Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Tax loss carryforwards	$ 53,834	$ 50,183
Accrued foreign taxes	0	1,044
Research and development and other tax credit carryforwards	1,149	792
Uniform capitalization, contract and inventory related reserves	1,388	982
Accrued vacation	327	301
Allowance for doubtful accounts	147	161
Stock-based compensation	2,309	2,029
Capitalized research and development	2,688	3,043
Tax basis depreciation less book depreciation	1,888	1,492
Pension assets	992	1,022
Deferred revenue	798	175
163(j) interest limitation	1,135	0
Other	1,741	2,367
Total	68,396	63,591
Deferred tax liabilities:
Withholding tax on undistributed earnings of foreign subsidiary	0	(4,879)
Intangible assets	978	1,514
Foreign exchange gains/losses	(183)	(351)
Total	(1,161)	(6,744)
Net deferred tax assets before valuation allowance	67,235	56,847
Valuation allowance	(67,012)	(61,403)
Net deferred tax assets	$ 223
Net deferred tax liabilities		$ (4,556)