Income Taxes - Schedule of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets (liabilities):
Net operating loss carryforwards	$ 216,377	$ 159,192
Research and orphan drug credit carryforwards	40,077	31,142
Stock-based compensation	15,701	8,928
Accrued expenses	9,560	3,779
Operating lease liabilities	9,019	9,321
Operating lease assets	(8,904)	(8,811)
Property and equipment	(322)	(387)
Other	1,028	16
Total gross deferred tax assets	282,536	203,180
Valuation allowance	(282,536)	(203,180)	$ (119,647)
Net deferred tax assets	$ 0	$ 0