Income Taxes - Schedule of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets (liabilities):
Net operating loss carryforwards	$ 85,632	$ 28,906
Research and orphan drug credit carryforwards	25,267	13,457
Stock-based compensation	5,535	4,681
Accrued expenses	2,951	1,270
Operating lease liabilities	5,841	3,200
Operating lease assets	(5,515)	0
Property and equipment	0	(3,200)
Other	(64)	(123)
Total gross deferred tax assets	119,647	48,191
Valuation allowance	(119,647)	(48,191)	$ (10,835)
Net deferred tax assets	$ 0	$ 0