Summary of Net Deferred Tax Assets and Net Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Capitalized intangible assets for tax purposes	$ (21)	$ 47
Reserves not currently deductible	1,130	2,117
Deferred revenue	5	8
Stock options	3,600	4,966
State taxes	6	9
Income tax credits	2,640	2,379
Inventory	495	940
Property and equipment	165	201
Other comprehensive income		252
Unrealized gain on foreign currency	84	136
Net operating losses	33,451	43,687
Total deferred tax assets	41,555	54,742
Valuation allowance	(40,866)	(54,310)
Net deferred tax assets	689	432
Capitalized intangible assets	(608)	(876)
Other	(185)	(354)
Total deferred tax liabilities	(793)	(1,230)
Net deferred tax liabilities	$ (104)	$ (798)