Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accounts receivable, due to allowance for doubtful accounts	$ 540	$ 1,104
Inventories	581	792
Operating loss carry-forwards	4,725	3,078
Stock-based compensation expense	696	1,231
Property and equipment, due to difference in depreciation	190	148
Research and development, alternative minimum tax credit carry-forwards	4,338	3,031
Foreign tax credit carry-forwards	2,958	754
State tax credit carry-forwards	2,958	2,277
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Warranty Reserves	495	822
Accrued expenses	334	432
Gross deferred tax assets	17,815	13,669
Less valuation allowance	(16,014)	(11,567)
Total deferred tax assets	1,801	2,102
Deferred tax liabilities:
Purchased intangible assets	(2,705)	(3,197)
Property and equipment, due to differences in depreciation	(1,681)	(2,003)
Other	(29)	(11)
Total deferred tax liabilities	(4,415)	(5,211)
Net deferred tax liability	(2,614)	(3,109)
Net deferred tax asset--non-current	20	24
Net deferred tax liability- non-current	$ (2,634)	$ (3,133)