Income Taxes - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
U.S. federal income tax at statutory rate	21.00%	21.00%	21.00%
State taxes (net of federal benefit)	25.23%	4.71%	5.67%
Stock-based compensation	69.14%	1.20%	7.51%
Permanent differences	(1.03%)	(0.97%)	(0.57%)
Federal research and development credit	(20.42%)	(2.45%)	(4.26%)
Change in valuation allowance	(132.88%)	(29.73%)	(37.33%)
Other	1.32%	1.34%	(0.54%)
Extinguishment of 2023 Notes	(3.20%)	0.00%	0.00%
Effective income tax rate	0.00%	0.00%	0.00%

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

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage 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 changes in the valuation allowance for deferred tax assets.

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