Income Taxes - Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 24,137	$ 18,257
Tax credits	6,571	2,287
Accruals and reserves	854	697
Stock-based expense	876	439
Start-up costs	698	829
Gross deferred tax assets	33,136	22,509
Valuation allowance	(33,136)	(22,509)
Net deferred tax assets	$ 0	$ 0