Income Taxes - Schedule of Significant Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 98,445	$ 49,689
Research and development reimbursement carryforwards	23,061	12,908
Stock-based compensation	8,861	4,349
Start-up costs	22,896	1,237
Depreciation	(1,027)	(959)
Other	2,646	1,858
Total deferred tax assets	154,882	69,082
Less: valuation allowance	$ (154,882)	$ (69,082)