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Income Taxes - Effective Income Tax Rate (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of income tax rate      
Income tax computed at federal statutory tax rate 21.00% 21.00% 21.00%
State taxes, net of federal benefit 5.20% 5.20% 6.40%
General business credit carryovers 4.80% 2.80% 4.40%
Stock Options (1.80%) (2.20%) 0.70%
Non-deductible expenses (0.10%) (0.10%) (0.10%)
Change in valuation allowance (29.10%) (26.70%) (32.40%)