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Income Taxes - Reconciliation (Details)
12 Months Ended
Dec. 22, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
A reconciliation of the U.S. statutory income tax rate to the Company's effective tax rate        
Federal income tax (benefit) at statutory rate 35.00% 21.00% 21.00% 34.00%
Permanent differences   1.11% 1.71% 0.49%
Federal research and development credits   0.77% 1.79% 0.97%
Federal orphan drug credits   6.90% 18.06% 6.40%
State income tax, net of federal benefit   7.46% 7.08% 5.40%
Other   2.13% 0.04% 0.83%
Foreign rate differential   (0.03%)    
Deferred rate change   (0.08%)   (27.58%)
Change in valuation allowance   (39.26%) (49.68%) (20.54%)
Effective income tax rate   0.00% 0.00% (0.03%)