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Income Taxes - Reconciliation of effective tax rate (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of the effective tax rate to the U.S. Federal statutory rate      
Federal tax benefit at statutory rate 21.00% 21.00% 21.00%
State tax, net of Federal benefit 32.00% 6.00% 9.00%
Change in valuation allowance (83.00%) (28.00%) (41.00%)
Research and development tax credits 65.00% 4.00% 3.00%
Share based Compensation (34.00%) (1.00%) (1.00%)
162(m) covered employees compensation limitation (20.00%)    
FIN48 Reserve (552.00%) (1.00%) (1.00%)
Deconsolidation gain 597.00%    
Prior year federal true-up (25.00%)   10.00%
Other (1.00%) (1.00%)  
Effective income tax rate 0.00% 0.00% 0.00%