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Income Taxes - Reconciliation of effective tax rate (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of the effective tax rate to the U.S. Federal statutory rate      
Federal tax benefit at statutory rate 21.00% 21.00% 34.00%
State tax, net of Federal benefit 6.00% 9.00% 7.00%
Foreign rate differential     (17.00%)
Federal rate change impact     (9.00%)
Change in valuation allowance (28.00%) (41.00%) (16.00%)
Research and development tax credits 3.00% 2.00% 2.00%
Prior year federal true-up   10.00% (3.00%)
Other (2.00%) (1.00%) 2.00%
Effective income tax rate 0.00% 0.00% 0.00%