Inventories - Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials and subassemblies	$ 20,455	$ 20,448
Work-in-process	1,342	1,146
Finished goods	7,967	7,953
Reserve for excess and obsolete inventories	(15,036)	(15,039)
Total	$ 14,728	$ 14,508