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Income Taxes - Rate reconciliation (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Rate reconciliation      
Federal tax benefit at statutory rate 21.00% 21.00% 34.00%
State rate, net of federal benefit 2.90% 4.70% 5.20%
Permanent differences 0.004 0.005 0.055
Change in federal rate     48.00%
Research and development 0.70% 0.80%  
Change in valuation allowance 24.20% 26.30% (15.10%)
Other   (0.30%) 0.80%
Total tax provision 0.00% 0.00% 0.00%