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Income Taxes - Rate reconciliation (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Rate reconciliation      
Federal tax benefit at statutory rate (21.00%) (21.00%) (21.00%)
State rate, net of federal benefit (4.20%) (2.90%) (4.70%)
Permanent differences 0.6 0.4 0.5
Research and development 0.70% (0.70%) (0.80%)
Change in valuation allowance 24.00% 24.20% 26.30%
Other (0.10%) 0.00% (0.30%)
Total tax provision