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Income Taxes - Reconciliation of U.S. Statutory Income Tax Rate to Effective Tax Rate (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Tax Disclosure [Abstract]      
Tax effected at statutory rate 21.00% 21.00% 21.00%
State taxes, net of federal benefit 3.60% 7.90% 6.30%
Non-deductible items (0.80%) 1.60% 0.00%
Change in valuation allowance (47.40%) (33.00%) (28.50%)
Federal research and development credits 3.80% 2.50% 1.50%
Foreign tax rate differential 0.00% (0.20%) (0.20%)
Stock compensation windfall 19.80% 0.00% 0.00%
Other (0.30%) 0.20% (0.20%)
Effective tax rate (0.30%) (0.00%) (0.10%)