Income Taxes - Schedule of Significant Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 49,689	$ 22,246
Research and development credits carryforwards	12,908	8,631
Stock-based compensation	4,349	890
Deferred revenue		9,391
Start-up costs	1,237	1,354
Depreciation	(959)	(457)
Other	1,858	1,327
Total deferred tax assets	69,082	43,382
Less: valuation allowance	$ (69,082)	$ (43,382)