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Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States federal tax at statutory rate	$ (27,245)	$ (31,880)	$ (36,005)
State taxes (net of federal benefit)	(8,100)	(9,524)	(11,133)
Deferred income tax adjustments	344	2,004	(4,435)
Research credit, net	(14,691)	(5,830)	(8,466)
Orphan drug credit, net	(528)	(301)	(872)
Other permanent items	1,156	1,206	148
Equity-based compensation	554	1,889	758
Change in valuation allowance	48,510	42,436	60,005
Income tax expense/(benefit)	$ 0	$ 0	$ 0

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deferred tax adjustments.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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