INVENTORY - (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
On-hand final assemblies and finished goods inventories	$ 18,374	$ 15,346
Deferred cost of goods sold	4,569	4,237
Gross inventory	22,943	19,583
Less current portion	(21,303)	(18,283)
Noncurrent portion (included in Other assets)	$ 1,640	$ 1,300