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Income Taxes - Effective Tax Rate of Provision for Income Taxes from Federal Statutory Rate (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]      
Federal statutory income tax rate 21.00% 21.00% 21.00%
State income taxes, net of federal benefit   (0.50%)  
Federal tax credits 13.70% 5.20% 9.50%
Other (0.50%) (0.20%) (0.20%)
Premium on equity issuance 2.20%    
Nondeductible permanent items (0.90%) (0.40%) (0.80%)
Stock-based compensation 0.90% (1.00%) (0.80%)
Uncertain tax positions (2.70%) (1.00%) (1.90%)
Change in valuation allowance (33.90%) (23.60%) (26.80%)
Foreign rate differential (0.50%) (0.30%) (0.30%)
Provision for income taxes (0.70%) (0.80%) (0.30%)