Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Net operating loss carryforward	$ 20,276	$ 12,170
Intangible assets	38	29
Accrued expense	508	333
Stock-based compensation	682	386
Other	73	75
Total deferred tax assets	29,196	19,389
Deferred tax liabilities
Depreciable assets	(74)	(63)
Total deferred tax liabilities	(74)	(63)
Less: Valuation allowance	(29,122)	(19,326)
Deferred tax assets, net	0	0
Federal
Deferred tax assets
Tax credits	7,260	5,572
State
Deferred tax assets
Tax credits	$ 359	$ 824