Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 18,347	$ 9,183
Stock based compensation	3,642	2,444
License fee	2,008	1,346
59 (e) expenditures and amortization	1,435	1,435
Research and development credits	1,137	365
Other	972	698
Gross deferred tax assets	27,541	15,471
Valuation allowance	(27,317)	(15,395)
Total deferred tax assets	224	76
Deferred tax liabilities:
Other	39	13
Total deferred tax liabilities	39	13
Total net deferred tax assets	$ 185	$ 63