Fair value of securities pledged as collateral	$ 240,348,000		$ 240,348,000		$ 259,557,000
Number of held-to-maturity securities sold	0	0	0	0