Income Taxes - Components of Net Deferred Income Tax Asset and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net deferred income tax asset - Non-current
Warranty cost	$ 695	$ 1,121
Inventory reserve	419	456
Unearned service revenue	5,364	7,088
Employee stock options	4,366	4,501
Tax Credits	1,785	2,035
Loss carryforwards	8,782	8,005
Other, net	1,479	1,213
Deferred income tax asset - Non-current	22,890	24,419
Valuation Allowance	(1,631)	(876)
Net deferred income tax asset - Non-current	21,259	23,543
Bad debt reserve	(2)	(159)
Depreciation	(3,675)	(6,799)
Goodwill amortization	(1,574)	(2,279)
Intangible assets	(1,097)	(1,409)
Deferred Tax Liabilities, Gross, Noncurrent	6,348	10,646
Net deferred tax assets	$ 14,911	$ 12,897