SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Changes in Accumulated Other Comprehensive Income (Loss)
Beginning Balance		$ (1,422)		$ (750)	$ (1,422)	$ (750)
Other comprehensive income (loss) before reclassifications					3,822	(1,242)
Amounts reclassified from accumulated other comprehensive loss					344	114
Net current period change in accumulated other comprehensive loss, before tax					4,166
Income tax expense related to items of other comprehensive income					(984)
Net current period change in accumulated other comprehensive income (loss)	$ 1,406	1,776	$ 458	(1,586)		(1,128)
Ending Balance	1,760		(1,878)		1,760	(1,878)
Foreign Currency Translation Adjustment
Changes in Accumulated Other Comprehensive Income (Loss)
Beginning Balance		(25)		(61)	(25)	(61)
Other comprehensive income (loss) before reclassifications					38	2
Net current period change in accumulated other comprehensive loss, before tax					38
Net current period change in accumulated other comprehensive income (loss)						2
Ending Balance	13		(59)		13	(59)
Unrealized Gain (Loss) on Marketable Securities
Changes in Accumulated Other Comprehensive Income (Loss)
Beginning Balance		$ (1,397)		$ (689)	(1,397)	(689)
Other comprehensive income (loss) before reclassifications					3,784	(1,244)
Amounts reclassified from accumulated other comprehensive loss					344	114
Net current period change in accumulated other comprehensive loss, before tax					4,128
Income tax expense related to items of other comprehensive income					(984)
Net current period change in accumulated other comprehensive income (loss)						(1,130)
Ending Balance	$ 1,747		$ (1,819)		$ 1,747	$ (1,819)