Income Taxes - Components of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Stock based compensation	$ 998	$ 926
Federal and state net operating losses	34,873	26,064
Goodwill	341	423
Accrued compensation	11	11
Deferred rent	290	281
Federal and state research and experimentation credit	7,528	6,055
Intangible asset differences	185	222
Other	76	75
Total gross deferred tax assets	44,302	34,057
Less valuation allowance	(42,349)	(32,325)
Net deferred tax assets	1,953	1,732
Deferred tax liabilities:
Patent expenditures	(1,442)	(1,427)
Fixed asset differences	(511)	(305)
Total gross deferred tax liabilities	(1,953)	(1,732)
Total net deferred tax assets	$ 0	$ 0