Income Taxes - Significant Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Net deferred tax asset:
Net operating loss carryforwards	$ 31,744	$ 26,138
Research and development credits	4,615	3,863
Accrued expenses	332	239
Intangibles	69	110
Property and equipment	(18)	(2)
Other, net	54	22
Total net deferred tax asset	36,796	30,370
Valuation allowance for deferred tax asset	(36,796)	(30,370)
Deferred tax assets, net of valuation allowance	$ 0	$ 0