Income Taxes - Components of the Company's Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Deferred income tax assets:
Federal U.S. net operating loss carryforward	$ 140,706	$ 163,071
State net operating loss carryforward	33,350	44,784
Research and development credit, net	68,251	65,084
Orphan drug credit, net	33,330	35,703
Operating lease liabilities	9,078	9,585
Deferred revenue	17,442	0
Section 174 deferred tax asset	48,421	43,192
Depreciation	16,217	15,228
Other	6,617	4,663
Gross deferred income tax assets	373,412	381,310
Valuation allowance	(365,010)	(372,267)	$ (327,592)	$ (263,400)
Net deferred income tax assets	8,402	9,043
Deferred income tax liabilities:
Operating lease ROU assets	(6,372)	(7,522)
Prepaid expenditures	(2,030)	(1,521)
Gross deferred income tax liabilities	(8,402)	(9,043)
Net deferred income tax asset/(liability)	$ 0	$ 0