Income Taxes - Components of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Stock based compensation	$ 926	$ 1,524
Federal and state net operating losses	26,064	21,089
Goodwill	423	715
Accrued compensation	11	19
Deferred rent	281	344
Federal and state research and experimentation credit	6,055	3,434
AMT credit	0	92
Intangible asset differences	222	253
Other	75	39
Total gross deferred tax assets	34,057	27,509
Less valuation allowance	(32,325)	(24,888)
Net deferred tax assets	1,732	2,621
Deferred tax liabilities:
Patent expenditures	(1,427)	(2,012)
Fixed asset differences	(305)	(609)
Total gross deferred tax liabilities	(1,732)	(2,621)
Total net deferred tax assets	$ 0	$ 0