Income Taxes - Schedule of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 46,352	$ 41,430
Research and development credits	3,208	3,059
Share-based compensation	523	316
Accruals	853	753
Interest expense - 163(j)	592
Capitalized start-up costs	1,295	1,530
Other temporary differences	951	253
Gross deferred tax assets	53,774	47,341
Less: Valuation allowance	$ (53,774)	$ (47,341)