Deferred Warranty Revenue and Warranty Reserve - Schedule of Warranty Activity (Details) - Accrued expenses - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Activity in the warranty reserve account
Balance, beginning of period	$ 153	$ 43	$ 85
Provision for warranty claims	80	165	165
Product or cash issued to settle claims	(139)	(55)	(207)
Balance, end of period	$ 94	$ 153	$ 43